The Timing of Floods and Storms as a Controlling Mechanism for Shelf Deposit Morphology

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The Timing of Floods and Storms as a Controlling Mechanism for Shelf DepositMorphologyAuthor(s): Cindy M. PalinkasSource: Journal of Coastal Research, Number 255:1122-1129. 2009.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/08-1041.1URL: http://www.bioone.org/doi/full/10.2112/08-1041.1

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Journal of Coastal Research 25 5 1122–1129 West Palm Beach, Florida September 2009

The Timing of Floods and Storms as a ControllingMechanism for Shelf Deposit MorphologyCindy M. Palinkas

Horn Point LaboratoryUniversity of Maryland Center for Environmental ScienceCambridge, MD 21613, [email protected]

ABSTRACT

PALINKAS, C.M., 2009. The timing of floods and storms as a controlling mechanism for shelf deposit morphology.Journal of Coastal Research, 25(5), 1122–1129. West Palm Beach (Florida), ISSN 0749-0208.

Deltas and clinoforms are accretionary deposits with subaerial and subaqueous topsets, respectively. Various factorshave been proposed that may control their geometry, especially the interaction of sediment supply and physicaloceanographic energy. A conceptual model of how this interaction affects shelf sedimentation indicates that sedimentfrom most rivers should form deltaic features in quiescent environments. In more energetic settings, most sedimentfrom small rivers (sediment load of 106–107 t/y) should be dispersed, limiting significant shelf accumulation, but theincreased supply from larger rivers (sediment load of 108–109 t/y) could allow for sediment retention on the shelf,leading to clinoform development. However, in the Adriatic Sea, a river delta and a shelf clinoform (i.e., adjacent tothe Po River and the Apennine rivers, respectively) have developed from similar sediment supply and oceanographicenergy. This suggests that other factors are likely important, particularly differences in the shelf gradient and thetiming of floods and storms. The shelf gradient is lower near the Po River, favoring retention of sediment in shallowerwater as compared to the Apennine rivers. In addition, floods and storms are uncorrelated on the Po River shelf dueto the large size of the drainage basin, enhancing sediment deposition in shallow water. For the Apennine rivers, thedrainage basin of individual rivers is small, and floods and storms are generally correlated, facilitating offshoredeposition of sediment and leading to the development of a shelf clinoform.

ADDITIONAL INDEX WORDS: Delta, clinoform, Adriatic Sea, floods, storms.

INTRODUCTION

The term ‘‘subaqueous delta’’ was originally created to de-scribe the wedge-shaped prograding body (i.e., a clinoform) onthe Amazon River shelf (Nittrouer et al., 1986) but has alsobeen used for the submerged part of a river delta (e.g., deltafront and prodelta; Cattaneo et al., 2003), causing confusionin the literature. The primary difference between a delta anda clinoform is the location of morphological components (thetopset is subaerial and subaqueous for deltas and clinoforms,respectively) relative to sea level (Figure 1). Several control-ling factors have been proposed to explain the morphologyoffshore rivers, such as the relationship between sedimentinput and physical oceanographic energetics (e.g., waves,tides, and currents; Nittrouer et al., 1986; Walsh et al., 2004),tectonic setting (Goodbred and Kuehl, 2000), relative sea-lev-el fluctuations (Driscoll and Karner, 1999), timing of floodsand storms (Wright and Nittrouer, 1995), shelf gradient(Coleman and Wright, 1975), and accommodation space (Cat-taneo et al., 2003). The difference between deltas and clino-forms may be subtle in the stratigraphic record, but the po-sition of the shoreline is critical for interpreting past depo-sitional environments. For example, an erroneous shorelineplacement affects estimates of oceanographic energy and re-constructions of past sea-level change.

DOI: 10.2112/08-1041.1 received 11 March 2008; accepted in revision15 September 2008.

The objectives of this paper are first to discuss the primaryfactors influencing the morphology of seabed deposits in gen-eral and then to assess the relevance of these factors in aspecific setting. In the latter portion, the Adriatic Sea is usedas a case study, because both a river delta and a shelf cli-noform are present.

BACKGROUND

Shelf Sedimentary Deposits

Most inner continental shelves have relatively coarse ma-terial (sand) either remaining from the Holocene marinetransgression or recently supplied by rivers. If fine sediment(mud) escapes estuaries to reach the shelf, it is typically de-posited below the depth of intense wave reworking. On thewest coast of the United States (e.g., near the Eel River innorthern California), modern mud and sand are supplied tothe shelf but the relatively intense physical oceanographicenergy disperses most mud beyond the inner shelf (Cacchioneet al., 1999; Crockett and Nittrouer, 2004). With increasedsupply and decreased dispersal, sedimentation on the innershelf can occur, and the resulting sedimentary deposit iscalled a delta if it accretes to sea level. This is consistent withboth the first definition of delta by the early Greeks, whoused it to describe the land between branching channels ofthe Nile, and the first geological classification as ‘‘a deposit

1123Controls on Shelf Deposit Morphology

Journal of Coastal Research, Vol. 25, No. 5, 2009

Figure 1. (A) Typical cross-sectional profile of an accretionary deposit(modified from Walsh et al., 2004). The location of sea level is shown bythe arrows and is near the boundary between the topset and the foreset(i.e., the rollover point) for deltas and above the topset for clinoforms. (B)Schematic of a compound delta–clinoform system.

Figure 2. Map of the Adriatic Sea, with isobaths contoured in 10-m in-tervals. The Po River is located in the north, and the Apennine rivers arelocated primarily between the Ancona Promontory and the Gargano Pen-insula. Note the difference in coastline orientation between the Po andthe Apennine systems mentioned in the text.

partly subaerial built by a river into or against a body ofpermanent water’’ (Barrell, 1912, p. 387).

Most classic river deltas (Coleman and Wright, 1975) formin relatively quiescent settings (e.g., the Mediterranean andthe Gulf of Mexico) and were initiated 7400–9500 cal y BPby a deceleration in postglacial sea-level rise (Stanley andWarne, 1994). The fundamental components of a delta arethe delta plain, delta front, and prodelta, which correspondto topset, foreset, and bottomset deposits (Figure 1). The mor-phology and geometry of deltas are mainly affected by theriver flow and wave and tidal transport (Wright and Nit-trouer, 1995), as shown by the tripartite classification schemeof tide-, wave-, or fluvial-dominated deltas (Galloway, 1975).Tide-dominated deltas (e.g., the Fly River Delta; Harris et al.,1993) are characterized by linear, subaqueous sand ridgesformed by sediment reworking due to bidirectional tidal cur-rents (Coleman and Wright, 1975). In contrast, the high en-ergy associated with wave-dominated deltas produces asmooth cuspate delta shoreline with a single distributarychannel (e.g., the Brazos River Delta; Galloway, 1975). Whenfluvial sediment enters a relatively quiescent setting, anelongate delta is formed with distinct digitate features relat-ed to the major distributaries (i.e., a ‘‘birdfoot’’ delta), exem-plified by the Mississippi River Delta.

In open-marine settings, the increased shear stressescaused by more energetic physical oceanographic conditionsoften prohibit the seabed deposit from accreting to sea level,and this can result in creation of a clinoform. The topset,foreset, and bottomset deposits of deltas correspond to thestratigraphic regions comprising clinoforms, originallynamed the undaform, clinoform, and fondaform by Rich(1951) (all three are now commonly included in the term ‘‘cli-

noform’’). Examples of clinoforms include the features nearthe Amazon and the Ganges-Brahmaputra rivers, which areformed by the reworking of sediment from large rivers (an-nual sediment load of 1200 and 1000 � 106 t/y, respectively;Milliman and Syvitski, 1992) by energetic tidal and shore-parallel currents, respectively (mean current velocity of 150and 200 cm/s; Curtin, 1986; Kuehl et al., 1997). More recently,the clinoform feature in the Gulf of Papua was examined(Slingerland et al., 2008; Walsh et al., 2004), and the couplingof tidal currents and surface gravity waves was identified asthe cause of intense shear stresses in the shallow topset re-gion.

Mud deposition emanating from small rivers (sedimentload of 106–107 t/y) in energetic environments usually occursat midshelf water depths, but development of a distinct cli-noform geometry is hindered by increased shear stresses, lim-iting significant shelf accumulation (McCave, 1972). An ex-ample of a midshelf mud deposit occurs near the Eel River,where only approximately 20% of the fine-grained sedimentsupply is estimated to be retained on the shelf (Sommerfieldand Nittrouer, 1999).

Adriatic Sea Sedimentation Characteristics and PhysicalOceanography

Both a river delta and a shelf clinoform are present in theAdriatic Sea (Figure 2). The Adriatic is a shallow epiconti-

1124 Palinkas


Figure 3. (A) Conceptual energy and supply scenarios. (B) Qualitativeplot of sediment supply versus oceanographic energy for modern fluvialsystems. See text for details.

nental sea, corresponding to the most recent Apennine fore-land basin (Ori, Roveri, and Vannoni, 1986). Late Quaternarysedimentation in this basin has resulted in a shore-parallelmud wedge along the western coast, which is the major com-ponent of the late Holocene highstand systems tract recog-nized on seismic profiles (Trincardi et al., 1996) that containsboth the Po and the Apennine depositional systems. The lo-cation of the rollover point (the landward edge of the foreset)deepens southward from near the shoreline for the modernPo River Delta to a 30- to 35-m water depth along the Apen-nine shelf clinoform (Cattaneo et al., 2007; Correggiari et al.,2001).

The surface circulation is primarily thermohaline, drivenby freshwater input in the north and exchange with the Med-iterranean Sea in the south. It is characterized by a cyclonic(counterclockwise) gyre, with northward and southward flowalong the eastern and the western coasts, respectively (Ar-tegiani et al., 1997). Significant sediment transport generallyoccurs during episodic wind events (Fain, Ogston, and Stern-berg, 2007) that are dominated by the Bora and Sciroccowinds, especially in the winter. Bora winds are cold, conti-nental, northeasterly winds that cool surface waters, result-ing in downwelling and dense-water formation (Hendershottand Rizzoli, 1976). Scirocco winds are moist, southeasterlywinds that frequently result in storm surges and flooding incoastal towns such as Venice (Orlic, Kuzmic, and Pasaric,1994; Pirazzoli and Tomasin, 2002). Bottom-boundary-layermeasurements from instrumented tripods during the winterof 2002–03 indicated that Bora wind-driven events dominatethe sediment transport record in the Adriatic, although thecurrent velocities can vary at sites along the Italian coast(event-averaged velocities of 12 and 18 cm/s near the Po andthe Pescara rivers, respectively; Fain, Ogston, and Sternberg,2007).

The Po River is the primary source of sediment to thenorthern Adriatic, delivering 1.5 � 107 tons of sediment an-nually (Frignani et al., 2005). Modern (�100 y) sediment ac-cumulation rates are 4 to 5 cm/y near the distributaries, andapproximately 50% of the estimated Po River sediment loadlikely is transported southward from the delta under the pre-vailing circulation (Frignani et al., 2005; Palinkas and Nit-trouer, 2007) to join with sediment from the Apennine rivers.These are a series of small, distributed fluvial sources (i.e., aline source), located primarily between Ancona and the Gar-gano Peninsula, that together discharge sediment at a rateof approximately 3 � 107 t/y (Cattaneo et al., 2003; Frignaniet al., 2005; Syvitski and Kettner, 2007). Maximum modernsediment accumulation rates occur far from the fluvial sourc-es near the Gargano Peninsula (�1.7 cm/y; Palinkas and Nit-trouer, 2006) due to advection by a strong, southward-flowingcoastal current that progressively adds material.

DISCUSSION

Conceptual Scenarios of Shelf Deposit Formation

As previously stated, the primary geomorphic differencebetween deltas and clinoforms is whether the topset is sub-aerial or subaqueous, and previous studies have indicatedthat the primary determinant is the balance between sedi-

ment supply and oceanographic energy (Alexander, Demas-ter, and Nittrouer, 1991; Nittrouer et al., 1986; Pirmez, Prat-son, and Steckler, 1998; Walsh et al., 2004). This can be eval-uated by considering the following conceptual scenarios (Fig-ure 3A), assuming that sedimentation is limited only byoceanographic energy or sediment supply: For fluvial systemswith equal supply but unequal energy, shear stresses in-crease with oceanographic energy such that deposition is in-hibited first in shallow water and then in deeper water. Sed-iment dispersal processes are also enhanced with increasedenergy, further limiting shelf accumulation. For example,sediment can be resuspended within the thin (1–10 cm) wave-boundary layer, forming wave-supported gravity flows thatcan be an important mode of cross-shelf transport (Scully,Friedrichs, and Wright, 2002; Traykovski et al., 2000; Wrightet al., 2001). Therefore, deltas are expected to form in low-energy environments, followed by clinoforms and midshelfmuds as energy increases. For fluvial systems with equal en-ergy but unequal supply, shelf accumulation increases withincreasing supply, and the opposite trend occurs—midshelfmuds are formed first and are followed by clinoforms anddeltas as supply increases.

Several difficulties arise in quantitatively assessing bothsediment supply and oceanographic energy for natural riversystems. For example, constraining the sediment supply termis challenging in areas with significant anthropogenic im-pacts on historical loads (e.g., decreases in supply for the Ebroand Mississippi rivers; Milliman and Syvitski, 1992) or inline-source systems (e.g., in the Gulf of Papua; Walsh et al.,2004). Also, many continental shelves are flood-dominatedshelves (e.g., the shelves adjacent to the Po and Eel rivers;Palinkas and Nittrouer, 2007; Sommerfield and Nittrouer,1999), and most sediment is delivered during short-durationevents (days to weeks) that can have sediment discharges



Table 1. Maximum monthly wave height ( Wa ) and tidal range ( Ti ) fromSyvitski and Saito (2007) for the global river systems considered in thispaper.

River

WaveHeight

(m)

TidalRange

(m) Pm* ‘‘Energy’’†

Amazon 2.0 6.0 40.00 HighBrazos 1.5 0.7 2.74 LowChangjiang 2.0 1.5 6.25 LowColumbia‡ 1.5 3.0 11.25 MediumEbro 1.5 0.2 2.29 LowEel 3.5 3.0 21.25 HighFly 1.5 4.0 18.25 MediumGanges-Brahmaputra 1.0 3.6 13.96 MediumHuanghe 1.5 0.8 2.89 LowMississippi 0.5 0.4 0.41 LowPescara 1.5 0.6 2.61 LowPo 1.5 0.7 2.74 LowRhone 2.0 0.5 4.25 Low

* Pm, the marine power, is defined as Pm � W � T .2 2a i

† ‘‘Energy’’ is defined as follows, using the Pm values: low � 1–10; medium� 10–20; high � 20 and above.‡ Data for the Columbia River is from Wright and Nittrouer (1995). Forthis river, wave height is the root-mean-square height, and tidal range isfor spring tides.

Table 2. Qualitative relationship of sediment supply and oceanographic energy for (a) global river systems and (b) the Po River and the Apennine rivers.

River Sediment Supply* (Load � 106)† Oceanographic Energy‡ Shelf Deposit

(a) Global River SystemsAmazon High (1200) High ClinoformBrazos Low (16) Low DeltaChangjiang Medium (480) Low CompoundColumbia Low (10) Medium Midshelf mudEbro Low (1.5) Low DeltaEel Low (14) High Midshelf mudFly Medium (115) Medium DeltaGanges-Brahmaputra High (1060) Medium CompoundHuanghe High (1100) Low CompoundMississippi Medium (210) Low DeltaPescara Low (0.9) Low ClinoformPo Low (13) Low DeltaRhone Low (31) Low Delta

(b) Po River and Apennine RiversPo Low (15)§ Low DeltaApennines Low (0.02–2.2)§ Low Clinoform

* Note that the estimated range in sediment supply given for the Apennine rivers is for individual rivers (e.g., the Pescara River); the combined sedimentload from the Apennine rivers is �30 � 106 t/y (Frignani et al., 2005).† Data from Milliman and Syvitski (1992).‡ The criteria used in characterizing physical oceanographic energy are given in Table 1.§ Data from Frignani et al., 2005.

many times higher than the average annual load. Difficultiesin characterizing the oceanographic energy of modern riversystems arise because many types of physical processes areactive that are spatially and temporally variable. For exam-ple, sediment is redistributed on the shelf by waves near theEel and Columbia rivers (Kachel and Smith, 1989; Ogstonand Sternberg, 1999), by tidal currents near the Fly and Am-azon rivers (Curtin, 1986; Wolanski, Norro, and King, 1995),and by shore-parallel oceanic currents near the Apennine andHuanghe rivers (Cattaneo et al., 2003; Wright et al., 1990).Temporal variations are especially important in storm-dom-

inated settings (e.g., near the Ganges-Brahmaputra River;Michels et al., 1998), and spatial variations can occur acrossand along the shelf and within the water column.

Nonetheless, comparisons among river systems can bemade by considering the ‘‘marine power’’ of the receiving ba-sin, following Syvitski and Saito (2007). This parameter iscalculated by summing the squares of the maximum monthlywave height and the tidal range (Table 1). Using this dataand estimates of sediment load for each river (Table 2a), aqualitative supply-versus-energy plot can be developed (Fig-ure 3B), reflecting the complex interaction of sediment supplyand oceanographic energy in forming shelf deposits. In rela-tively quiescent environments, the sediment from both smallrivers (sediment load of 106–107 t/y) and medium rivers (sed-iment load of 108 t/y) is expected to form deltaic features (e.g.,Ebro and Mississippi rivers). However, if the receiving basinadjacent to a small–medium river is highly energetic, sedi-ment can be widely dispersed, preventing significant shelfaccumulation and forming a midshelf mud deposit (e.g., EelRiver). The sediment supply from large rivers (sediment loadof 109 t/y) could compensate for this increased oceanographicenergy, although the high shear stresses would prohibit thedevelopment of a subaerial topset, producing a shelf clino-form (e.g., Amazon River). Compound systems, consisting ofboth well-developed subaerial deltaic topsets and subaqueousclinoform units, can result from medium and large rivers thatdischarge into low- and medium-energy environments (e.g.,Huanghe and Ganges-Brahmaputra rivers; Alexander, De-master, and Nittrouer, 1991; Goodbred and Kuehl, 2000).

This conceptual scheme can account for much of the vari-ability in natural shelf deposits. However, other factors (seeIntroduction) are important for determining shelf morpholo-gy. Their influence can be seen in the differing deposits thatcan emanate from fluvial systems with relatively similar sed-

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iment load and oceanographic energy, as are present in theAdriatic Sea (Table 2b).

The Adriatic Sea Example

Potential Differences in Sediment Supply andOceanographic Energy

The Po and Apennine fluvial systems differ in their styleof sediment delivery—point versus line source, respectively.Because the sediment input from the Apennine margin is de-livered by many small sources, oceanographic energy mayhave a larger role in shaping the resulting sedimentary de-posit than it would if the input occurred from a single, largersource, as it is for the Po River. This may also affect thesediment concentration in the river plume, which could inturn influence flocculation dynamics, because floc formationis controlled by a balance between parameters that bring par-ticles together (e.g., high concentrations) and those that breakclumps of particles apart (e.g., high energy) (Hill, Milligan,and Geyer, 2000).

Floc formation has been observed upstream of the Po Rivermouth (Fox et al., 2004), resulting in rapid sedimentation ofthis material immediately offshore of the river mouth, whichin turn contributes to deltaic growth. The potential for flocformation within individual Apennine rivers has not been ex-amined; however, George, Hill, and Milligan (2007) deter-mined the fraction of sediment within the seabed that depos-ited as flocs (termed ‘‘floc fraction,’’ or fs) from the disaggre-gated inorganic grain-size distribution (DIGS; following Cur-ran et al., 2004). At equivalent depths on the shelf, the flocfraction of surficial sediment near the Po River is greaterthan that near the Tronto and Pescara rivers (Apennineshelf; see Figure 7 in George, Hill, and Milligan, 2007). If theenergy of both shelf environments is assumed to be equiva-lent (see discussion that follows), the difference in floc frac-tions can be attributed to differences in sediment concentra-tion.

However, this is indicative of present conditions only. Oth-er studies have noted that sedimentation processes in theAdriatic Sea, particularly on the Apennine shelf, havechanged dramatically over the last few decades, primarilydue to damming (Palinkas and Nittrouer, 2006; Syvitski andKettner, 2007). This has resulted in reduced sediment loadsand a clinoform deposit that is likely not actively accretingeverywhere along the Apennine shelf at present (Palinkasand Nittrouer, 2006). While no direct comparisons have beenmade of historical flocculation dynamics for the Po and Apen-nine systems, past sediment concentrations in the Apenninerivers were likely much higher than at present, especiallyduring extreme sedimentation events that build much of thestratigraphic record (Sommerfield and Nittrouer, 1999). Us-ing the climate-driven hydrological model HydroTrend, Sy-vitski and Kettner (2007) simulated natural discharge andsediment loads from the Apennine rivers and found that 20to 40% of the historical sediment loads from these rivers werelikely delivered as hyperpycnal flows because of the high re-lief and small drainage basins of these rivers. Hyperpycnalflows require suspended-sediment concentrations of approx-imately 40 kg/m3 to overcome the density difference between

freshwater and ocean water (Mulder and Syvitski, 1995), al-though they may form at significantly lower concentrationsif convective settling is established (Parsons, Bush, and Sy-vitski, 2001). The magnitude of the flux to the Adriatic hasalso been affected, and reservoirs currently trap about 30 to50% of the Apennine sediment load (Syvitski and Kettner,2007). Therefore, although delivery by the present-day Apen-nine rivers is relatively low, it is likely to have been higherin the past when the shelf clinoform was developed.

Another potential difference between the Po and the Apen-nine systems relates to the physical oceanographic energeticson the adjacent shelf. This is suggested by the slightly dif-fering tidal ranges presented Table 1, which agree with ob-servations during winter 2002–03 that greater average tidalrange and current speed occur on the shelf near the Po River(64 cm and 6.8 cm/s, respectively) than near the Pescara Riv-er (30 cm and 4.9 cm/s, respectively; Fain, Ogston, and Stern-berg, 2007). However, tides in both locations are relativelysmall (microtidal), and sediment transport occurs largely dur-ing a series of wind-driven resuspension events during wintermonths. On the Po shelf, these events occur during both thenortheasterly Bora and the southwesterly Scirocco winds. Itappears that the Pescara shelf may be protected from theScirocco winds due to its orientation (Figure 2), resulting inlow wave-orbital velocities and little sediment resuspensionat these times. However, the along-shelf currents duringBora winds tend to be stronger on the Pescara shelf due tointeractions with the Western Adriatic Current (Fain, Og-ston, and Sternberg, 2007). On both shelves, currents arestronger in the along-shelf direction than in the across-shelfdirection, which is typical of low-energy environments (e.g.,Ebro; Palanques et al., 2002).

While some differences appear in the magnitude of sedi-ment supply and oceanographic energy between the Po andthe Apennine systems, they are relatively small in compari-son to the diversity in global fluvial systems, in which supplyand energy can vary over many orders of magnitude. Greaterdifferences between the Po River and the Apennine rivers arethe size of their drainage basins (70 � 103 and 0.5–3.3 � 103

km2, respectively; Nelson, 1970; Syvitski and Kettner, 2007),which affects the timing of fluvial sediment delivery in rela-tion to the energy of the receiving basin, and the gradient ofthe adjacent shelf (increasing from �0.050 to �0.20 near thePo River and the Apennine rivers, respectively; Cattaneo etal., 2007). These are likely to be the primary controllingmechanisms in the Adriatic, over which the subtle differencesin supply and energy discussed earlier are imprinted.

Other Differences: Shelf Gradient and the Timingof Floods and Storms

The seabed gradient is a measure of the shelf geometry,which influences the magnitude and direction of physical pro-cesses and controls the volume available for shelf accumula-tion (i.e., the accommodation space; Walsh and Nittrouer,2003). Traditionally, the accommodation space is the area be-tween the seafloor and the sea level (Jervey, 1988), but shelfdeposits do not exploit all of this space. Instead, the relevantarea is the dynamic accommodation space, which is the por-



Figure 4. River-discharge and wave-height measurements for the Po,Pescara, and Eel rivers showing the relationship of floods (high discharge)and storms (high waves; Ogston, 2005). For the Po River, peak dischargeoccurs during a lull in the wave activity, allowing sediment to deposit inshallow water. For the Pescara and Eel rivers, peak discharge and waveheights occur nearly simultaneously, maintaining sediment in suspensionand facilitating transport to deeper waters.

tion of accommodation below the depth of sediment resus-pension and transport (Cattaneo et al., 2003) and, as such, isconstrained by oceanographic energy and shelf geometry (i.e.,shelf gradient). More sediment can be retained on shelveswith lower gradients, facilitating deltaic sedimentation,whereas sediment dispersal is enhanced for higher gradients,leading to clinoform development. The seabed gradient steep-ens between the north and the central Adriatic, from about0.020 near the Po River to about 0.50 near the Apennine riv-ers, and is likely an important factor in determining shelfdeposit morphology.

Another important factor is the timing of floods andstorms. When high sediment discharge occurs during periodsof increased physical oceanographic energetics, sediment canbe widely dispersed, as is the case on the shelf near the EelRiver (Wheatcroft and Borgeld, 2000; Figure 4). For the De-cember 1995 flood event of the Eel, peak discharge (2295.1m3/s) occurred approximately 1 day after the peak waveheights on the shelf (7.7 m); note that discharge data is adaily average, whereas wave data is reported hourly. How-ever, on the shelf near the Po River Delta, increased sedimentdischarge occurs over a range of conditions, and depositiontypically occurs in shallow water near the river mouth (Pal-inkas et al., 2005). For example, during the October 2000flood event, peak discharge (9650 m3/s) occurred approxi-mately 7 days after peak wave heights on the shelf (2.66 m).During peak discharge, waves were about 0.4 m and re-mained less than 1 m until approximately 14 days after peakdischarge (discharge values for the Po are daily averages,whereas wave data is reported every 3 hours). When sedi-ment is deposited into relatively quiescent waters, it can be-

gin to consolidate, and higher near-bottom shear stresses arerequired for erosion. This threshold is exceeded on the Poshelf an average of only 14 days per winter (compared to anaverage of 53 days per winter on the Eel shelf; Fain, Ogston,and Sternberg, 2007; Guerra, 2004), further limiting sedi-ment resuspension. Therefore, much sediment remains inshallow water over long time scales, leading to the develop-ment of the Po River Delta.

The timing of floods and storms near the Apennine riversis somewhat difficult to assess, because most of these small,coastal rivers are hydrologically controlled (Syvitski andKettner, 2007). These controls have resulted in river hydro-graphs with relatively uniform low flows and a shelf clino-form that may be a relict feature in some areas (Palinkas andNittrouer, 2006). For the flood event in March 2000 of thePescara River highlighted in Figure 4, peak discharge (61.1m3/s) occurred approximately 1 day after peak wave heights(3.02 m) on the shelf (as it is for the Po, discharge data isdaily averaged, whereas wave heights are reported every 3hours). However, the predam Apennine rivers are expectedto have experienced frequent high-discharge events (Syvitskiand Kettner, 2007), behaving much like the present-day EelRiver. Both the individual Apennine rivers and the Eel Riverhave small drainage basins (0.5–3.3 � 103 and 9 � 103 km2,respectively; Nittrouer, 1999; Syvitski and Kettner, 2007) inwhich most sediment is discharged during periods of in-creased precipitation occurring with elevated winds andwaves (i.e., sediment is discharged during the storm deliver-ing the increased precipitation). In contrast, for rivers withlarger drainage basins, like the Po River, flood waves cantake longer to travel through the basin, and sediment is dis-charged under physical oceanographic energetics that may bemore reflective of fair-weather conditions, allowing for sedi-ment deposition in shallower water.

The difference in the timing of floods and storms for thePo River and the Apennine rivers (i.e., uncorrelated and cor-related, respectively) can help explain the differences in themorphology of the seabed deposits. For the Po River, sedi-ment is discharged during relatively quiescent conditions andsediment deposits in shallow water, facilitating deltaic sedi-mentation. For the Apennine rivers, especially before damconstruction, the small drainage basins allow a rapid re-sponse of the rivers to periods of increased precipitation, andsediment is discharged during more energetic conditions.This enhances dispersal so that the locus of sedimentation isfurther offshore, leading to development of a shelf clinoform.

In summary, the Po is dominated by a large input of rap-idly settling particles into relatively quiescent physical–oceanographic conditions and onto a low-gradient shelf, al-lowing significant accumulation in shallow water and devel-opment of a subaerial deposit. However, about 50% of the PoRiver sediment load is transported southward to the Apen-nine region (Palinkas and Nittrouer, 2006), where it is redis-tributed largely during Bora wind–induced sediment resus-pension events. This redistributed sediment joins with Apen-nine sediment, most of which is delivered during floods thattend to coincide with energetic conditions, among a relativelyhigh-gradient shelf, limiting accretion in shallow water andfacilitating clinoform development.

1128 Palinkas


SUMMARY

The interaction of sediment supply and oceanographic en-ergy generally determines the geometry of seabed deposits.Quantitatively constraining the sediment supply and physi-cal oceanographic energy of natural river systems is difficult;however, qualitative assessments can be made. Deltaic sedi-mentation is expected in quiescent conditions, because sedi-ment can deposit in shallow water and subsequent resuspen-sion is limited. In higher-energy areas, most sediment fromsmall rivers is dispersed off shelf, forming a midshelf muddeposit, but the increased supply from large rivers can com-pensate for the increased near-bottom shear stresses and aclinoform geometry can develop.

Sedimentation in the Adriatic Sea presents a challenge tothe preceding ideas. A river delta and a shelf clinoform havedeveloped near the Po River and the Apennine rivers, re-spectively, from similar sediment loads and physical ocean-ographic conditions, although along-shelf currents are stron-ger on the shelf adjacent to the Apennine rivers. This sug-gests that other factors are important, particularly differenc-es in shelf gradient and timing of floods and storms.Low-gradient shelves tend to favor retention of sediment inshallow water, whereas high-gradient shelves can enhancesediment dispersal to deeper water. When floods and stormsare uncorrelated (e.g., on the Po River shelf), sediment can bedeposited in shallow water, facilitating deltaic sedimentation.But when floods and storms are correlated (e.g., near theApennine rivers, which have small drainage basins), sedi-ment can be dispersed offshore, leading to clinoform devel-opment.

ACKNOWLEDGMENTS

The author would like to thank many colleagues in theEuroSTRATAFORM project for fruitful discussions on Adri-atic Sea sedimentation. In particular, Chuck Nittrouer, An-drea Ogston, Jeff Parsons, and Craig Lee provided valuableinsight and guidance during the preparation of this manu-script. Reviews of an earlier version by Courtney Harris andSteve Goodbred, as well as two anonymous reviewers, greatlyimproved the final manuscript. This work is based on resultsfrom the Po and Apennine Sediment Transport and Accu-mulation project funded by the Office of Naval Research. Thisis University of Maryland Center for Environmental Sciencecontribution No. 4217.

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