Journal of Field Archaeology 1991 Waters

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The Geoarchaeology of Gullies and Arroyos in Southern Arizona

Michael R. Waters

Journal of Field Archaeology, Vol. 18, No. 2. (Summer, 1991), pp. 141-159.

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The Geoarchaeologyof Gullies andArroyos

in Southern Arizona

Michael R. Waters

Texas A&M University

College Station, Texas

Many of the major rivers and streams in the arid and semiarid American Southwest can beclassified asgullies and awoyos. The histoly of deposition, stability, and erosion ofgully andawoyo environments has had a profound injuence on the archaeological recmd containedwithin valley and bajada (piedmont) alluvium. The tempmal and spatial sample of archae-ological sites in southern Arizona is as much a reFeccion ofgeolog.ical processes as it is of cul-tural processes. Where bothgeolog.ical and archaeological data sets are well preserved, de-tailed lanhcape reconstructions are possible and prehistmzmzcctivity can be placed in the

context of the prehistwic lanhcape. In southern Arizona, the location of Hohokam agricul-tural settlements at any particular time, changes in position of these settlements throughtime, and agricultural technolog-y were affected by the confi;quration of the lanhcape, by the

hydrologic rgime of the bajadugullies and valley awqos, and by lanhcape changes thathave occuwed through time. In act, prebistoric Hohokam agriculturalists may have initi-

ated some of the environmental degradation observed in thegeological record.

Introduction that merge downstream into a single entrenched channel

Arroyos and gullies are dynamic components of the (FIGS.3,4; Leopold, Wolman, and Miller 1964; Packard

semiarid landscape of southern Arizona (FIGS. , 2 ) . Their 1974; Graf 1987). The walls of the entrenched channel

distribution, hydrologic characteristics, and history have gradually decrease in height downslope from the headcut,

shaped the archaeological record of the major alluvial val- because the slope of the channel bed is less than the slopeleys and affected the location of the late prehistoric agri- of the original valley floor into which the channel is en-

cultural settlements. Conversely, prehistoric human activ- trenched. Eventually the walls and bottom of the channel

ities have in turn affected these fluvial environments and merge downslope at a location known as the intersection

caused the landscape to change. While these concepts are point. Downslope of the intersection point sediment

illustrated with examples from southern Arizona, they are eroded from the channel accumulates to form an alluvial

broadly applicable to other arid and semiarid areas of the fan (FIGS. ,4 ; Graf 1987; Leopold, Wolman, and Miller

American Southwest where arroyos and gullies are com- 1964; Packard 1974). This gully-mouth fan is the terres-

mon. Before the relationship between the archaeological trial equivalent of a river delta (i.e., it forms on the land

record, prehistoric human behavior, and fluvial processes surface instead of underwater).

are dscussed, it is necessary to understand these processes Discharge through a gully system is initiated after rain-

and the alluvial deposits of gullies and arroyos. fall occurs in its watershed. Upslope of the intersection

point, flow is generally confined to the channel, withGullies and Arroyos floodwaters rarely overflowing the channel banks. During

Gullies and arroyos are entrenched channels that are dry times of flow, gravel and sand are transported and accu-

most of the year and flow for only a few hours or days mulated in the channel bed. Downslope of the intersection

after heavy tainfall occurs within their associated drainage point, shallow braided channels radiate across the proxi-

basins. As a result, fluvial activity is sporadic, with erosion mal fan surface. Because the channels are shallow, uncon-

and deposition occurring rapidly during catastrophic flash fined sheetflow is generated. Sheetflows transport a com-

flood events. Gullies and arroyos are characterized by sim- bination of sand and gravel bedload and a suspended load

ilar processes and deposits, differing only in scale: gullies of silt and clay. Vegetation on the fan slows the movement

are smaller than arroyos. of the water and deposition occurs. Coarse sedunent ac-

G d e s begin upslope at a headcut or series of headcuts cumulates in the braided channels and finer sediments on



142 Gztllzes and A w q o s in ArizonalWaters

Figure 1. Ma p of SE Arizona show ing the locations mentioned in the text. Study areas: 1 ) Marana area; 2)Schuk Toak area shown in Figure 11; 3) Sierrita Mountain bajada area shown in Figure 8; 4) San Xavier reach

shown in Figure 10; 5) Corona de Tucson; 6 )Whitewater Draw alluvial sections. Inset map of Arizona shows

boundary of H ohoka m culture area. Mountains are hatched, blank areas are Qu aternary alluvium. Note that

scale and symbols apply to the large base map.

the adjacent surfaces. These shallow channels frequently

shifi position on the fan surface as they become choked

with sedment. The resultant fan deposits are laterally ex-

tensive layers of sand and silt interbedded with shallow,

sand- and gravel-filled channels. The sand and silt layers

are commonly mixed into a massive deposit of silty sand

as a result of bioturbation. Sedment-laden water traveling

over the proximal fan surface continues to spread down-slope over the distal fan. Because vegetation on the prox-

imal fan filters the coarser sediments, only the very fine

sand, silt, and clay that remain in suspension reach the

distal fan, where they accumulate into massive and lami-

nated deposits.

Gullies occur along the axis of a valley, but are most

common on the alluvial piedmonts or bajadas that extend

from the base of the mountains (FIG. 2) . Bajadas have a

stratigraphy of superimposed channel and fan sedments,

because gullies are continually shif'ting position as one

gully pirates the flow away from another.

Arroyos are large gullies, created by the deepening and

expansion of a single gully or by the merger of a series of

gullies into an uninterrupted channel (Leopold, Wolman,

and Miller 1964).This occurs when the headcut of the

gully immediately downslope erodes upslope and connects

with the gully lying upstream. Once the gullies are con-nected, the newly formed arroyo will deepen and widen

its channel. Arroyos are commonly found along the axis

of most valleys in the American Southwest.

Arroyos are characterized by steep vertical channel

banks, usually several meters high, in unconsolidated sed-

iments (e.g., FIGS. 1, 2: Altar Wash, Santa Cruz fiver).

Most flows are confined to the channel, but during un-

usually heavy floods, water will overflow the arroyo banks

and spread laterally over the valley floor. Because of the



Figure 2. High altitude aerial photograph of s~ Arizona and northern Mexico. T: Tucson; SC: Santa

Cruz River; SPR: San Pedro River; CC: Cienega Creek; WP: Willcox Playa; AW: Altar Wash; AWF:

Altar Wash Fan; B: bajadas; SM: Sierrita Mountains; RM : Rincon M ountains; SCM : Santa Catalina

Mountains.

heavy sediment loads and ephemeral high-energy dis- with layers of imbricated gravels, cross-bedded to ripple-

charge within the arroyo channel, the streambed has a laminated sands, and occasional clay and silt layers (Has-

braided pattern, with channels diverging and rejoining san 1985).Commonly, arroyo channel sequences become

around channel bars. Therefore, arroyo channel sediments upwardly finer, with coarse gravel and sand at the base,

are similar to those of sandy and gravelly braided rivers, then silty sand, and finally clay at the top. As with gullies,



144 Gullies and Af-rqos n Arizona1 Waters

IC FLOW

INTERSECTIONPOINT

I / OLDER CHANNEL AND FAN SEDIMENT S 1,'/. ~ , > , , , , > , ,

Figure 3. Generalized plan view (a ) and cross section (b ) of gully and

arroyo systems. hk-Chin field indicated.

arroyo-mouth fans develop at the downslope terminus of

arroyo channels (e.g., FIG. : the Altar Wash arroyo-mouthfan). Arroyo-mouth fans are, however, much larger than

those of gullies, forming broad valley fills. Arroyos, like

gullies, are unstable and through time have undergone

numerous episodes of channel cutting and filling to create

a complex alluvial sequence.

Because arroyos are devoid of streamflow for long pe-

riods, their dnr channel beds are subject to wind erosion.

This windblown sand often accumulates in the form of

dunes on the alluvial floodplain. If not destroyed by later

flooding, these eolian sediments will become incorporated

into the alluvial sequence.

Marshlands, called cienegas in the American Southwest(Hendrickson and Mincklev 1984; Melton 1956),develop

locally within an arroyo if the streambed intersects the

water table. At these locations water seeps to the surface

and flows sluggishly through a shallow channel, which

becomes heavily vegetated. Laminated, organic-rich, fine-

grained sediments are commonly deposited in the shallow

channel, and massive deposits of silt and clay accumulate

on the adjacent wet meadows. Cienega sediments are fre-

quentlv interbedded within arroyo channel fills.

Fluvial Landscape Evolution and the

Archaeological RecordDuring the late Quaternary, gully and arroyo environ-

ments went through periods of stability, followed by chan-

nel entrenchment, eventual backfilling, and a return to

stability as a result of changes in climate, tectonic activity,

internal geomorphic adjustments, or human land use.

These changes created a stratigraphic record of erosional

unconformities, channel fill sequences, terraces, and paleo-

sols. The nature and structure of an alluvial stratigraphic

sequence (i.e., how many sedimentary units, paleosols,

and erosional surfaces are present, their spatial distribu-

tion, sequencing, and how much time they represent) is

determined by the number, magnitude, duration, areal

extent, and timing of periods of deposition, erosion, and

stability. These factors determine how much of the time

continuum of a stratigraphic sequence is recorded by ma-terial depositional units as opposed to that represented by

erosional unconformities and surfaces of stability (paleo-

sols) (FIG. ). Episodes of landscape degradation and sta-

bility produce gaps within a depositional sequence, thus

creating an incomplete geologic record of material sedi-

ment units. A ratio of the total amount of time recorded

by the deposition of physical strata to the amount of time

represented by nondepositional and erosional breaks be-

tween sediments provides a measure of the completeness

of the stratigraphic sequence (Sadler 1981). In general,

this ratio is low because the contacts between and within

the sediment units of a stratigraphic sequence representthe passage of more time than do the phvsical sediments

themselves (Kraus and Bown 1986; ~a dle r 981). These

concepts are crucial to understanding the archaeological

record of alluvial valleys because the same processes that

have molded the landscape have also shaped the archaeo-

logical record (FIG. ; Butzer 1982; Gladfelter 1985).

When the landscape is stable, characterized by negligible

erosion and deposition as well as the formation of soils,

archaeological debris will accumulate on the surface until

it is buried. Consequently, if a fluvial landscape is char-

acterized by repeated periods of stability and each episode

of stability is lengthy, much of the archaeological recordis compressed and mixed on common surfaces before bur-

ial. If ;site is situated in an area of active deposition, it

will be buried within alluvium soon after abandonment.

Because of this, the greater the number and duration of

depositional episodes, the greater the likelihood that as-

semblages of discrete occupations will become buried

within sediments as spatially separated occupation surfaces

(Ferring 1986).

If a site is situated in an area dominated bv erosion,

however, all or part of the site will be destroyed soon after

abandonment. Furthermore, even if sites are initially pre-

served, later degradation of the landscape affects theirchances of survival. For example, if an arroyo channel

downcuts into its floodplain and widens its channel, pre-

viously preserved sites within the zone of entrenchment

will be eroded. Each subsequent degradational event dt-

minishes the completeness of those portions of the geo-

logical and archaeological record that have survived into

the present (FIG. ) . The greater the number and duration

of erosional events, the greater the destruction. For ex-

ample, the Santa Cruz k ver has entrenched its floodplain

five times since 5500 B.P. (FIG.6; Haynes and Huckell



Journal$Field ArchaeologyVol. 18,1991 145

Figure 4. Aerial photograph of a section of the bajada emanating from the Tortolita Mountains. Dis-continuous @es and gully-mouth fans radiate over the lower late Holocene bajada. Features identifiedare entrenched channels (C), intersection point (IP), proximal fan (PF), distal fan (DF), Pleistocenepiedmont (PP). Hohokarn settlements occur on the lower bajada.



146 Gullies and Arvcyos in Arizona1Waters

[A& S C A L E ,

4

1 1 , 3 00 - 1 0 8 0 0 B P 1 0 . 2 00 - 1 0 , 0 0 0 8 P 9 0 0 0 8 0 0 0 B P 7 0 0 0 5 0 0 0 B P 4 5 0 0 - 5 O O B P

GENERALIZED COMPOSITEGEOLOGICAL SECTION I GEOLOGICAL SECTION 2 GEOLOGICAL SECTION 3 G E O L O G I C A L S E Q U E N C E

T IM E INTE RV A L S O F

M A T E R I A L M A T E R I A L M A TE RIA L M A TE RIA L

S TRA TIG RA P HIC S TRA TIG RA P HIC S TRA TIG RA P HIC S TRA TIG RA P HIC

U N I T S L N I T S U N I T S U N I T S

CULTURE S

2 0 0 0 I-W

3 0 0 0

n

4 0 0 0

IL 5 0 0 0 2

M I D D L E m

Figure 5. Sequence o f landscape changes in a hy pothetical valley shows how an alluvial sequence is

created and interpreted in terms of time (i.e., periods of deposition, erosion, and stability). Also shown

is how the archaeological record is shaped within an alluvial environment. T he three-dimensional block

diagrams in the upper portion of the figure illustrate landscape changes over the last 11,500 years. Sites

located on the landscape during these intervals are shown by various symbols. Some of these sitesbecome buried and eroded within the dynamic floodplain environment. Evidence of all periods of

occupation will be fou nd o n the surface of the stable Pleistocene terrace ov e rl o o h g the floodplain.

Th e lower p ortion of the figure illustrates the material stratigraphic record preserved at three localities

designated in block diagram 11. Next to these stratigraphic sections are diagrams that interpret the

stratigraphic section in terms of time. Each section records 11,500 years of time as a combination of

depositional, erosional, an d stability intervals. Th e preserved material units an d comb ination of time

represented will vary from section to section. The fourth diagram illustrates a composite section of the

alluvial stratigraphy. C omp arison of th e valley alluvial stratigraphy w ith the archaeological culture se-

qyence shows that Paleoindian and initial Early Archaic remains have been eroded from the valley;

transitional material between the Early and Middle Archaic and some later Middle Archaic remains

were either eroded or may be found on soil surfaces; Late Archaic and ceramic period sites will be

found compressed onto a common surface on the S3 soil and buried by a thin layer of historical

alluvium; the dep osits will contain only Early and M iddle Archaic, protohistorical, and historical re-

mains.

198 6; Waters 19 88a, 19 88b ). Each time the river down- older alluvial sediments. In general, the fewer episodes of

cut in to its floodplain and widened its channel, it eroded erosion and the shorter their duration, th e more complete

and reworked older alluvial sediments and their archaeo- the stratigraphic sequence of material units and the con-

logical contents. In this case, repeated channel entrench- tained archaeological record.

ment fragmented the record of older alluvial sediments No single alluvial stratigraphic sequence, however, has

and archaeological sites. The depth of channel cutting, a complete record of late Pleistocene and Holocene sedi-

degree of channel widenin g, pos ition of channel entrench- ments, let alone a representative sample of the archaeolog-

ment, and the volume o f preexisting sediments all help to ical record. Commoniy, different portions of the geolog-

determine the effect of repeated channel entrenchment o n ical and archaeological record are contained in a number



of individual stratigraphic sections over a region (F IG . 5 ) .

Compilation of these isolated stratigraphic sequences per-

rnits the construction of a composite stratigraphic record

of preserved material units and archaeological sites ( F IG .

5) . For example, in Whitewater Draw, an arroyo in SE

Arizona, no single exposure preserved a complete se-

quence of stratigraphic units spanning the last 15,000

Journal of Field ArchaeologyiVol. 18, 1991 147

years ( F I G . 6; Waters 1986).Instead, individual exposures

over the 25-km-long arroyo preserved different segments

of the late Quaternary record. By correlating one strati-

graphic section with the next, a composite stratigraphic

sequence representing most of the late Quaternary wasconstructed. Since archaeological remains of different ages

were preserved within these sections, a composite archae-

Figure 6. Correlation of the alluvial sequences for the Santa Cruz fiver, Cienega Creek, San Pedro

fiver, and Whitewater Draw. These are compared with the generalized alluvial chronologies proposed

for arroyos in the Southwest and the archaeological culture sequence for southern Arizona. Major

periods of channel downcutting and widening that created major unconformities are hatched. Implica-

tions for the archaeological record are discussed in the text.

TIM E ARCHAEOLOGICAL SANTA CRUZ

8 . C /A . D Y R. B P CULTURESRIVER

(Waters,1988a)A.D.1950-0

3 05 OB .C .- 5 0 0 0

Middle -

- - n

70508.C: 9000

- 8 0 5 0 8.C:IROOO

P a l e o - I n d i a n

P 2 = Cienega Formation

GENERALIZEDCIENEGA SAN PEDRO WHITEWATER ALLUVIAL

CREEK RIVER DRAW CHRONOLOGIES

(Eddy Cwley, l983) ( Haynes.1981,1982) (Wa ter s, 1986b ) (Hoynes,1968)(Knox,l983)

Cienega. Col luvial

S h o l l o w S t re a m

C i e n e g a E ro s i on

Cutting and Fi l l ing

(Units J.K.L.M.N,BO)

C h a n n e l F i l l i n p

Eolian Sediments

[D o n n e t S il t - a d o )

B ra i d e d S t re a m

(Units Da B Db)

P 3 0 * Ch an ne l C ut B F ~ l l

P 3 b ' F a n C ut 8 F i l l

P 4 S a n d D un e F o rm a ti on



148 Gullies and Arvoyos in ArizonalWa ters

ological record also emerged, which illustrated the long

continuum of Archaic hunter-gatherer occupation along

Whitewater Draw.

In some cases, regional erosional events completely re-

move older sediments and soils that may contain archae-ological remains, as shown by Turnbaugh (1978);

Brookes, Levine, and D enn ell(1 982 ); and Thompson and

Bettis (1 98 2) . In these cases, no deposits or soils of a

certain time interval are present anywhere in a valley,

which creates a break in the material stratigraphic record

and, correspondingly, in the archaeological record. This is

well illustrated by the regional stratigraphic sequence of

the Santa Cruz River near Tucson, Arizona, where sedi-

ments older than 55 00 B .P. were eroded from the valley

during a regional episode when the channel of the Santa

Cruz k v e r downcut in to its floodplain, widened, and

scoured the valley. Deposition resumed only after 5500B.P.(FIG.6) . This erosional event was responsible for the

destruction o f most alluvial units and paleosols of greater

age together with their potentially associated artifact de-

bris (Haynes and Huckell 1986; Waters 1988a, 1988b).

As a result, there is no record of Paleoindian or Early

Archaic sites in the alluvial stratigraphic sequence of the

Santa Cruz River. Furthermore, the erosion appears to

have been a regional event that occurred throughout the

greater Tucson Basin. Quaternary sediments in the tribu-

tary drainages of the Santa Cruz k v er , such as the Rillito

and Pantano Rivers, also appear to have been eroded

during this middle Holocene erosional event. An uncon-formity spanning the period between 10,000 and 4000

B .P .occurs in the stratigrap hy of Cienega Creek, the head-

waters of the ~ a n t a n o iver (F IG. 6; Eddy and Cooley

19 83 ). Clearly, gaps in occupa tion at a specific locality or

within a valley may just as likely be the result of erosion

as of intentional abandonment.

The number, magnitude, duration, areal extent, and

timing of erosional, depositional, and stability intervals

may be different or the same between valleys. Spatially

separated valleys will have similar and synchronous land-

scape histories if the geomorphic variables of each valley

(e.g., relief, position o f the w ater table, lithology, sedimentyield from the hillslopes, internal landscape thresholds,

vegetation cover, tectonics, human land use) are similar

and if external changes in climate triggered similar re-

sponses within each valley. This circumstance will create

stratigraphic sequences that are similar and can be corre-

lated from one valley to the next (Haynes 1968; Knox

19 83 ). In this situ ation com parable archaeological se-

quences may be potentially preserved in adjacent valleys.

Spatially separated valleys, however, may have different

and nonsynch ronous landscape histories if the geom orphic

variables of each valley are different or if during the late

Quaternary climatic conditions varied from one valley to

the next (Butzer 198 0; Patton and Schurnm 19 75; Waters

198 5). This situation may trigger degradation, aggrada-

tion, or stability in one valley and not the other. This inturn results in stratigraphic sequences that are different

from one valley to the next (i.e., there is no one-to-one

correlation of depositional units, unconformities, and pa-

leosols between the valleys). Consequently, dfferent ar-

chaeological sequences may be potentially preserved in

adjacent valleys. For example, the late Quaternary strati-

graphic sequences preserved in four adjacent river valleys

in southern Arizona-the Santa Cruz k v e r , Cienega

Creek, San Pedro kver, and Whitewater Draw-show

only minimal similarity because each had an independent

landscape history (FIG.6). Correspondingly, the archaeo-

logical sample preserved in the alluvial sequence of eachvalley is unique (FIG.6). Because of the m iddle Holocene

period of erosion in the Santa Cruz k v e r Valley, only

archaeological remains dating from 550 0 B .P. o historical

times are present. A similar period o f erosion affected the

archaeological record along Cienega C reek (E ddy and

Cooley 1 98 3). In the San P edro drainage, the alluvial

sequence is well preserved, and as a result a nearly com-

plete record of Holocene occupation from 11,500 B.P. o

historical times is preserved in the sediments (Haynes

1981, 1982). The a lluvia l s e b e n t s in Whitewater Draw

are similarly well preserved and contain an archaeological

record dating from 10,000 B.P . through the historicalperiod.

The differential structure of the archaeological record

within and betw een valleys has im por tant implications for

archaeological interpretations. Archaeologists must con-

sider whether the observed patterns of occupation within

and between valleys accurately reflects the distribution of

human activity or the biases of geological preservation

processes. For example, consider the Paleoindlan record

of southern Arizona. In the San Pedro Valley a large

number of undisturbed Paleoindian sites are preserved

because of the favorable geological conditions that existed

in the valley during the late Pleistocene (Haynes 1981,1982). Clovis sites were rapidly buried beneath an or-

ganic-rich clay (the black mat) in a low-energy cienega

environment, and subsequent Holocene erosion did not

remove this record. On the other hand, no Paleoindan

sites have been found in Whitewater Draw, even though

sediments of this age are exposed in the arroyo (units Da

and D b in FIG. 6). This is in large part due to the fact

that, even though units Da and Db are time transgressive,

ranging in age from 15,000 to 8000 B.P., most of those

exposures date between 8 00 0 and 10,0 00 B.P. and only a



few of the sand and gravel deposits are older than 10,000

B.P. What is more, if Paleoindan remains were found in

units Da and D b they would lie in secondary contexts,

because these sands and gravels were deposited in a high-

energy braided stream environment unconducive to thepreservation of und isturbed archaeological associations. In

both the Santa Cruz h v e r and Cienega Creek, erosional

episodes have removed any Paleoindan record that may

once have existed. Therefore, the intensity of Paleoindian

utilization of the Santa Cruz h v e r Valley, Whitewater

Draw, and Cienega Creek cannot be gauged. Conse-

quently, it cannot be determined if the Paleoindian record

of the San Pedro Valley represents a unique, intensive

occupation o f this valley alone dur ing the late Pleistocene

or if it reflects the biases imposed by different intervalley

geological processes. If the latter is true, then perhaps a

similar level of Paleoindlan activity occurred within theother valleys of sout hern A rizona. This, however, we shall

never know.

Changes in th e landscape also impose limitations on th e

discovery of archaeological sites (Bettis and Benn 1984;

Gladfelter 1985; Thom pson and Bettis 1 982 ). Many pre-

served sites are not visible or detectable at the surface

because of deep burial. Fo r example, along the San Xavier

reach of the Santa Cruz h v e r , late Archaic remains occur

at a depth of 7 m, Hohokam remains at depths of 1.25

to 5.5 m, and protohistorical remains at a depth of 0.5 m

below the surface (Waters 1988 a, 19 88b ). Further, a thin,

0. 5 m thick layer o f historical alluvium overlies the flood-plain; as a result, prehistoric sites are not visible at the

surface of the floodplain except in areas that are biotur-

bated. Buried archaeological sites are observable only in

the channel bank exposures. The same holds true for the

bajadas extendng from the mountains, where many sites

are not readly visible through regular surface survey be-

cause of their depth of burial. Yet, even though sites are

visible in the late Quaternary alluvium exposed along the

banks of arroyos such as the Santa Cruz River, this allu-

vium is only a small portion of the total volume of late

Quaternary sediments stored within the floodplain. The

majority of the Santa Cruz h v e r floodplain, Holocenefloodplain sediments in other valleys, and large tracts of

the late Quaternary alluvium covering the bajadas extend-

ing from the mountains are unentrenched, and this allu-

vium m ust surely contain archaeological sites. Therefore,

an indeterminate nu mb er o f undete cted archaeological re-

sources must lie beneath much of the undssected Holo-

cene valley and bajada alluvium. Buried Paleoindian and

Archaic sites are notably unknown on the bajadas ema-

nating from many of the m ountains in the Tucson Basin.

Sites of this age may be deeply buried in the bajada allu-

Journal ofField ArchaeologylVol. 18, 1991 149

vium and thus go undetected. Also, the depth of gully

and arroyo channel entrenchment is not uniform along its

length; older sediments may not be exposed everywhere

along the channel or they may be covered in places by

recent alluvial deposits, or w here the banks have slumped.Therefore, deep burial and limited exposures affect our

perception of the archaeological record. In some cases,

even if subsurface sites are detected, they may not be

accessible because they are buried at a depth beyond fea-

sible archaeological investigation.

To deal with the problem of differential site preserva-

tion, visibility, and detection, researchers must modify

traditional archaeological survey techniques to include

geomorphic site prediction modeling and testing (Bettis

and Benn 1984 ; Gardner and Donahue 1985; Thompson

and B ettis 19 82 ). Because the alluvial stratigraphic frame-

work dictates the spatial and tempo ral structuring of thearchaeological record, it provides the framework needed

to determine w hich parts of th e archaeological continuum

are absent or potentially preserved, and how fragmentary

the preserved portions of the record m ay be. By mapping

the spatial dstribution of the stratigraphic units and as-

sociated landforms of known age, a model is established

to predict the mos t probable 1o;ations of potential surface

and buried cultural resources. This stratigraphic frame-

work can be used to delineate s e b e n t s that are too old

to contain archaeological remains and to identify those

sediments dating to the time of human habitation of

North America. What is more, attention can be focusedon the low-energy depositional settings where sites would

most likely be preserved and away from high-energy de-

positional settings where artifacts would occur only in

secondary contexts. Geoarchaeological modeling maxi-

mizes valuable field time and allows archaeologists to ef-

fectively prospect for archaeological sites in alluvial set-

tings. Survey crews do not have to waste energy

prospecting for sites in deposits th at are to o old t o contain

sites and instead can direct their efforts to those deposits

that do. This procedure optimizes site recordng during

archaeological survey and helps obtain a representative

sample of buried sites. In s outh ern Arizona site predictionmodels must be valley specific, because each valley has a

different and unique sequence of s e b e n t s (FIG. 6) .

To summarize, the archaeological record of utilization

of the alluvial environments of southern Arizona has in

large part been shaped by the sam e processes that molded

the fluvial landscape. Deposition, erosion, and stability

worked in concert to preserve, arrange, and fragment the

evidence of human activity associated with gullies and

arroyos. The degree to which these geological processes

have affected the temporal and spatial sample of archaeo-



150 Gullies and Awoyos in Arizona1Waters

logical sites must be evaluated before meaningful inter-

pretations of the archaeological record are possible (Butze r

198 2; Schiffer 19 87 ). In places where geoarchaeological

studies demonstrate that portions of the geological and

archaeological record are fairly complete and visible, de-tailed landscape reconstructions are possible, and prehis-

toric activity can be placed in its landscape context. In

southern Arizona, the late Holocene geological and ar-

chaeological records of bajadas, arroyo floodplains, and

arroyo-mouth fans are well preserved. Hence, analysis of

human-land interaction is possible.

Prehistoric Settlement Patterning Associatedwith GulliesandArroyos

A group of agriculturalists known as the Hohokam

occupied the Tucson Basin from approximately A.C. 300

to 15 00 (F IG.1). These prehistoric peop le exploited manydifferent environments, but were heavily dependent upon

bajada gullies, arroyo floodplains, and arroyo-mouth fans.

The location of late prehistoric settlements at a n y partic-

ular time, and changes in settlement location through

time, are closely tied t o th e distribu tion of fluvial systems,

the evolution of fluvial environments, and the farming

potential of the ancient landscape. Also to be considered

is the effect of human activity on the fluvial landscape.

T o clari@ the terminology used in this paper, the H o-

hokam cultural sequence is divided into four periods. In

sequence from oldest to youngest, these periods are the

Pioneer, Colonial, Sedental?; and Classic (FIG.6 ) . Theseperiods are further subdivided into the following phases:

the Colonial period is divided into the Caiiada del Oro

and Rillito phases; the Sedentary period into the Rincon

phase (which is subdivided into early, middle, and late

subphases); and the Classic period into the Tanque V erde

and Tucson phases. For a discussion of Tucson Basin

Ho hokam prehistory see Czaplicki and Ravesloot (19 89),

Fish (1 989), Doelle and Wallace (19 86) , and Dart (19 87).

Hohokanz Utilization of the Bajada Gzelly

Envi~onlnent

In the semiarid Tucson Basin, rainfall is sporadic andunevenly distributed over a wide area. Historical Indian

groups knew that the m oisture provided t o fields by direct

rainfall was inadequate for crops to mature on a reliable

basis and thus sustain a sedentary population. Additional

and mo re reliable sources of moisture w ere required for

successful farming. Consequently, historical peoples

turned to areas on the bajada that were naturally flooded

on a regular basis to pursue farming. The areas most

commonly used by the Papago (Bryan 192 9) and Pima

(Wilson 1985 ) were the fan environments (bo th proximal

Figure 7. Two typical Ak-Chin fields farmed by the Hopi in the Talla-

hogan Valley, Arizona (from Hack 1942).Note that the house is lo-

cated slightly above intersection point.

and distal) at the terminus of ephemeral gullies on thebajadas (FIGS . .4 .7; Nabhan 1979, 1986a, 19 86b). These

gully-mouth fan surfaces were especially desirable for

farming because the runoff generated by rainfall in any

part of the watershed, even miles away from the fan,

would be funneled into the main gully channel and even-

tually make its way to fields on the fan surface. Th e gully-

m ou th fans are calledAk-Chin,a Papago term for "arroyo-

mouth" (Bryan 1929 ; Hack 194 2; Nabhan 1 986 b).These

areas required little or no modification to direct the

streamflow, except perhaps brush or rock structures to

slow and spread the water over a broader area.

Reconstructing the paleolandscape of several bajadas,delineating stable Pleistocene alluvial fan surfaces and late

Holocene gully and gully-mouth fan environments, and

comparing this information to the distribution of Hoho-

kam settlements, showed tha t H oho kam settlements were

consistently situated within areas dominated by gully-

mo uth fans (the prehistoric environmen t that was optimal

for floodwater farming; FIG.8) and n ot in areas dominated

by gully channels. This site distribution is analogous to

the po sition of historical sites where A k-Ch in farming was

pursued. This pattern is found on the bajadas (FIG. 1)



Journal of Field Archaeology1 Vol. 18, 1991 151

Q F a P le i s t o c e n e f a n s e d im e n t s H o h o k a m s e t t l e m e n t s

Q F b l E r o d e d P l e i st o ce n e f a n r e m n a n t s a n d H o l o c e n e g u l l y c h a n n e l s e d im e n t s H o h o k a m a c t l v l t y a r e a s

Q F b 2 H o l o c e ne g u l l y - m o u t h f a n s e d ~ m e n t s S u r v e y a r e a b o u n d a r y

p s P h a ne r oz o ic m e t a s e d i m e n t b e d r o c k - - - - - C o n t a c t b e t w e e n g e o m o r p h i c u n i t s- W a s h e s

Figure 8. Sierrita Mountains study area (see FIG. 1 for location) showing the distribution of H ohok am

settlements on the lower bajada where Ak-Chin farming could have been pursued. Generally, only

specialized activity sites occur in the area do minated by entrenched gullies; Ho hok am settlements clus-

ter in the lower bajada where proximal and distal fan environments occur at the mouths of these gullies

(after Waters 19 87a ).

emanating from the Tortolita Mountains (Marana sites; inated by gully-mouth fan environments on the lowerFish 1989; Rice 1987; Waters and Field 1986), Sierrita bajada (FIG. 4; Fish 1989; Waters and Field 1986). Hab-

Mountains (Waters 1987a; Dart 1987), Tucson Moun- itation sites are not present on the upper stable Pleistocenetains (Czapliclu and Ravesloot 1989), Picacho Mountains surfaces. Instead these areas were used for gathering wild(sites around Brady Wash and McClellan Wash; Ciolek- plant resources and growing agave (a plant with low mois-Torrello 1987; Field and Lombard 1987), and Santa f i t a ture and minimal soil requirements; Fish et al. 1985; Fish

Mountains (Huckell et al. 1987; Phillips 1984; Waters 1989). A regional archaeological survey of 1650 sq krn

198%). north of Marana to the Picacho Mountains and comple-The correspondence between the position of Hohokam mentary geomorphic mapping show that the lower bajada

agricultural settlements and gully-mouth fans becomes alluvial surfaces conducive to floodwater farming make upclearer when the bajada environment is placed within the only 23.5% of the total alluvial landscape (all Pleistocene

context of other landscape elements. Near Marana, Ari- and Holocene alluvial surfaces), but were the loci ofzona, large habitation sites are concentrated in areas dom- 56.3% of the Hohokam sites (Field and Lombard 1987).



152 Gullies andArvoyos in Arizona1 Waters

This is more than would be expected by chance. By anal-

ogy with the historical Ak-Chin floodwater farming tech-

nique, strong correspondence between Hohokam settle-

ments and the distribution of late Holocene gully-mouth

fans, and avoidance of areas dominated by channelized

flow, it is clear that the Hohokam farmers preferred to

locate their settlements on the bajada in those areas well

suited for floodwater Ak-Chin farming. At the time of

occupation, Hohokam settlements were probably located

slightly above the intersection point, and farming was

pursued on both the distal and proximal gully-mouth fan

surfaces.

Not all bajadas were equally utilized, nor were settle-

ments evenly distributed over a single bajada. Other fac-

tors, especially the nature and thickness of the sediments

comprising the gully-mouth fan, fan slope (Dart 1987),

the substrate underlying it, and drainage basin character-

istics (e.g., size, the distance from the drainage basin to

the fan field, and hillslope composition) made some areas

better suited for farming than others and thus affected the

location of agricultural settlements. In general, the opti-

mum areas, characterized by soils with high water holding

and availability characteristics and desirable drainage basin

features (e.g., those of small size, with short distance from

field to drainage basin, and with impermeable slopes),

were used heavily while less desirable areas on the bajada

were used only marginally.

Ak-Chin fields do not occupy fixed locations and would

have been abandoned frequently because of migration of

headcuts through the fan surface (FIG. ). When entrench-

ment destroyed the usefulness of a field by channelizing

the former sheetflow, old fields were abandoned and new

ones selected for planting. This shifting occurred on a

small scale, displacing fields and settlements on the order

of tens or hundreds of meters on the bajada surface. As a

consequence, Hohokam settlements appear concentrated

around the same position on the bajada through time and

became buried as the gullies shifted position.

Ak-Chin farming appears to have been widely used by

the Tucson Basin Hohokam, as evidenced by the numer-

ous bajada-Hohokam associations. Furthermore, this ag-

ricultural strategy appears to have been long lived; it was

practiced from the Pioneer through the Classic Hohokam

periods (ca. A.C. 300-1500). For example, bajada sites

near the Sierrita Mountains date from the Pioneer through

Classic periods (Dart 1987); hose near the Tucson Moun-

tains span the Pioneer through the Colonial periods

(Downum, Rankin, and Czaplicki 1986); those near the

Tortolita Mountains date from the late Sedentary to the

early Classic periods (Rice 1987; Fish 1989); those near

Meters-ontour nterval 1 meter

0

Figure 9. Typical Ak-Chin field farmed by the Hopi in the Tallahogan

Valley, Arizona (from Hack 1942).Abandoned Ak-Chin field area is

shown. Apparently, a downstream gully headcut migrated through the

former fan area lying upslope and destroyed the former Ak-Chin field.

A new field was located on the downslope fan.

the Picacho Mountains belong to the Colonial and Classic

periods (Ciolek-Torrello 1987); and those near the Santa

Rita Mountains date from the Sedentary and early Classic

periods (Huckell, Tagg, and Huckell 1987). Long term

use of the bajada environment for agriculture was possible

because deposition of sediments on the Ak-Chin fields

replenished nutrients with each runoff event. Clearly, this

was a dominant agricultural strategy practiced by the Tuc-

son Basin Hohokam through time.

The Santa Cruz River: Hohokam Use of a Floodplain

Environment

The Santa Cruz River floodplain was also heavily uti-

lized by the Hohokam. Changes in the landscape and

hydrologic conditions on the floodplain influenced the

position of Hohokam settlements and agricultural strate-

gies at any particular time. This is well illustrated along

the 15-km segment of the Santa Cruz arroyo traversing

the San Xavier Indian Reservation, south ofTucson (FIGS.

1, 2) . Here, the locations of settlements are well docu-

mented for several Hohokam phases (Rillito, Rincon,

Tanque Verde, and Tucson) dating between A.C. 800 and

1500 (Doelle, Dart, and Wallace 1985; Doelle and Wal-

lace 1986). (Because of problems of site burial and visi-



Journal $Field ArchueologyIVol. 18,1991 15 3

bility, pre-kllito settlement patterns are poorly known.)

The geomorphic history of the San Xavier reach, especially

for the period between A.C.800 and 1450, is well estab-

lished through stratigraphic and geochronologic studies

(Haynes and Huckell1986; Waters 1988a, 1988b).Whenthe settlement patterns are superimposed over the corre-

sponding landscape reconstructions, it becomes clear that

the varying distribution of processes and landforms on the

floodplain influenced prehistoric Hohokam utilization of

the riverine environment, and changes to the floodplain

affected the regional stability, dsruption, and reorgani-

zation of settlement patterns (FIG.lo).

The channel of the Santa Cruz kv e r was entrenched

around 50 B.c., prior to the kllito phase. Over the next

1000 years, the channel filled with alluvium, then the

floodplain aggraded and eventually stabilized by A.C.950.

The last 150 years of aggradation, from A.C. 800 to 950,

coincided with the kl li to phase when settlement was char-

acterized by occupation at five primary villages and a num-

ber of hamlets on the western side of the floodplain (FIG.

lo). Floodplain stability continued into the early Rincon

(A.c. 950-1000; FIG. 10) and the settlement pattern

closely resembles that during the Rillito phase, but the

number of hamlets and the intensity of occupation at

primary villages increased.The floodplain during both the

kllito and early Rincon was characterized by a broad,

sandy surface that was probably traversed by a shallow

channel or draw. Discharge across most of the floodplain

would have been ephemeral; however, small localized

seeps may have been present. This environment was well

suited for floodwater farming. Crops could have beenplanted on the floodplain, where they would have been

watered during overbank flows. Crops could also have

been planted along the margin of the floodplain, where

gullies from the bajada intersected the floodplain and in-

termittent runoff would have provided water to the crops

planted there. Villages were located immediately adjacent

to the floodplain, next to major washes entering it, but

above areas of active flooding. Additional floodwater Ak-

Chin farming could have been pursued on the gully-

mouth fans on the bajada to the west.

At the beginning of the middle Rincon subphase (A.c.

1000-1100; FIG. lo), arroyo cutting occurred in the cen-

tral portion of the floodplain and extended southward.

Channel entrenchment led concurrently to the formation

of an arroyo-mouth fan and sand dunes to the north. The

arroyo destroyed arable land in the south, but in the

process created arable land to the north. Ak-Chin flood-

water farming was possible on the gully-mouth fan and

dry farming was possible on the sand dunes. These land-

scape changes appear to have triggered settlement reor-

ganization. During the middle kncon the number of

Figure 10. Landscape changes along the San Xavier reach of the SantaCruz

hver (see FIG. 1) fromA.C. 800 to 1450 and their effect on Hohokarn settlement patterns (modhed from Waters 1988a,

1988b).

1 4 5 0+Rtl l i to Phase -Ear ly R ~ n c onSubphase=k Middle Rtnco n Subpha seSc Late Rtnco n Subphase+Tanque Verde Phase+Tucson Phase*

I+----- Unen trenched F loodp la in Sou the rn Por t lon o f Floodplain E n t r e n ch e d "I, Arroyo Channel Ftl l ing- A r ro yo Channe l Ar ro yo M o u t h Fan C re a te d ::, wi th A l luv tum

In Nor th e rn Par t o f F loo dp la ln>- Z

wZ-7A.7,Sandy F loodp la in San ta Cruz Rlve r

,,,u". tsconttnuous arroyo Pr tmary village?U

= - A r r o yo - m o u th fa n . Hamle t -Dune Format io n and Stab i l i ty zwSand dunes a Seasona l camp c+

%? Spr tng /c tenega 0 Trtncheras si te LU 100 f t con tou r tn te rva l Diagnost ic she rd +----- Ctenega For rna t lon and Stab~ l t ty 4



154 Gullies and Aw qo s in Arizona1Waters

primary villages decreased from five to one and settlements

generally shifted northward. A second entrenchment epi-

sode during the late Rincon subphase (A.c. 1100 to 1150;

FIG. lo ) resulted in the formation of a cienega in the

northern part of the floodplain. Also, the arroyo in the

southern portion of the floodplain continued to destroy

arable land to the south. Correspondingly, Hohokam set-

tlements shifted to the northern and eastern edges of the

floodplain.

During the subsequent Tanque Verde phase (A.c.

1150-1130; FIG. lo), the landscape remained much like

that of the previous 150 years. The cienega expanded and

stabilized, and the arroyo channel in the southern portion

of the floodplain began to backfill. During this time, the

number of settlements continued to decline on the western

side and increase on the eastern side of the floodplain.

New primary villages were established around Martinez

Hill and the dune complex, and the number of hamlets

also increased.

The settlement shift to the northern and eastern sides

of the floodplain, completed by the Tanque Verde phase,

appears to have been a response to the destruction of

arable land in the southern and western parts of the flood-

plain and the creation of environments suitable for farm-

ing to the north. Major villages were established next to

the cienega and sand dunes, and smaller sites occurred

around and within these two environments that repre-

sented the optimum areas for farming in the newly created

landscape. Water could have been drawn from the cienega

to fields situated on its margin, and dry farming could

have been pursued on the sand dunes. Floodwater farming

could have been conducted on the gully-mouth fan and

along the undissected portions of the floodplain-bajada

interface on the eastern side of the floodplain. Clearly, the

unentrenched northern and eastern portions of the flood-

plain offered favorable agricultural conditions.

By the beginning of the Tucson phase (A.c. 1300 to

1450; FIG. lo), the arroyo had filled with alluvium and

the floodplain was no longer entrenched, while the cienega

and sand dune environments remained. The population

appears to have become concentrated in the primary vil-

lages established during the Tanque Verde phase. A few

small sites are present and diagnostic sherds of the Tucson

phase are found on activity loci on the floodplain.

Major entrenchment of the Santa Cruz floodplain fol-

lowed the Tucson phase around A.C. 1450. This wide-

spread environmental degradation would have again ren-

dered the floodplain unfarmable; following the entrench-

ment, the large primary villages were abandoned. Occu-

pation is not documented along the San Xavier reach of

the Santa Cruz River until sometime after A.C.1650 when

the arroyo channel was largely filled (Ravesloot 1987).

From this, we can see that the landscape of the San

Xavier reach of the Santa Cruz fi ve r changed dramatically

during the period of Hohokam occupation and clearly

influenced Hohokam settlement patterns. In addition to

landscape changes, social factors may account for some of

the observed patterns. However, the adverse effect of

channel entrenchment on floodplain farming and the

emergence of the cienega, arroyo fan, and sand dune en-

vironments as alternative locations for agriculture were

probably the most important factors responsible for ds -

ruption of settlement stability.

Altar Wash: Hohokam Utilization of an Awoyo-

mouth Fan Environment

The late Holocene alluvial sediments comprising the

floor of the Avra Valley were deposited on the arroyo-

mouth fan at the terminus of the Altar Wash (FIGS. , 2 ) .

In most places a 1.5-m-thick layer of late Holocene allu-

vium overlies a Pleistocene surface that is capped by a well

developed paleosol (Brakenridge and Schuster 1986;

Schuster and Brakenridge 1986; Waters 1987a). The Hol-

ocene sediments were deposited by repeated episodes of

sheetflooding over the arroyo fan. While the alluvial stra-

tigraphy of the Avra Valley is not completely known,

preliminary reconnaissance suggests that the floodplain

was not characterized by arroyo cutting and filling during

the Holocene (Waters 1987a). Exposures in a historical

arroyo (Brawley Wash) reveal no evidence of paleoar-

royos. Therefore, the hydrologic conditions during the

Holocene appear to have been dominated by sheettlood-

ing over the basin, with little channel cutting and filling.

The arroyo-mouth fan at the terminus of Altar Wash

was utilized by the Hohokam for farming from the Pi-

oneer through the Classic periods (FIG.11; Dart 1987).

Studies in the Schuk Toak area (FIGS. , i i ) , which tran-

sects the arroyo-mouth fan, indicate that many small sites

are near patches of silt-rich alluvium that were ideal for

plant growth and where floodwaters could easily have

been diverted onto these surfaces. Most Hohokam agri-

cultural sites are situated in the western part of the Schuk

Toak study area on and adjacent to these silt-rich sedi-

ments, especially near the bajada emanating from the Ros-

kruge Mountains (FIG. 11). In contrast, few Hohokam

sites are found in an analogous location on the eastern

side of the fan where it intersects the bajada emanating

from the Tucson Mountains. This distribution does not

appear to be the result of dfferential erosion, because



Journal $Field ArchaeologyIVol.18, 1991 155

Qala Holocene Brawley Wash arroyo-mouth fan (s ~lt y and and gravel)

Qalb Holocene Brawley Wash arr oyo-mouth fan (sandy s~l t nd %It)

I\QfT Holocene alluv~al ans emanating from the Tucson Mountains I

QfRb Holocene alluv~al ans emanating from the Roskruge Mount a~ns Kilometers

QfRa Ple~stocenealluv~al ans emanating from the Roskruge Mountai ns 0 1

Figure 11. Schuk Toak study area (see FIG. 1 for location) showing the distribution of Hohokam

settlements and activity areas on the arroyo-mouth fan at the termhus of Altar Wash. Hohokam sites

cluster on the western side of the arroyo-mouth fan as discussed, where floodwater Ak-Chin farming

could be effectively pursued (modified from Waters 1987a).

some intact Archaic and Hohokam features and sites are

found on the eastern portion of the fan. This indicates

that intense destruction of sites by floods has not occurred

and that the distribution reflects behavioral patterns

(Waters 1987a; Dart 1987).

The distribution of Hohokam sites appears to be related

to the floodwater farming potential of the Altar Wash

arroyo-mouth fan. Two factors make the western edge of

the fan more desirable for floodwater farming than the

eastern side. First, the western side of the Avra Valley is

higher, so floodwaters derived from Altar Wash are &-

rected by the slope and concentrated on the eastern side

of the valley. Discharge across the Altar Wash fan was

probably infrequent, but when it &d occur was cata-

strophic and therefore not conducive to the location of

fields. Thus, settlements and farmland on the western side

of the valley would have been relatively unaffected by large

floods. Second, fine-grained alluvium which would have

provided excellent farmland is widespread on the western

side of the valley. Consequently, the areas most utilized

by the Hohokam on the Altar arroyo-mouth fan were

adjacent to the Roskruge Mountains where runoff from

the gullies traversing the bajada could be funneled onto

floodwater fields on the arroyo-mouth fan (FIG. 11). This

runoff would have been more frequent, less severe, and of

short duration.

Prehistoric Human Effects o n the Eluvial

Landscape

Historical agriculturalists in southern Arizona mo&fied

their environment and triggered changes in the landscape.

Arroyo entrenchment of the Tucson Basin during the late

19th century was the unintentional result of human activ-

ity (Betancourt 1986; Cooke and Reeves 1976; Dobyns

1981) spechcally, agricultural modifications, ditch &g-

ging, and road construction on oversteepened reaches of

the floodplain. Similarly, some of the prehistoric landscape

changes on the Santa Cruz and the Cienega Creek flood-

plains may be attributed to humans.

Along the San Xavier reach of the Santa Cruz k ve r the

frequency of floodplain entrenchment increased after Ho-

hokam occupation intensified along the river. Over the

500-year period between A.C. 950-1450, the floodplain

was entrenched twice. In contrast, during the preceding

7000 years the floodplain was entrenched only three times

(FIG. 6; Haynes and Huckell 1986; Waters 1988a). The

increased frequency of entrenchment during the late pre-

historic period may be attributed in part to the creation

of repeated, unstable, internal geomorphic threshold con-

ditions. The oversteepening of reaches of the floodplain

by sedunent deposition would have created loci on the

floodplain that were susceptible to entrenchment. En-



156 Cullies and Arvoyos in Arizona1 Waters

trenchment of these unstable reaches of the floodplain

could have been triggered during the late Holocene by

flooding associated with short climatic iterations lasting

less than 10 years or by changes in the rainfall patterns

caused by long-term climatic fluctuations lasting 100 to

1000 years. In conjunction with flooding, prehistoric hu-

man activity on the floodplain may have been a major

factor in late Holocene channel cutting.

As agriculturalists, the Hohokam modified the land-

scape. They probably cleared natural vegetation from fields

on the floodplain; concentrated the runoff from drainages

on the bajada to the edge of the floodplain; collected

undergrowth and deadfall from mesquite thickets for fire-

wood; created well worn, compacted paths across the

floodplain; and perhaps constructed small diversion struc-

tures, short canals, and ditches on the floodplain.All these

modifications would have unintentionally made the

ground more susceptible to erosion and allowed water

flowing over the floodplain to increase its velocity. The

disturbed areas and ditches, if present, could have served

as loci for the initiation of entrenchment that could have

expanded during floods to become the arroyos visible in

the alluvial record. Although there is no direct evidence

to support the hypothesis that human actions on the flood-

plain resulted in entrenchment, the temporal correlation

between the occupation of the Santa Cruz hver by the

Hohokam agriculturalists and the increased frequency of

floodplain entrenchment is strilung. It appears that the

Hohokam agriculturalists may have made the same mis-

takes that historical farmers made in the late 19th century,

which led to floodplain entrenchment and creation of the

modern Santa Cruz arroyo.

Late Holocene arroyo cutting is also documented along

Cienega Creek (FIG. 6; Eddy and Cooley 1983). This

occurred sometime during the late hn con and early Tan-

que Verde phases when there was an increase in both

arable land and intensity of occupation. It may be that the

entrenchment of Cienega Creek was also triggered by the

intensified use of the region for agriculture. Interestingly,

late Holocene channel cutting on Cienega Wash and the

Santa Cruz h ver floodplains was not synchronous. Ar-

royo entrenchment of the Cienega Creek floodplain began

at the end of the late hncon phase while the first en-

trenchment along the San Xavier reach began during the

early Rmcon phase. Because these arroyo cutting events

are out of phase, they were probably not triggered by a

simultaneous climatic iteration. Instead, the utilization of

both environments by agriculturalists and historical ana-

logs suggest that the nonsynchronous arroyo cutting ap-

pears to have been the result of human activity. A second

entrenchment episode occurred along the Santa Cruz

River at the end of the Tucson Phase at A.C. 1450, but

there was no similar event on the Cienega Creek flood-

plain. This later entrenchment of the Santa Cruz hver

floodplain also may have been human induced.

Conclusions

Geoarchaeological studies are crucial for the proper in-

terpretation of the archaeological record and provide the

landscape context for elucidation of human ecology. With-

out the crucial information geoarchaeological studes pro-

vide, archaeological interpretations will be incomplete and

in some cases inaccurate. In southern Arizona, hydrologic

conditions and landscape evolution clearly affected the

archaeological record, influenced the location of Hoho-

kam settlements, and accounted for changes in settlement

patterns through time. Finally, prehistoric Hohokam uti-

lization of arroyo floodplains may have led to environ-

mental degradation.

Acknowledgments

The geoarchaeological research in southern Arizona that

provided many of the specific examples in this paper was

funded by the U.S. Bureau of Reclamation, the Arizona

Department of Transportation, and the Wenner-Gren

Foundation. A preliminary version of this paper was pre-

pared for the Tucson Aqueduct Archaeological Synthesis

volume prepared for the Bureau of Reclamation by theArizona State Museum. John Ravesloot and Jon Czapliclu

are thanked for funds used to prepare this report and the

illustrations. Helpful criticisms of earlier drafts of this

manuscript were received from John Ravesloot, Jon Cza-

pliclu, Robert E. Dewar, Michael B. Schiffer, Julie K.

Stein, Randall H. McGuire, Jack Donahue, and Lain Ellis.

Randy McGuire and Robert Dewar provided helpful sug-

gestions that improved this manuscript. A1 Wesolowsky

improved both the text and illustrations. Dora Lopez

typed the many drafis of this paper. All are thanked for

their assistance.

Michael R. Watersis an assistant professm ofAnthropology

and Geography at TexasAOM University, where he has been

teaching since 1985. Waters received his doctmatefrom the

Department of Geosciences at the University ofArizona in

1983.His cuwent research interests includegeoarchaeolog~

peopling of the Americas, and prehistoric settlement patterns

in the American Southwest. Mazfing address: Depa~ment f

Anthropology, TexasAOM University, College Station, TX77843.



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Documents

Journal of Field Archaeology 1991 Waters