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Title: Treatise on Geomorphology (MORP)
Article Title/Article ID: Streams of the Montane Humid Tropics/00256
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Non Print Items
KeywordsBoulders; Mountain streams; Steep gradients; Tropical.
Author and Co-author Contact Information
Fred N ScatenaAU15Earth and Environmental Science240 S 33rd StreetPhiladelphiaPA 19104-6316USA
A Gupta
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c0010 9.31 Streams of the Montane Humid TropicsFN ScatenaAU1 , Earth and Environmental Science, Philadelphia, PA, USAA Gupta
r 2012 Elsevier Inc. All rights reserved.
9.31.1 Introduction 19.31.1.1 Historic Perspective 29.31.1.2 Environmental Settings of TMSs 29.31.1.3 Tectonic Settings 39.31.1.4 Modern Climate 39.31.1.5 Paleoclimate 49.31.1.6 Vegetation of Tropical Montane Watersheds 59.31.2 Hydrology and Aquatic Ecology of TMSs 59.31.2.1 Runoff Generation in TMS 59.31.2.2 Floods and Storm Flows 69.31.2.3 Aquatic Ecology of Tropical Rivers 69.31.3 Water Quality and Denudation 79.31.3.1 Water Quality 79.31.3.2 Denudation 79.31.4 Channel Morphology of TMSs 89.31.4.1 Drainage Networks of TMSs 89.31.4.2 Longitudinal Profiles and Hydraulic Geometry 89.31.4.3 Channel Features 99.31.4.4 Floodplains and Riparian Zones 99.31.4.5 Role of Instream Wood 109.31.5 Response to Anthropogenic Disturbances 119.31.5.1 Land-Use Change 119.31.5.2 Dams and Water Diversions 119.31.5.3 Climate Change 119.31.6 Conclusions 12References 13
Abstract
TropicalAU2 montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastalregions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not
been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive
2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfallsthat alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these
streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic
landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging
view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a highfrequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic
processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to
transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in
other fluvial systems.
s0010 9.31.1 Introduction
p0010 Tropical landscapes have played an important role in the sci-
entific development of geomorphology and in evaluating the
relative roles of climate, structure, process, and time in land-
scape development. Understanding the fluvial geomorphology
of tropical streams in general and tropical montane streams
(TMSs) in particular is essential, as they produce a dis-
proportionately large amount of the sediment, carbon, and
material that reaches coastal regions (Milliman and Syvitski,
1992; Lyons et al., 2002; Meade, 2007; Goldsmith et al.,
2008). TMSs are also important water sources and drain some
of the planet’s most diverse ecosystems and areas that are
Scatena, F.N., Gupta, A., 2012. Streams of the montane humid tropics. In:
Shroder, J., Jr., Wohl, E. (Eds.), Treatise on Geomorphology. Academic Press,
San Diego, CA, vol. 9, pp. xx–xx. [Please replace ‘xx’ by correct page number
when available.]
MORP 00256
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considered especially sensitive to environmental and climate
change (Emanuel et al., 1985; Pepin and Lundquist, 2008;
Colwell et al., 2008). Despite the importance of TMSs, their
fundamental properties have received only limited systematic
description (Wohl, 2006). This chapter reviews the current
state of knowledge of the fluvial geomorphology of streams in
montane tropical environments, starting with a historical
perspective and their environmental settings. This is followed
by a review of their morphology and their known responses to
disturbances. The chapter concludes with discussion of
knowledge gaps and research needs.
s0015 9.31.1.1 Historic Perspective
p0015 Studies of early tropical geomorphology were dominated by
the observations of short-term visitors from temperate lati-
tudes. This early work was inherently descriptive and typically
focused on attention-grabbing features such as inselbergs (see
Wirthmann, 2000; Thomas, 2006). In general, tropical land-
scapes were considered unique assemblages of landforms de-
veloped from long periods of intense chemical weathering in
climatically and tectonically stable areas. Thus, many tropical
landscapes were considered to be the end products of a
Davisian-type of landscape evolution. The highly productive
and diverse forests that covered these landscapes were recog-
nized as the unique end products of millions of years of
relatively undisturbed evolution. The rivers that drain these
landscapes have also been considered unique since Alexander
von Humboldt described the bedrock rapids of the Orinoco
lowlands (Wirthmann, 2000). Subsequently, many writers
have opined that the combination of intense chemical wea-
thering, forceful rainfalls, and assumed climatic and tectonic
stability has caused tropical rivers to incise and produce
unique assemblages of bedrock-lined rapids and low-gradient
reaches that flow over weathered bedrock covered by a thin
layer of boulders and alluvium (see Wirthmann (2000) for an
excellent review).
p0020Until the mid-1950s most researchers in tropical geo-
morphology were based in Europe and the research focused
on defining climatic–landform assemblages in cratonic set-
tings (Budel, 1982; Kesel, 1985). In these studies, tropical
mountain valleys were commonly described as narrow and
V-shaped. Lowland rivers were thought to exhibit little lateral
erosion and were expected to be dominated by incision (Kesel,
1985). In the past 50 years, tropical geomorphology has
shifted away from its historic fixation on steady change and
the hills and plains of the Gondwana continent and toward an
explicit recognition of the dynamic and diverse nature of the
tropics and the acknowledgment that landscapes are sculpted
by a range of formative events and the restorative processes
between these events (Wolman and Gerson, 1978; Scatena,
1995; Brunsden, 1996). Although inselbergs and planation
surfaces are still in vogue (Coltorto et al., 2007), a much larger
emphasis is currently focused on (1) quantifying the role of
tropical rivers in global biogeochemical budgets (Milliman
and Syvitski, 1992; Douglas and Guyot, 2004; Carey et al.,
2005; Meade, 2007; Goldsmith et al., 2008) and (2) evaluating
the relative roles of climate and tectonics in weathering and
landscape evolution (White et al., 1995 AU3, 1998; Riebe et al.,
2001; Hsieh and Knuepfer, 2001; Whipple, 2004; Latrubesse,
2006). Rivers of the humid tropical mountains play a central
role in these debates.
s00209.31.1.2 Environmental Settings of TMSs
p0025This chapter is focused on streams in mountainous regions of
the humid tropics that currently receive 2000–3000 mm or
more of precipitation each year. Figure 1 contains a general-
ized map of their occurrence that was developed from our
knowledge of their distribution and by identifying ecoregions
of tropical and subtropical humid montane forests (Olson
et al., 2001). Nevertheless, identifying TMS can be just as
challenging and as conceptually useful as defining large rivers
(Potter, 1978; Miall, 2006). As described in detail later, the
TMSs considered here are located in forested montane areas
30° North
30° South
0 2500 50001250Kilometers Zones of tropical montane streams
Isothermality >=50%
Major tracks of cyclones
N
S
EW
f0010 Figure 1 The distribution
AU16
of environments with tropical montane streams, paths of major cyclones, and areas where isothermality is Z50%.Isothermality is defined as mean diurnal air temperature range/monthly air temperature range and reflects tropical and subtropical climates. Seetext for details.
MORP 00256
2 Streams of the Montane Humid Tropics
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below the alpine tree line and in areas that have cooler tem-
peratures and higher rainfall than adjacent lowland regions.
They also tend to drain spatially diverse and complex geology
before they enter the lowlands and coastal plain. For example,
in Central America, TMS streams flowing over Pleistocene
glacial deposits can be a few kilometers away from streams
draining highly weathered oxisols and carbonate platforms.
s0025 9.31.1.3 Tectonic Settings
p0030 In general, TMS streams drain continental and insular
mountains in active subduction zones, collision belts, rift
zones, and volcanic arcs. Transform faults have also been
important in the development of TMSs in the Caribbean,
Taiwan, and Indonesia. They also have diverse tectonic his-
tories and ages that have influenced their climate and geo-
morphic development in complex ways (Thomas, 2006).
India has traversed the equator through much of the Cenozoic
to form the Himalayan collision belt and the Western Ghat
escarpment that are now drained by TMS. This collision also
resulted in a series of strike-slip faults that traverse Southeast
Asia and influenced the location of many low mountain and
hill streams that are currently drained by TMS. Tropical South
America has TMS associated with Pacific and Car-
ibbean–South American plate boundary interactions that date
from the Cretaceous. Africa has slowly moved northward with
TMS draining rift zones and uplifted sedimentary rocks, while
Australia has moved from a near-polar position to its current
subtropical location and has TMS draining Paleozoic bedrock.
This diversity in geologic and tectonic history contradicts the
historic notion of the old tropical Earth and indicates that
many tropical landscapes and TMS have not developed under
fixed climatically or latitudinally defined conditions.
s0030 9.31.1.4 Modern Climate
p0035 Superimposed upon the geologic and tectonic variability of
TMS is a diverse set of climatic conditions. The tropics can be
defined as the area of surplus radiative energy that is bounded
by anticyclonic circulations near the 301 north and southlatitudes (Reynolds, 1985; McGregor and Nieuwolt, 1998;
Callaghan and Bonell, 2004). The climate of TMSs lacks very
cold seasons and they have a consistent diurnal range of air
temperature throughout the year (Hijmans et al., 2005). They
also have spatially complex patterns of precipitation that result
from the interaction of low-level circulation patterns, cyclonic
circulation, easterly waves, and the seasonal march of the
Intertropical Convergence Zone (ITCZ). Locally, precipitation
patterns are also influenced by land–sea breezes, orographic
uplifts, and the trade wind inversions. In many tropical
mountains, these processes interact such that the zone of
maximum annual rainfall occurs between 1000 and 1500 masl
(McGregor and Nieuwolt, 1998) and within the catchments of
TMS. The climates of these watersheds are typically within the
Af and Am groups in the Köppen–Geiger climate classification
system. In the Holdridge Life Zone system, these areas are
within the lower montane to upper montane altitudinal belts
of the tropical and subtropical moist, wet, and rain-forest life
zones (Holdridge, 1967).
p0040The major climatic feature that distinguishes the humid
tropics from the dry tropics is that average annual rainfall is
greater than potential evapotranspiration and there is enough
precipitation to support evergreen or semi-deciduous forests.
Many, if not most, TMS streams drain areas that receive an
annual precipitation greater than 2000 or 3000 mm yr�1. Be-
cause of the considerable seasonal variations in precipitation
and runoff, it is also common in the geomorphic literature to
explicitly acknowledge the presence of the seasonal and
aseasonal humid tropics (Gupta, 1975, 1988, 1995). The
seasonal humid tropics can be broadly defined as areas that
have a marked seasonal concentration of rainfall and runoff.
These areas are typically influenced by the ITCZ or the mon-
soonal rains and it is not uncommon that 80% of their annual
stream flow occurs in 4 or 5 months of the year. Whereas most
areas in the aseasonal humid tropics have a mean annual
rainfall between 2000 and 4000 mm yr�1, the mean annual
rainfall of the seasonal tropics is more variable and ranges
between 1000 and 6000 mm. In these areas, the interannual
variability of runoff is large (Mahe et al., 2004) and stream
channel geometry can change dramatically between wet and
dry seasons (Gupta, 1995).
p0045Interannual- to millennial-scale variability in rainfall,
flooding, drought and hurricane intensity, sediment transport
and deposition, water quality, and the structure of aquatic
populations of TMS have all been related, albeit complexly in
many cases, to changes in sea-surface temperatures, the El
Niño–Southern Oscillation (ENSO), monsoons, and other
global-scale circulation systems (see Douglas et al., 1999;
Rodbell et al., 1999; Giannini et al., 2001; Aalto et al., 2003;
Donnelly and Woodruff, 2007). In general, the interannual
variability of rainfall that influences TMS increases with de-
creasing rainfall, decreasing latitude, and the influence of the
ITCZ and ENSO (Dewar and Wallis, 1999). There is a general
impression, and in some places a misconception, that the
humid tropics are characterized by a domain of steady but
low-intensity rains, while the seasonal tropics have a higher
frequency of intense rainfalls. Although inverse relationships
between rainfall intensity and total rainfall have been shown
in some tropical areas, these relationships are not universal
(Yu, 1995).
p0050Frequent landscape-altering rainfalls are characteristic of
the watersheds of TMS and can occur in both dry and wet
seasons. Multiday rainfalls over 2000 mm are not uncommon
(Table 1 AU4), rainfalls greater than 500 mm d�1 typically have
recurrence intervals of 20 years or less, and daily totals greater
than 75 mm d�1 occur in most years (Gupta, 1988; Scatena
et al., 2004; Chu et al., 2009). Rainfall intensities and event
totals are commonly an order of magnitude higher in the
humid tropics compared to humid temperate regions and
rainfalls with intensities of 25 mm h�1 or more can account
for more than 30% of annual rainfall, whereas they typically
account for less than 5% in the temperate areas (Bonell,
2004). Maximum rainfall intensities also tend to occur in
tropical highlands below 1500 masl (McGregor and Nieuwolt,
1998), an elevation that is drained by TMS.
p0055Hurricanes and cyclonic depressions traverse many TMSs
but are rare in Africa, South America, and most of Southeast
Asia (Figure 1). In regions where they are common, an average
of 5–25 cyclonic storms can occur each year (Scatena et al.,
MORP 00256
Streams of the Montane Humid Tropics 3
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2004). Where hurricanes pass directly over a TM watershed,
defoliation, landslides, and flooding are widespread and can
alter local and regional-scale hydrologic and nutrient cycles
(Scatena and Lugo, 1995; McDowell et al., 1996; Schaffer
et al., 2000; Gupta, 2000; Lyons et al., 2002; Carey et al., 2005;
Goldsmith et al., 2008). Although event rainfalls can exceed
2000 mm, average rainfalls within 222 km of the eye of a
hurricane are on the order of 100 mm d�1 (Anthes, 1982;
McGregor and Nieuwolt, 1998). Locally their impact is
strongly influenced by their storm tracks and physiography
and topography (Gupta, 1988). Likewise, not all the intense
rainfalls or peak discharges of TMS are associated with hurri-
canes or cyclonic depressions. Multiday rainfall events asso-
ciated with annual monsoons or the seasonal passage of the
ITCZ may actually generate more geomorphic work and have a
larger overall influence on fluvial landscapes than cyclonic
systems that have recurrence intervals on the orders of decades
in any location.
p0060 One characteristic that appears to be relatively common in
TMS is the basin-wide nature of the intense storms. Moreover,
monsoons, hurricanes, and ITZC-related storms tend to cover
such large regions that even large drainage basins receive
geomorphic significant rainfalls at the same time. This results
in large parts of the basin contributing to and experiencing
channel-modifying discharges at the same time. This is in
contrast to the spatially restricted contributions of snowmelt
or convective storms that often cause important but relatively
localized geomorphic impacts in temperate montane streams
(see Chapter 9.28 Specific Fluvial Environments: Steep
Headwater Channels (00253)).
p0065 Prolonged droughts and fires are an important, but com-
monly underestimated, disturbance in humid tropical forests
and in TMS (Walsh and Newbery, 1999; Grau, 2001; Malmer
et al., 2004; Sherman et al., 2008). Fires are less common than
droughts and the highest fire frequencies occur below the
cloud forest zone and where annual precipitation is seasonal
and/or less than 1000 mm yr�1. In most humid tropical for-
ests, cumulative rainfall deficits between 5% and 10% of mean
annual precipitation are common on annual and decadal
timescales (Scatena et al., 2004). During droughts with re-
currence intervals approaching a decade, riffles in headwater
TMS can dry up, pools can be isolated and reduced in volume,
and there can be localized crowding of benthic invertebrates
(Covich et al., 1998, 2003, 2006).
s00359.31.1.5 Paleoclimate
p0070Geomorphic legacies of past climates are widely recognized in
the fluvial environments of the mid-latitudes. By contrast,
because the tropics were historically considered as being cli-
matically stable, an explicit consideration of the geomorphic
legacies of past climates has not been a tradition in tropical
studies. It is now known that many tropical landscapes have
undergone considerable climatic changes during the Quater-
nary. Prior to about 28 000 14C year BP, the TMSs of Africa,
South America, and Australia had experienced humid forested
conditions for approximately 104 years (Thomas, 2003).
During the Last Glacial Maximum (LGM), between 21 000 and
18 000 14C yr BP, large parts of the tropics were cooler, rainfall
was reduced by 30–60%, and there was a reduction in the
extent of humid tropical forests (Servant et al., 1993; Mahe
et al., 2004; Kale et al., 2003;Goodbred, 2003; Thomas, 2003
and references therein). As the glaciers retreated, rainfall and
the extent of humid tropical forests increased and major
changes in fluvial activity apparently took place in several
tropical basins.
t0010 Table 1 Select examples of extreme rainfall events that have influences on tropical montane streams
Total rainfall (mm per event) Average mm d�1 during event Location Dates Source
Hurricanes5678 568 La Reunion 18–27 Jan 1980 Landsea, 19993240 1080 La Reunion 24–27 Jan 1980 Landsea, 19992467 1233 La Reunion 8–10 Apr 1958 Landsea, 19992287 327 Jamaica 4–11 Nov 1909 Gupta, 19882025 405 Cuba 3–8 Oct 1963 Gupta, 19881825 1825 La Reunion 7–8 Jan 1966 Landsea, 19991524 762 Jamaica 5–7 Oct 1963 Gupta, 19881248 1248 Taiwan 10–11 Sept 1963 Gupta, 19881168 1168 Philippines 14–15 July 1911 Gupta, 1988
Monsoon3388 484 India 9–16 June 1876 Gupta, 19883213 536 India 24–30 June 1932 Gupta, 19881036 1036 India 14 June 1876 Gupta, 1988
Frontal systems and the ITCZ2789 930 Jamaica 22–25 Jan 1960 Gupta, 19881109 1109 Jamaica 23 Jan 1960 Gupta, 1988911 304 Venezuela 14–16 Dec 1999 PAHO, 1999867 867 Cuba 1 June 1996 Planos, 2003
Modified from Scatena, F.N., Planos-Gutierrez, E., Schellekens, J., 2004. Impacts of natural disturbances on the hydrology of tropical forests. In: Bonell, M., Bruijnzeel, L.A. (Eds.),
Forest, Water and People in the Humid Tropics. International Hydrology Series. Cambridge University Press, Cambridge, ch. 19, pp. 489–513.
MORP 00256
4 Streams of the Montane Humid Tropics
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p0075 The early Holocene in many TMSs was relatively wet. In
some areas, precipitation may have been elevated 20–35%
above recent means, and 40–80% greater than the LGM
minima (Goodbred, 2003; Thomas, 2003; Mahe et al., 2004
and references therein). Consequently, early Holocene chan-
ges in the fluvial environments were probably rapid and er-
ratic and there is some evidence that many tropical rivers
excavated deep rocky channels as a result of high discharges
that occurred during the Pleistocene–Holocene transition. The
maximum extension of modern tropical rainforests is thought
to have occurred in the early Holocene (i.e., 9500–8500 14C
year BP). This was followed by a mid-Holocene dry period that
created favorable conditions for forest contraction, forest fires,
and cut-and-fill episodes in alluvial reaches of lowland tropi-
cal streams. Humid tropical forests in Africa, Asia, Amazonia,
Central America, and the Caribbean have all experienced fires
and extended droughts during the past 10 000 years (Sanford
et al., 1985; Hodel et al., 1991; Guilderson et al., 1994).
Variations in the Holocene frequency of hurricanes and cata-
strophic rainfalls have also been linked to terrace formation
and channel incision in Taiwan (Hsieh and Knuepfer, 2001)
and coastal processes on Caribbean islands (Donnelly and
Woodruff, 2007).
p0080 Comparisons of the present hydrological and climatic
setting with the climate regime necessary to maintain full lake
levels at steady state can be used to gauge the degree to which
tropical drainages are adjusted to their present climatic regime
(Burrough and Thomas, 2009). This disequilibrium lake basin
index suggests that equatorial lakes are currently closer to their
full lake conditions at steady state than lakes in the subtropics.
A similar spatial pattern of channel disequilibrium may apply
to TMSs and their lowland counterparts. In any case, there is
ample evidence to suggest that many TMSs have fluvial fea-
tures related to Pleistocene climates and many may still be
adjusting to previous climatic regimes.
s0040 9.31.1.6 Vegetation of Tropical Montane Watersheds
p0085 Process-based classifications of tropical forests are farther ad-
vanced than classifications of tropical fluvial systems. Dis-
tinctions between forests are typically based on phenology
(i.e., evergreen or deciduous), climate (i.e., rain, wet, moist, or
dry forests), physiography (i.e., upland and lowland), and
hydrologic conditions (i.e., cloud forests, riparian, and wet-
land). The natural vegetation that typically covers the water-
sheds of the TMS discussed here are evergreen rain, wet, moist,
or cloud forests. On large equatorial mountains, the transition
from montane forests to subalpine forests or grasslands is
generally observed at elevations between 2800 and 3200 m
(Bruijnzeel, 2001). As such, this type of land cover is only
encountered in TMSs that drain the highest mountains, most
of which occur in Latin America, the Himalayas, and Papua
New Guinea.
p0090 Currently, most watersheds drained by TMS are covered by
mixtures of cut-over forests, pasture, coffee plantations, crop-
land, and small communities. Historically, the steep and
rugged terrain of TMSs provided them with some basic level of
protection. However, by the early 1990s tropical montane
forests were high on the list of the world’s most threatened
ecosystems and they were being deforested at a rate that was
considerably greater than that of lowland tropical forests
(1.1% yr�1 vs. 0.8% yr�1; Doumenge et al., 1995). The total
potential area of tropical montane forests has been estimated
by various methods to be between 3 and 5 million km2
(Bruijnzeel et al., 2010). Approximately 45–56% of these
forests remain and are drained by TMSs. Although protecting
these forests is still considered a critical conservation need, the
recognition of their importance as biodiversity centers and
water sources has resulted in some increased legal protection.
Abandonment and subsequent reforestation of watersheds
drained by TMSs is also occurring in some areas (Aide and
Grau, 2004). Although the impact of this reforestation on TMS
morphology is uncertain, hydrologic analysis suggests that
reforestation can decrease sediment yields, low-flow dis-
charges, annual runoff, and the proportion of rainfall that
contributes to stream flow (Bruijnzeel, 2004; Wu et al., 2007;
Bruijnzeel et al., 2010). The influence of reforestation on peak
storm flow discharge and stream power is less certain but may
be proportionately less than the influence of reforestation on
water quality and low stream flows.
s00459.31.2 Hydrology and Aquatic Ecology of TMSs
s00509.31.2.1 Runoff Generation in TMS
p0095It is commonly assumed that humid tropical landscapes have
quick hydrologic response times and high runoff coefficients
that result in flashy streams with high peak discharges that
ultimately incise channels. In practice, infiltration rates can
range from 0 to over 200 mm h�1 (Harden et al., 2003 AU5) and
runoff generation is complex and dependent on land cover,
antecedent conditions, bedrock lithology, and basin and ri-
parian morphology (Walsh, 1980; McDowell et al., 1992;
Dykes and Thornes, 2000; Elsenbeer, 2001; Schellekens et al.,
2004; Bonell, 2004; Niedzialek and Ogden, 2005; Saunders
et al., 2006 and references therein). Multivariate analysis has
been used to determine the relative influence of physical
characteristics and land cover on the hydrology and water
quality of several watersheds that contain TMS (Santos-Román
et al., 2003; Rivera-Ramirez et al., 2002; Soldner et al., 2004;
Harmon et al., 2009). The factors most commonly linked to
runoff quantity and water quality are bedrock geology, dom-
inant land cover (i.e., forest, agriculture, and urban), and
elevation, which is typically a cross-correlated surrogate of
precipitation and/or land use.
p0100The most characteristic feature of the response of TMS to
precipitation is the rapid and extremely flashy nature of
catchment runoff that is attributed to a variety of shallow
subsurface flow paths. US AU6DA Soil Conservation curve num-
bers (CNs) calculated for 28 storms in the predominantly
forested Rio Chagres Basin of Panama ranged from 64 to 98
and depended on storm intensity and antecedent conditions
(Calvo et al., 2005). These authors recommend a CN of 75 for
extreme storms in the wet season. The area-weighted CN for
TMS and their associated lowlands reaches of northeastern
Puerto Rico decreased from 74 to 60.7 as barren agricultural
lands and pastures were reforested (Wu et al., 2007). However,
stream flow response times and the ratio of runoff to rainfall
MORP 00256
Streams of the Montane Humid Tropics 5
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can vary within and between seasons and for some areas these
figures can be relatively high at the end of the dry season when
the soils are cracked and macro-pores are abundant (Nied-
zialek and Ogden, 2005).
s0055 9.31.2.2 Floods and Storm Flows
p0105 Flooding is common throughout the tropics and several dis-
tinct flood regimes have previously been distinguished and
include: (1) occasional short-term flood; (2) frequent or an-
nual short-term flooding; (3) annual long-term flooding; and
(4) annual submersion by floodwaters (Salo et al., 1986;
Scatena et al., 2004). Only the first two of these regimes are
common in TMSs and TMSs in both seasonal and non-
seasonal environments and are characterized by a regime
where short-term events capable of transporting bedload (see
Chapter 9.8 Bedload Kinematics and Fluxes (00233) and
Chapter 9.28 Specific Fluvial Environments: Steep Head-
water Channels (00253)) and removing periphyton (see
Chapter 9.12 Influence of Aquatic and Semi-Aquatic Or-
ganisms on Channel Forms and Processes (00237)) occur
several times each year. Peak discharges are several orders of
magnitude larger than base flow but average annual peak
flows are generally not channel-forming or -modifying events,
especially in areas with boulders and bedrock-lined channels
(Scatena et al., 2004; Pike et al., 2010; see Chapter
9.29 Bedrock Rivers (00254)). Major channel-modifying
events have been associated with peak discharges that range
from 20.9 to 65 m3 s�1 km�2 and have recurrence intervals on
the order of decades (Gupta, 1988; O’Connor and Costa,
2004; Garcin et al., 2005). The defoliation and uprooting as-
sociated with hurricanes can also produce considerable
amounts of nutrient-rich green litter and wood (see Chapter
9.11 Wood in Fluvial Systems (00236)) that can clog chan-
nels and even cause temporary reductions in suspended
sediment yields (Lodge et al., 1991; Gellis, 1993; see Chapter
9.9 Suspended Load (00234)).
p0110 Although the importance of flooding to TMS is widely
acknowledged, so is the difficulty in estimating the recurrence
intervals of moderate to extreme floods in ungauged TMS
(Pike and Scatena, 2010). In a well-gauged TMS network in
Puerto Rico, flood discharges that are close to the annual peak
are commonly experienced several times in a year. Compara-
tive analysis of these streams also showed that in these flashy
and relatively small streams, annual maximum flow series
analysis fails to capture the intra-annual flows that are re-
sponsible for structuring the vegetation in and adjacent to the
channels. A partial duration series based on 15-min discharges
is recommended for most analyses.
s0060 9.31.2.3 Aquatic Ecology of Tropical Rivers
p0115 Research on the aquatic ecology of tropical rivers has high-
lighted differences between tropical and temperate streams
(see Chapter 9.12 Influence of Aquatic and Semi-Aquatic
Organisms on Channel Forms and Processes (00237)), in-
cluding the latitudinal variations in diversity, radiation, tem-
perature, geostrophic effects, and the influences of continuous
litter inputs, warm water, the lack of ice, and common high
flows (Payne, 1986; Jackson and Sweeney, 1995; Talling and
Lemoalle, 1998; Dudgeon, 2008). Recent efforts have focused
on the spatial variability in ecological processes in relation to
waterfalls and other geomorphic conditions that influence
within-channel habitats and the migration and distribution of
species (Wantzen et al., 2006; Boyero et al., 2009).
p0120Although studies of tropical stream metabolism that ex-
tend for at least 1 year and/or extend along the longitudinal
profile of a basin are scarce, available information suggests
that rates of in-stream photosynthesis in forested TMS are
similar to those of similarly sized streams draining temperate-
deciduous forests (Ortiz-Zayas et al., 2005). However, con-
tinual herbivory and a high frequency of bedload-transporting
storms interact to suppress the abundance of periphyton and
submerged aquatic plants in TMS. Consequently, their rates of
respiration are much higher than in most temperate streams
and they can have ratios of photosynthesis to respiration of
less than 1 from their headwaters to their lower reaches.
Nevertheless, where tropical rainforest vegetation is present, it
can provide streams with sufficient amounts of labile organic
carbon to support high rates of respiration over long distances
and make tropical streams globally important sources of car-
bon inputs to oceans (Kao and Liu, 1996; Lyons et al., 2002;
Ortiz-Zayas et al., 2005).
p0125Short-term floods and droughts cause significant in-
vertebrate mortality and shifts in population-age distributions
in TMS streams in Malaysia, Hong-Kong, India, Ecuador, tro-
pical Australia, the Andean piedmont of Venezuela, and the
Caribbean (Flechter and Feifarek, 1994; Scatena et al., 2004).
However, native populations have numerous morphological
and behavioral adaptations to deal with common, substrate-
disturbing stream flows, including suction-cup-like append-
ages that can cling to bedrock surfaces and the ability to hide
under large boulders or occupy shallow-water channel-margin
habitats during floods and droughts. Some pan-tropical
shrimp and fish species can migrate past vertical waterfalls that
are tens of meters high by migrating in bedrock joints and in
areas with moss coverings and laminar sheet flow.
p0130Although frequent floods and steep gradients characterize
TMS, many, if not the majority, of the native macro-fauna
living in TMSs migrate between rivers and coastal zones over
the course of their lives (March et al., 1998, 2003; March and
Pringle, 2003; Crook et al., 2009). Consequently, waterfalls,
dams, and other geomorphic or anthropogenic migration
barriers can have important roles in determining the com-
munity composition and longitudinal variation of aquatic
species in TMS. Coastal conditions, the distribution of
waterfalls and migration barriers, altitude, drainage area, ri-
parian and watershed land cover, water quality, substrate size,
and pool volume have all been correlated to the abundance of
aquatic organisms and the structure of aquatic communities in
TMSs (Pyron et al., 1999; Fievet et al., 2001; Zimmerman and
Covich, 2003; Soldner et al., 2004; Blanco and Scatena, 2005,
2006).
p0135Because of the importance of floods, waterfalls, and other
geomorphic barriers to the distribution of aquatic organisms
in TMSs, the question has been raised whether the river con-
tinuum concept (RCC), which successfully explains longi-
tudinal patterns in species distributions and food webs in
temperate streams (Vannote et al., 1980), also applies to TMS.
MORP 00256
6 Streams of the Montane Humid Tropics
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In general, the RCC suggests that the longitudinal distri-
butions of aquatic species reflect downstream changes in
channel morphology, discharge, sediment load, riparian cover,
and incident radiation on the water surface. Although geo-
morphic barriers do cause discontinuities in the distribution
of organisms in TMS, the few existing controlled studies sug-
gest that the general patterns of food web and resource
availability that are predicted by the RCC do exist (March and
Pringle, 2003; Greathouse and Pringle, 2006). It has also been
hypothesized that the ecology of TMS might be functionally
closer to montane temperate streams than to their lowland
counterparts and that lowland tropical streams might differ
substantially from lowland temperate streams (Boyero et al.,
2009). Increased knowledge of the life histories and habitat
requirements of tropical species are desperately needed to
quantify and understand longitudinal patterns and differences
between TMS and temperate streams.
s0065 9.31.3 Water Quality and Denudation
s0070 9.31.3.1 Water Quality
p0140 TMS streams have warmer water, higher annual exports of
dissolved constituents and sediment, and less seasonal dif-
ferences in water temperature and water chemistry than their
temperate counterparts. Their water quality is not influenced
by freeze–thaw cycles, ice-induced bank erosion, or seasonal
pulses of plant litter (although hurricane deforestation can
create significant pulses). Several studies have related stream
water concentrations of certain elements to either shallow
near-surface flow or longer and deeper flow paths (Schellekens
et al., 2004; Bonell, 2004; Bhatt and McDowell, 2007; Saun-
ders et al., 2006). Surface soils drained by TMSs are typically
wet, commonly saturated, and have intensive microbial ac-
tivity that can reduce and remove nitrogen from soil and
groundwater before it enters the stream channel (Chestnut
et al., 1999; Chestnut and McDowell, 2000). Local riparian
nitrogen dynamics and their ability to remove nitrogen before
it enters the stream channel depend on local lithology and
geomorphology (McDowell et al., 1992) and some of the
planet’s highest basin-average rates of denitrification are found
in the humid tropic systems in South America and Africa
(Seitzinger et al., 2006).
p0145 Most carbon exports from TMSs are in the form of dis-
solved organic carbon (McDowell and Asbury, 1994; Chestnut
et al., 1999; Lyons et al., 2002). Land-use change and large-
scale hurricane-related defoliation can result in significant
increases in stream water cation and carbon exports. However,
post-hurricane exports are less than a few percentages of the
hurricane-derived plant litter inputs, which reflects the tight
nutrient retention these systems can have (Schaffer et al.,
2000). Nevertheless, because of their high carbon exports and
storm-initiated CO2 consumption from silicate weathering,
some TMSs subject to tropical cyclones may be important
global sinks of CO2 transport to ocean burial (Kao and Liu,
1996; Lyons et al., 2002; Goldsmith et al., 2008; Draut et al.,
2009).
s00759.31.3.2 Denudation
p0150The average rate of ground surface lowering of TMS can be
well over 100 m per million years, but typically ranges be-
tween 50 and 75 m per million years (White et al., 1998;
Hsieh and Knuepfer, 2001; Hartshorn et al., 2002; Thomas,
2003; Riebe et al., 2004 AU7; Whipple, 2004 and references
therein). The sediment in most TMS is ultimately derived from
weathered saprolite, which typically contains meter-diameter
core stone boulders in a matrix of clays, silts, and sands.
Saprolite thickness varies widely and ranges from a few meters
to more than 100 m and comparisons of long-term denuda-
tion rates with the rate of saprolite advance suggest that the
saprolites drained by some TMSs have reached their steady-
state thickness (see Buss et al., 2008 and references therein).
p0155Rates of chemical denudation in TMSs are some of the
largest in the world (Milliman and Syvitski, 1992; White and
Blum, 1995; Syvitski and Milliman, 2007) and TMSs under-
lain by granite or on volcanic islands where meteorologic
waters are impacted by high subsurface temperatures are
among the highest of TMSs (Brown et al., 1995; Riebe et al.,
2001; Rad et al., 2007). The ratio of physical denudation to
total denudation in the drainages of TMS is variable and there
are insufficient studies for a definitive analysis. Nevertheless,
available rates suggest that physical denudation for TMS
drainages can range between 40% and 75% of total denuda-
tion and averages around 60% (White et al., 1998; Riebe et al.,
2001; Buss et al., 2008; Harmon et al., 2009 and references
therein).
p0160Slope failures tend to contribute most of the river sediment
in TMS, irrespective of the scale of the basin. In some areas,
landslides producing rain storms occur on an average of once
every 1–2 years and rainfall intensity-duration curves indicate
that slope failures can occur with most types of tropical rain
events, including the annual migration of the ITCZ, hurri-
canes, convective storms, and cold fronts (Scatena et al.,
2004). Available comparisons suggest that the rainfall
thresholds needed to trigger slope failures may be higher in
the humid tropics than in temperate areas (Larsen and Simon,
1993; Gabet et al., 1994 AU8). However, because the frequency of
these rains is higher, landslides have a significant influence on
hillslopes and channel processes. The most common slope
failures are shallow translational failures that have depths less
than 10 m (Simon et al., 1990; Larsen and Torres Sánchez,
1992; Maharaj, 1993; Paolini et al., 2005). Whereas most
slope failures are moisture driven and occur in the wet season
or following large tropical storms, earthquake-generated
landslides can also be significant in many TMS drainages
(Garwood et al., 1979).
p0165It is also common in TMSs that large flows can transport
years to decades worth of average annual sediment and ma-
terial flux in a single event. Modeling studies on the humid
tropics of Australia indicate that 30% of the total rainfall
contributes approximately 87% of the total transported sedi-
ment but only 45% of the total runoff (Yu, 1995). In Puerto
Rican streams, the highest recorded daily sediment discharges
are 1–3.6 times the annual suspended-sediment discharge,
and runoff from major storms transport 1–32 times the me-
dian annual sediment load (Warne et al., 2005). In eastern
Jamaica, stream flows associated with Hurricane Gilbert
MORP 00256
Streams of the Montane Humid Tropics 7
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transported large quantities of coarse bed sediment and over
1700 times the daily suspended load of the dry season (Tho-
mas, 1991; Gupta, 2000). In Taiwan, individual typhoons can
have hyperpycnal sediment concentrations (440 g l�1) andcan transport between 72% and 95% of the annual particulate
organic carbon fluxes of the highest yielding world rivers
(Goldsmith et al., 2008). Studies in Taiwan also suggest that
valley lowering and channel incision are driven by relatively
frequent flows of low to moderate intensity, whereas large and
rare floods are more important in widening bedrock channels
(Hartshorn et al., 2002).
p0170 The highest sediment yields in the humid tropics also
originate from TMS in tectonically active regions (Douglas and
Guyot, 2004). Whereas the global-scale battle regarding who is
responsible for delivering the most fluvial sediment to the
ocean may be fought between geology, geography, and
humans (Syvitski and Milliman, 2007), TMSs underlain by
highly weatherable bedrock and in areas subject to hurricanes
and occupied by marginalized farmers appear to be the win-
ners. Moreover, soil erosion in anthropogenically disturbed
TM watersheds can be several orders of magnitude larger than
pre-disturbed rates (Anderson and Spencer, 1991; Douglas
et al., 1992; Hewawasam et al., 2003; Douglas and Guyot,
2004; Sidle et al., 2004; Warne et al., 2005). These studies
suggest that TMS draining undisturbed forested watersheds
typically have sediment yields around 100–500 t km�2 yr�1.
Large, mixed land-use watersheds can have sediment yields
between 1000 and 3000 t km�2 yr�1. Small, highly disturbed
areas associated with logging can have yields greater than
25 000 t km�2 yr�1 (Sidle et al., 2004).
s0080 9.31.4 Channel Morphology of TMSs
s0085 9.31.4.1 Drainage Networks of TMSs
p0175 The hillslopes that drain into TMS are typically characterized
by landslide scars and a dense network of intermittent swales
and channels that dissect the landscape into narrow inter-
fluves and deep valleys. Reported drainage densities of TMSs
range from 2.6 to over 20 km/km�2 (Scatena and Lugo, 1995;
Walsh, 1996; Terry, 1999). In general, drainage densities in
humid tropical areas are considered to be higher than in
humid temperate areas because of higher precipitation inten-
sities but lower than in semi-arid areas because of greater
vegetation cover (Chorley et al., 1984). Detailed analyses of
several tropical areas further suggest that the relationship be-
tween drainage density and annual rainfall in TMS networks is
nonlinear and influenced by extreme daily rainfall totals and
the permeability, mineralogy, and storage capacity of soils
(Walsh, 1996). Analysis of several TMS networks indicates that
drainage density increases relatively rapidly until approxi-
mately 2500–3000 mm yr�1, at which point it increases at a
reduced rate with further increases in annual rainfall (Walsh,
1996). These studies also indicate that these networks com-
monly failed to conform to Horton’s laws of stream numbers
and that while the high channel densities can develop in less
than 50 years, major changes in basin or network shape do
not occur before 14 000 years.
p0180The drainage networks of TMSs are commonly described as
rectangular and structurally controlled and as having straight
nonaccordant tributaries that join the main channels at high
angles (Ahmad et al., 1993; Hare and Gardner, 1995 AU9; Ng,
2006). On the Nicoya Peninsula of Costa Rica, drainage basin
asymmetry has been used to identify centers of uplift and
direction of tilt (Hare and Gardner, 1995). In the Greater
Antilles, the rectangular network morphology of TMS has been
related to strike-slip, plate-boundary tectonics (Ahmad et al.,
1993) and in Hong Kong the headwater progression of the
drainage network has been related to systematic variations on
landslide morphology and density (Ng, 2006). It is typically
unclear whether the slope breaks and steps at the junctions of
a tributary and the mainstem are actively retreating knick-
points, structural, or high-flow features. These nonaccordant
tributary junctions have been known to influence the up-
stream migration of aquatic species.
s00909.31.4.2 Longitudinal Profiles and Hydraulic Geometry
p0185Average stream gradients of TMS are typically well above the
0.002 m m�1 threshold that has typically been used to define
montane streams (Wohl and Merritt, 2005, 2008). Their lon-
gitudinal profiles are typically described as being segmented
by waterfalls and alternating steep and lower gradient seg-
ments with morphology correlated to bedrock morphology.
For example, along the upper Rio Chagres watershed of Pan-
ama, reaches flowing across granites, diorites, and tonalites
have lower gradients and wider channels than reaches flowing
across gabbros and diorites (Wohl, 2005). In the Luquillo
Mountains of Puerto Rico, lower stream gradients are associ-
ated with granodiorite, whereas steep gradients are associated
with more erosion-resistant contact metamorphic rocks (Pike
et al., 2010). In Fiji and Hawaii, waterfalls can also occur
where stream flows across more resistant lava flows (Terry,
1999).
p0190Hydraulic geometries of many mountain streams are
highly variable and complex (Wohl and Merritt, 2005; see
Chapter 9.18 Hydraulic Geometry (00243) and Chapter
9.28 Specific Fluvial Environments: Steep Headwater Chan-
nels (00253)). Nevertheless, the few available studies suggest
that TMSs may have better developed downstream hydraulic
relationships than their temperate counterparts, especially
when compared to montane streams in temperate areas that
have been recently glaciated (Pike et al., 2010). Unfortunately,
the interpretation of hydraulic geometry in mountain streams
in general, and TMS in particular, is complicated because of
the lack of identifiable bankfull or reference discharges to
compare channel geometry at constant flow frequencies. Be-
cause of the lack of well-defined floodplains or other bank-full
features, most studies of the hydraulic geometry of montane
streams have based hydraulic geometries on reference dis-
charges that are associated with recent floods (Pike et al.,
2010). Nevertheless, in TMS reaches of Puerto Rico that lack a
floodplain, a definable channel boundary that is characterized
by the incipient presence of soil, woody shrubs, and trees
corresponds to the same flow frequency as the bankfull dis-
charge of nearby alluvial channels. The reference discharge
based on these riparian features has an average exceedance
MORP 00256
8 Streams of the Montane Humid Tropics
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probability between 0.09% and 0.30%, and a recurrence
interval between 40 and 90 days.
s0095 9.31.4.3 Channel Features
p0195 Bedrock channels (see Chapter 9.29 Bedrock Rivers
(00254)), boulder bars and boulder-lined channels, step–-
pools (see Chapter 9.20 Step–Pool (00245)), pool–riffle se-
quences (see Chapter 9.21 Pool-Riffle (00246)), and all of
the morphologic features observed in other mountain streams
(Montgomery and Buffington, 1997; Thompson et al., 2006)
have all been observed in TMSs. Although it is generally
considered that the abundance of bedrock channels or bed-
rock–alluvial channels (sensu Whipple, 2004) is relatively high
in TMSs, a comprehensive data set does not exist to verify this
quantitatively. Nevertheless, reaches with a continuous or
deep cover of alluvial sediments are generally lacking, whereas
reaches with accumulations of large-diameter boulders as well
as boulder leeves, boulder bars, and boulder steps are com-
mon (Figures 2 and 3). The origin of the boulders varies, as
some large boulders are exhumed core stones whereas others
have been transported to channels during slope failures and
debris flows (Ahmad et al., 1993; Terry, 1999).
p0200 Globally, TMSs in the following areas are considered to
have the intense rainfalls, steep slopes, and the geologic sub-
strate that produce coarse-grained material to maintain the
morphology created during large floods (Gupta, 1988): (1)
river valleys of East Asia, especially Taiwan and the Philip-
pines; (2) upland areas of Vietnam, Sumatra, Java, and Burma;
(3) humid areas of the Indian subcontinent; (4) Madagascar
and neighboring parts of coastal East Africa; (5) North and
Northeast Australia; and (6) Caribbean basin and highlands of
Central America.
p0205 In some regions, the morphology of tropical stream
channels has also been related to a pronounced seasonality in
stream flow. Streams in the seasonal dry Kimberley Plateau of
tropical Australia have a unique channel system where narrow
bedrock-lined reaches alternate with wider alluvial reaches
that have sandy ridges and anabranching channels (Wende
and Nanson, 1998). In areas with large storms and large
seasonal fluctuations in discharge, alluvial reaches of TMSs can
have a nested morphology that consists of a large storm flow
channel and a smaller channel that carries interstorm dis-
charges (Gupta, 1995). The interstorm channels are box
shaped and have high banks and high width–depth ratios. The
high-magnitude floods can occupy the entire valley bottom
and are sufficiently frequent that the high-flow channels fea-
tures are maintained. This type of multiple low- and high-flow
channels appears to be most common and pronounced in
areas subject to monsoon rains.
s01009.31.4.4 Floodplains and Riparian Zones
p0210Continuous alluvial floodplains (see Chapter 9.22 Flood-
plains (00247)) or riparian zones (see Chapter 9.14 Re-
ciprocal Relations between Riparian Vegetation, Fluvial
Landforms, and Channel Processes (00239)) are rare in TMSs.
Instead, channel margins are most commonly boulder-lined,
steeply sloping hillslopes, or consist of smaller patches of al-
luvium associated with tributary junctions, slope breaks, or
former slope failures. Floodplains and associated terraces are
more common in middle and lower reaches of the drainages.
In Fiji, cesium profiles in floodplain sediments indicate ac-
cretion rates of 3.2 cm yr�1 over the past 45 years (Terry et al.,
2002). These high accretion rates are attributed to the high
frequency of tropical cyclones that pass the area (40 between
1970 and 2002).f0015 Figure 2 The Rio Mameyes River in the Luquillo Mountains of
Puerto Rico.
f0020Figure 3 Headwater tropical montane stream in the LuquilloMountains of Puerto Rico.
MORP 00256
Streams of the Montane Humid Tropics 9
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p0215 Although alluvial riparian zones along TMSs are dis-
continuous and typically occupy less than 10–15% of the
landscape, they can be significant sources of storm flow
(Schellekens et al., 2004) and have large influences on light,
temperature, and the carbon and nitrogen chemistry of TMS
water (Chestnut and McDowell, 2000; Chestnut et al., 1999;
Heartsill-Scalley and Aide, 2003; MacKenzie, 2008). In the
subtropical wet forests of the Luquillo Mountains in Puerto
Rico, steep topographic and hydraulic gradients between the
riparian zone and the stream were responsible for nearly
constant inputs of groundwater to these first- to third-order
streams (McDowell et al., 1992, 1996). However, the abun-
dance and hydrologic and biogeochemical influence of ripar-
ian zones were highly dependent on geology such that areas
underlain by granodiorite that weather into deep sandy soils
have stronger hydrologic and biogeochemical connection than
areas underlain by volcanic rocks that weather into clay. These
floodplains tend to be smaller and are characterized by surface
drainage and periodically dry surface soils.
p0220 The floodplains of larger lowland tropical streams offer
striking and well-documented examples of how the frequency
and duration of flooding and floodplain soil saturation are
linked to patterns of forest structure and biodiversity (Salo
et al., 1986; Kalliola et al., 1991; Mertes et al. 1995; Hamilton
et al., 2007 and others). Relationships between fluvial pro-
cesses and riparian vegetation have also been documented in a
few TMSs. Riparian zones along the first- to third-order TMS of
the Luquillo Mountains of Puerto Rico do not have distinct
riparian species or riparian communities but do have distinct
understory species (Heartsill-Scalley et al., 2009). Relation-
ships between the flood frequency and the structure of ripar-
ian vegetation have been documented in these streams (Pike
and Scatena, 2010). The width of their riparian zones defined
on the basis of canopy cover, understory vegetation, and soil
drainage averages 22 m for perennial channels and 10 m for
intermittent channels (Scatena, 1990). For comparison, tim-
ber harvesting guidelines for Australian tropical forests require
leaving a minimum strip of undisturbed forests of 10 m for
streams draining less that 60 ha and 20 m for channels
draining 100 ha or more. No buffer protection is required
where channels are less than 5m wide. In Peninsular Malaysia,
the amount of logging-derived sediment reaching the channel
declined after 40 m but the overall effectiveness of riparian
buffers depends on the hydrologic connectivity between hill-
slope and channel buffers (Gomi et al., 2006). In summary,
these studies indicate that TMSs can have distinct zones of
riparian vegetation that are on the order of 10–40 m wide. For
comparison, in the relatively flat terra firme landscape of the
Central Amazon, riparian zones defined by a distinct, high-
diversity riparian herb community can be 100 m wide
(Drucker et al., 2008).
s0105 9.31.4.5 Role of Instream Wood
p0225 Logjams and accumulations of coarse woody debris (CWD)
are known to play an important role in structuring the
morphology and habitat in forested temperate streams (see
Chapter 9.11 Wood in Fluvial Systems (00236)). The few
studies that have investigated CWD in tropical streams
indicate that although TMSs lack beavers and other large river
dwellers, debris packs created by CWD, palm fronds, and fine
litter do provide important habitat and food resources to the
detrital-based aquatic food webs of TMSs (Covich and Crowl,
1990). However, CWD appears to be less abundant in TMSs
than in some temperate counterparts. A detailed survey of 26
montane to lowland stream reaches in the Dominican Re-
public indicated that 62% had measurable CWD, but no reach
has more than 5% woody debris cover (Soldner et al., 2004).
In first-order TMS in a pasture–forest landscape mosaic in
Puerto Rico, the amount of CWD tended to increase with
forest cover and there were positive relationships between tree
cover and percentage of dissolved oxygen, and negative rela-
tionships between tree cover and percentage of substrata
covered by fine-grained sediments from eroded soil (Heartsill-
Scalley and Aide, 2003). A 4-month CWD addition experi-
ment in pools in headwater TMS streams also indicated that
the CWD additions were correlated to changes in aquatic
species composition but had no effect on the total number of
freshwater shrimp per pool area (Pyron et al., 1999).
p0230A study of the transport of numbered, 2-cm-diameter
hardwood dowels in a second-order, boulder-lined stream in
the Luquillo Mountains indicated the dowels are dispersed in
a negative exponential pattern and have high retention at the
reach scale, even during large, hurricane-related storm flows
(Covich and Crowl, 1990). This high retention is attributed to
CWD, palm fronds, and other plant material being entangled
in crevices between boulders. It has also been noted that
suspended sediment concentrations during hurricanes can be
lower than predicted from concentration–discharge relation-
ships derived from nonhurricane storms of similar magni-
tudes (Gellis, 1993). Apparently, defoliation by the hurricane-
force winds created temporary debris dams that trapped
sediment and reduced suspended sediment concentrations.
Nevertheless, because relatively high stream flow can persist
for several days, the total sediment transported during the
passage of a hurricane can be significant (Gupta, 2000; Warne
et al., 2005).
p0235Unlike some cold temperate streams where CWD dams can
last for decades or centuries, the CWD in TMS is removed on
the order of years. In the Malaysian State of Sabah, CWD dams
have an average life span of approximately 1 year, although
some can exist over 10 years (Spencer et al., 1990). In the
Upper Rio Chagres Basin of Panama, large wood dams pro-
duced by a widespread flooding and landsliding event lasted 2
years or less and the fluvial system appears to alternate be-
tween brief periods of moderate wood load and long periods
without (Wohl et al., 2009). Although a few of the CWD dams
that are produced by hurricane defoliation and uprooting in
the Luquillo Mountains of Puerto Rico last as long as 5 years
in some headwater reaches, majority of CWD dams in head-
water streams were broken and redistributed within less than 6
months and CWD dams have not been observed in third- and
fourth-order streams.
p0240The long-term average rate of CWD inputs into most TMSs
is unknown, but it should be similar to that observed in
temperate environments because tree mortality and the rate of
stand turnover are similar in tropical and temperate forests
(Lugo and Scatena, 1996). However, in hurricane-impacted
areas, and in areas undergoing deforestation, the average
MORP 00256
10 Streams of the Montane Humid Tropics
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annual rate may be larger or at least more episodic. Available
decay rates of CWD tissue in warm tropical streams indicate
they can decay between 16% and 30% over 3 years (Beard
et al., 2005). These relatively rapid decay rates, combined with
the high frequency of storm flows and episodic inputs, ap-
parently interact to reduce the life span of CWD accumu-
lations and their ultimate influence on channel morphology.
s0110 9.31.5 Response to Anthropogenic Disturbances
p0245 Because TMSs drain hillslopes with abundant weathering
products and because they have many storms per year and
high sediment loads, it could be assumed that they can recover
from formative events or adjust to new environmental con-
ditions faster than their temperate or arid counterparts.
However, because many TMSs are also supply-limited with
respect to fine sediments, they may not have the material
needed to rapidly reform and adjust their channels in re-
sponse to environmental changes. Although time will tell
whether the presumed effectiveness in restorative processes of
TMSs actually is true or part of the dynamic but
stable mythology associated with TMSs, there is no doubt that
TMSs are undergoing significant changes because of human
activities.
s0115 9.31.5.1 Land-Use Change
p0250 The influence of land-use change (see Chapter 9.39 Impacts
of Land-Use and Land-Cover Change on River Systems
(00264)) on hydrologic process and runoff of TMS has been
documented in several locations (see many examples in Bonell
and Bruijnzeel (2004)). In general, the conversion of forested
watersheds to pastures and cropland increases erosion, runoff,
and sediment yields. Available studies indicate that PO4, K,
and Mg concentrations increased considerably with urban-
ization and the water-quality changes associated with agri-
culture and urbanization in the humid tropics are of similar
magnitudes and directions as temperate streams (Santos-
Román et al., 2003; Ramirez et al., 2009). However, the long-
term geomorphic response of tropical stream channels to
land-cover change and urbanization (see Chapter 9.41 Ur-
banization (00266)) is poorly quantified and no studies have
focused on TMSs (Douglas, 1978; Gupta, 1982, 1984, 2010;
Gupta and Ahmad, 1999; Ebisemiju, 1989a, 1989b; Chin,
2006; Jeje and Ikeazota, 2002; Ramirez et al., 2009). In humid
temperate environments, streams commonly aggrade during
the initial phases of urbanization in response to the increased
sediment loads that are generated during construction. As the
urban landscape becomes established, channels then tend to
enlarge in response to an increased frequency of high-flow
events that carry less sediment. A recent review of the limited
studies from urban tropical areas suggests that channel en-
largement from tropical urbanization tends to be smaller in
magnitude compared to temperate counterparts (Chin, 2006).
Slight downstream decreases in channel size have also been
observed in coastal plain streams in Puerto Rico and have
been related to the presence of sediment deposited in earlier
agricultural periods (Clark and Wilcock, 2007AU10 ). By contrast, in
streams draining established urban areas of Puerto Rico, the
amount of channel incision does not appear to be correlated
with urbanization and river connectivity seems to be more
important than urbanization in determining fish assemblage
composition (Ramirez et al., 2009). Unfortunately, existing
studies on the impacts of urbanization on tropical streams
have been short term and may be biased toward the initial
stages of construction and aggradation. Thus, the long-term
influence of urbanization or other land-cover changes on the
morphology of TMS is uncertain. However, given that these
systems have few alluvial reaches and already capable of
transporting more fine sediment than is supplied, they are not
expected to undergo the same pattern of aggradation and
widening as alluvial reaches in humid temperate areas.
s01209.31.5.2 Dams and Water Diversions
p0255Because of their high runoff and montane settings, TMSs are
often well suited for hydroelectric generation or gravity-driven
water diversions (Benstead et al., 1999; Pringle et al., 2000;
Brasher, 2003; March et al., 2003). In Central America,
hydropower from TMSs already generates approximately 50%
of the electricity and the number of dams and diversions is
expected to continue to increase in the future (Anderson et al.,
2006a, 2006b). From an ecological view, dams and diversions
can be similar to extended droughts and result in a reduction
in resident and migratory habitat and the crowding and ac-
celerated mortality of individuals (see Chapter 9.40 Flow
Regulation (00265)). The cumulative effects of these alter-
ations can be a decrease in riffle habitats and in the number of
fish species immediately downstream from the dams. The
cumulative impacts of multiple hydroelectric dams releasing
water at the same time each day are unclear but of concern to
many residents who live downstream of TMSs.
s01259.31.5.3 Climate Change
p0260As noted earlier, the climate of most TMSs has changed in the
past and will change in the future (see Chapter 9.42 Climate
Change (00267)). The general expectation is that in the next
century tropical mountains will undergo warming and drying
that will result in an upward shift in life zones (Colwell et al.,
2008). Local and upwind deforestation can also influence
precipitation patterns in the watersheds of TMSs (Bruijnzeel
et al., 2010). The relatively high atmospheric inputs of nutri-
ents and efficient internal nutrient cycles suggest that the
biogeochemical systems of tropical montane forests will rap-
idly adjust to future environmental changes (Bruijnzeel et al.,
2010). How and when the morphology of TMSs will respond
to climatic and environmental change is less certain. However,
if the future resembles our admittedly poor understanding of
the past that is discussed above, widespread drying should
result in cut-and-fill episodes in alluvial reaches in the foot-
hills of TMSs. Increases in precipitation and forest cover
should promote relatively fixed, stable channels with riparian
areas covered by mature rainforest. Changes to the morph-
ology of the steeper gradient, boulder- and bedrock-lined
channels of TMSs are expected to reflect changes in the fre-
quency of slope failures and debris flows. Changes in
MORP 00256
Streams of the Montane Humid Tropics 11
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biogeochemical weathering rates and in the fluxes of water
and sediment from TMSs are also expected and may be good
geoindicators of environmental change in these systems
(Osterkamp, 2002).
s0130 9.31.6 Conclusions
p0265 Although TMSs have a long history of fascinating and con-
fusing geomorphologists, it is only recently that a rudimentary
understanding of their fluvial geomorphology can be de-
veloped from process-based case studies. Whereas the paucity
and restricted geographic distribution of available studies still
limit our ability to develop rigorous predictions of their be-
havior, the emerging view that is summarized below can be
used to guide future research and management:
•p0270 Most TMSs drain orogenic terrains that have not beenglaciated but have undergone climatic changes throughout
the Pleistocene and Holocene. In many areas, early Holo-
cene precipitation was 20–35% above recent means and
40–80% greater than during the drier LGM. Drying and an
upward shift in life zones are expected for the future and an
ongoing challenge is to identify the geomorphic legacies of
these past climatic fluctuations and the response of TMSs
to future changes.
•p0275 TMSs typically receive 2000–3000 mm yr�1 or more ofprecipitation and have a high frequency of intense and, in
some cases, prolonged rainfalls that commonly impact the
entire watershed at the same time. Rainfall and discharge
can be seasonal and temporal changes in runoff co-
efficients are common.
•p0280 TMSs drain steep hillslopes with high drainage densitiesand shallow subsurface storm flow paths that rapidly de-
liver precipitation to stream channels. Their rectangular
channel networks closely reflect regional geologic structure,
whereas the slopes and widths of their segmented longi-
tudinal profiles reflect underlying bedrock. High channel
densities can develop in a few decades but drainage net-
works commonly fail to conform to Horton’s laws of
stream numbers and length.
•p0285 TMSs have high material fluxes from both physical andchemical weathering and are the headwaters of streams
that may contribute between 20% and 40% of the global
fluxes of dissolved load and sediment to the oceans. Their
chemical denudation is strongly influenced by deeply
weathered and thick saprolite and tight internal biogeo-
chemical cycles. Available data suggest that physical de-
nudation averages around 60% of total denudation and the
recurrence interval of landslide-generating events is on the
order of years.
p0290 The net result of these interactions are storm-dominated
fluvial systems that are characterized by a high frequency of
short-duration events that efficiently transport dissolved ma-
terial and fine sediment. Most TMS streams are considered to
be supply-limited with respect to fine-grained sediment and
transport-limited with respect to the large boulders that enter
the channel during debris flows or by in situ weathering. The
stream channels in these systems are characterized by:
• p0295Steep-gradient streams with numerous boulders, rapids, andwaterfalls that alternate with low-gradient reaches flowing over
weathered rock or a thin veneer of coarse alluvium. Knick-point
migration, differential weathering rates, and debris flo-
w–hillslope interactions are all responsible for the devel-
opment of waterfalls and rapids in TMSs. A future
challenge will be to determine the relative importance of
these processes in different tectonic and geologic
environments.
• p0300Better developed downstream hydraulic geometries than theirtemperate montane counterparts. This may be due to some
combination of the lack of recent glaciations and because
the deeply weathered saprolite is relatively deformable
given the high frequency of intense storms they experience.
• p0305Lack of permanent CWD that structures channel and reachmorphology. Although the long-term supply of CWD to
TMSs may be similar or even larger than the supply to
temperate montane counterparts, the combination of epi-
sodic inputs, rapid decomposition, and mechanical
breakdown by a high frequency of storms apparently re-
duces the residence time and overall geomorphic influence
of CWD.
• p0310Poorly developed and discontinuous floodplains. Distinct ri-parian zones can be identified on the basis of soils and
vegetation and typically extend 20–40 m from either side
of headwater channels. Because shallow subsurface flow
commonly passes through wet and nearly saturated ripar-
ian soils before entering TMS channels, the biogeochemical
transformations within the riparian zone can have a dis-
proportionate influence on stream-water chemistry.
Therefore, establishing riparian buffers zones can be an
effective best management practice in these systems.
• p0315Migratory aquatic species that are well adapted to floods and tomigrating steep bedrock channels. Their spatial distribution
within TMS networks is directly related to the distribution
of waterfalls or anthropogenic barriers. The high frequency
of bedload-transporting storms combined with continual
herbivory interacts to suppress the abundance of periph-
yton and aquatic plants. Consequently, rates of respiration
are much higher than most forested temperate streams.
p0320As the above review suggests, TMSs do not have diagnostic
landforms that can be solely attributed to their low-latitude
locations. However, there is general agreement that TMSs are
distinct fluvial systems. The distinctiveness of TMSs appears to
result from a combination of high rates of chemical and
physical weathering and a high frequency of significant geo-
morphic events rather than the absolute magnitudes of indi-
vidual events or processes.
p0325It is generally assumed that stream channels tend toward
quasi-equilibrium morphologies because their beds and pro-
files deform and adjust in response to changes in discharge
and sediment supply. In many TMSs, described relationships
between bedrock lithology, channel slope, and channel
morphology suggest that these channels do adjust and estab-
lish quasi-equilibrium morphologies. However, the abun-
dance of large and relatively immobile boulders and their lack
of fine-grained alluvial deposits suggest that the restorative
processes in these systems may be less responsive than those
where fine-grained sediment is actively involved in rebuilding
MORP 00256
12 Streams of the Montane Humid Tropics
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and sculpting channels between formative events. A future
challenge in understanding and managing these systems
under ever-increasing anthropogenic pressures is to dis-
tinguish the formative and restorative events that sculpt these
landscapes and maintain their aquatic resources and contri-
butions to regional biogeochemical cycles.
Uncited references
p0330 Blum (2007), Clark and Wilcock (2000), Gabet et al. (2004),
Harden and Scruggs (2003), Hare and Gardner (1985), Har-
mon (2005).
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