causes of river avulsion - insights from the late holocene avulsion history of the mississippi...

Copyright q 2005, SEPM (Society for Sedimentary Geology) 1527-1404/05/075-650/$03.00

Journal ofSedimentaryResearch

Journal of Sedimentary Research, 2005, v. 75, 650–664DOI: 10.2110/jsr.2005.053

CAUSES OF RIVER AVULSION: INSIGHTS FROM THE LATE HOLOCENE AVULSION HISTORYOF THE MISSISSIPPI RIVER, U.S.A.

ANDRES ASLAN,1 WHITNEY J. AUTIN,2 AND MICHAEL D. BLUM3

1 Department of Physical and Environmental Sciences, Mesa State College, Grand Junction, Colorado 81501, U.S.A.2 Department of the Earth Sciences, State University of New York, College at Brockport,

Brockport, New York 14420, U.S.A.3 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

e-mail: [email protected]

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FIG. 1.—Geologic map of the southern Lower Mississippi Valley (LMV) showing the location of the Old River study areas (box). Regional geology is modified fromSaucier and Snead (1989). Inset map shows the location of the Mississippi and Atchafalaya rivers in Louisiana. Locations of Figures 5 and 10 are shown. Solid trianglesshow locations of gradient calculations in Table 2.

ABSTRACT: The emphasis on gradient advantages in studies of avul-sion is misleading. While gradient advantages are necessary for anavulsion to occur, the late Holocene avulsion history of the MississippiRiver in Louisiana suggests that factors such as substrate compositionand floodplain channel distributions are more important. Cross-valleyto down-valley slope ratios of the modern floodplain range from 16 to110 and are typically . 30. The slope ratio is 35 at the location of theMississippi–Atchafalaya diversion (Old River) yet slope ratios are 83to 110 immediately upvalley of Old River. All values of MississippiRiver floodplain slope ratios are significantly larger than values ofavulsion threshold calculated by numerical models. Shallow floodplaincores, 14C dating of organic remains, and geologic mapping show thatthe Mississippi River has avulsed only four times over the past 5 ky inthe southern Lower Mississippi Valley (LMV). Gradient advantagesare widespread, yet avulsions are rare. These observations indicate thatfactors other than gradient advantage control Mississippi River avul-sion.

Several examples of Mississippi and Red River avulsion by channelreoccupation support the idea that channel distributions and substratecompositions are primary influences on avulsion. Incipient MississippiRiver avulsion and development of the Atchafalaya River involved re-occupation of abandoned Mississippi River channels and a Red Rivercrevasse-splay complex. The modern Atchafalaya River also incisesburied Mississippi River channel-belt sands. Abandoned channel beltsand crevasse-splay complexes consist of sandy substrates that facilitatescour and the development of channels capable of capturing the Mis-sissippi River. Abandoned channels provide ready-made conduits forMississippi River flow that can efficiently develop into avulsive chan-nels. Multi-storied sheet sandstones in ancient fluvial deposits may pro-vide additional support for the idea that erodible substrates and flood-plain channel distributions are critical influences on avulsion. Thesefeatures record episodic reoccupation of channel belts, which at leastin some cases, may simply reflect successive avulsions rather than ma-jor changes in aggradation rate or extrabasinal factors such as climate.

INTRODUCTION

Avulsion plays a major role in the construction of floodplains in conti-nental interiors, and the alluvial–deltaic plains of continental margins. Thisprocess determines locations of river courses, influences sediment deliveryto coastal regions, controls local rates of sediment accumulation, andstrongly influences floodplain topography and alluvial architecture. Avul-sions typically consist of two phases: (1) development of initial conditionsthat set the stage for an avulsion (i.e., avulsion threshold sensu Jones andSchumm 1999) and (2) a trigger event such as a major flood (Mohrig etal. 2000). Well-documented examples of Holocene avulsions involve theSaskatchewan River in Canada and the Rhine–Meuse system in the Neth-erlands (N.D. Smith et al. 1989; Tornqvist 1994; N.D. Smith et al. 1998;Morozova and Smith 1999, 2000; Stouthamer 2001; Stouthamer and Ber-endsen 2000, 2001; Berendsen and Stouthamer 2002; Tornqvist and Bridge2002). While these studies have greatly improved our knowledge of avul-sion, our understanding of avulsion causes remains incomplete.

Most avulsion studies emphasize gradient advantages as the underlyingdriving mechanism for avulsion (e.g., Slingerland and Smith 1998). Chan-nel-belt aggradation leads to production of local floodplain relief (Mohriget al. 2000; Makaske 2001; Makaske et al. 2002; Tornqvist and Bridge2002) and results in cross-valley gradient advantages (Tornqvist and Bridge2002). Alternative views of avulsion include channel-capacity-limited mod-els of avulsion (Makaske 2001). For instance, Schumm et al. (1996) de-scribe how reductions in channel gradient caused by increased channelsinuosity lead to decreased sediment transport capacity and avulsion. Sev-eral important examples of avulsion due to channel blockage by sediment,ice, and vegetation have also been documented (King and Martini 1984;Schumann 1989; McCarthy et al. 1992; Harwood and Brown 1993; Eth-ridge et al. 1999). While gradient advantages appear to be necessary foravulsion (Slingerland and Smith 1998), the relative importance of this fac-tor compared to other conditions remains unclear.

Mississippi River avulsion has received relatively little attention (Autinet al. 1991; Guccione et al. 1999; Tornqvist and Bridge 2002) despite earlystudies by Fisk (1952), which recognized that the Mississippi–Atchafalaya

J S R 651MISSISSIPPI RIVER AVULSION

J S R652 A. ASLAN ET AL.

FIG. 2.—Chart showing chronology ofMississippi River subdeltas and meander belts inthe LMV. Note that the meander belt chronologyof the southern LMV differs from thegeneralized chronology of Saucier (1994). Datafrom Aslan and Autin (1998, 1999), Autin et al.(1991), Saucier (1994), and Tornqvist et al.(1996).

FIG. 3.—Map showing late HoloceneMississippi River courses, subdeltas, andlocations of avulsion nodes (numbered circles).The avulsions are numbered from 1 to 4; theAtchafalaya diversion (#4) is the most recentavulsion. Avulsion #2 occurred ; 328 159 Nnear Vicksburg, Mississippi. Modified from Fisk(1952) and Coleman et al. (1998).

River diversion represented an incipient avulsion. Furthermore, most dis-cussion of Mississippi River avulsion has focused on its relevance to delta-lobe switching (Saucier 1994; Roberts and Coleman 1996; Tornqvist et al.1996; Coleman et al. 1998). This paper provides new information on thelate Holocene avulsion histories of the Mississippi and Red rivers in thesouthern Lower Mississippi Valley with an emphasis on development ofthe Mississippi–Atchafalaya diversion (Fig. 1). We use a combination ofgeomorphic and stratigraphic data to reconstruct the avulsion history of theregion and to evaluate conditions that favor avulsion. In particular, wesuggest that gradient advantages, while necessary, may play a less signif-

icant role in avulsion of the Mississippi River than factors such as substratecomposition and distributions of floodplain channels.

AVULSION HISTORY

It has long been recognized that a series of avulsions during the Holocenesea-level highstand led to the development of a succession of Mississippichannel belts and subdeltas (Fig. 2). During the late Holocene in southLouisiana, the Mississippi River avulsed three times, with the Mississippi–Atchafalaya diversion representing a fourth incipient avulsion (Fig. 3).


FIG. 4.—A–D) Maps showing development of the Atchafalaya River. Modified from Fisk (1952).

These observations suggest that the frequency of late Holocene MississippiRiver avulsion is slightly larger than 1,000 years. Ca. 5000 yr B.P., theMississippi River flowed on the western side of its valley, through a chan-nel belt now occupied by Bayou Teche, and constructed the Teche subdelta(Saucier 1994). The Mississippi subsequently avulsed near Old River, Lou-isiana (; 318 N, Fig. 3, avulsion site #1), and flowed to the Gulf of Mexicoalong the eastern side of the valley. By 1500 yr B.P., the Lafourche sub-delta became active and prograded across the abandoned Teche subdelta(McFarlan 1961; Tornqvist et al. 1996). Avulsion upstream near Vicksburg,Mississippi (; 328 159 N) completed the eastward shift by the MississippiRiver. Avulsion near the Lafourche avulsion site (; 308 N, Fig. 3, avulsionsite #3) led to the development of the modern Balize subdelta (Fig. 3).Historic development of the Atchafalaya River represents the earliest stagesof a fourth avulsion.

Fisk (1952) suggested a four-step evolution for the Atchafalaya River.Initially, the Mississippi and Red rivers flowed south along separate coursesand joined downstream of Old River (Fig. 4A). A westward migratingmeander of the Mississippi River, Turnbull Bend, captured the Red River(Fig. 4B). Crevassing along the western edge of the meander initiated thedevelopment of the Atchafalaya River in the Eighteenth Century (Fisk1952), and by 1765, the Atchafalaya River was well established (Fig. 4B).In 1831, an artificial cutoff (Shreve’s cutoff) across Turnbull Bend mini-mized flow between the Mississippi and Atchafalaya rivers (Fig. 4C). Sub-sequent dredging of Lower Old River between the 1880s and 1930s main-

tained flow between these two major waterways. Upper Old River filledwith sediment during this time, which led to the capture of the Red Riverby the Atchafalaya River and its continued enlargement (Fig. 4D). By 1950,the Atchafalaya was transporting ; 25% of the Mississippi River dis-charge, and the growth of the Atchafalaya River and its down-valley gra-dient advantage over the Mississippi made clear that capture was imminent(Fisk 1952). Ensuing construction of the Old River Control Structures bythe U.S. Army Corps of Engineers has, at least temporarily, arrested thisavulsion.

STUDY AREA AND METHODS

The Atchafalaya River flows south from Old River near Simmesport,Louisiana, through the Atchafalaya Basin to Atchafalaya Bay (Fig. 1). TheAtchafalaya Basin is located between alluvial ridges of the modern andTeche meander belts (Meander Belts 1 and 3 of Saucier and Snead 1989)(L.M. Smith et al. 1986) (Fig. 3). Late Holocene progradation of the La-fourche subdelta across the Teche subdelta impounded surface water andcreated a vast network of wetlands and lakes in the Atchafalaya Basin (Tyeand Coleman 1989). The uppermost reach of the Atchafalaya River is lev-eed and consists of a relatively straight channel that is up to 1 km wideand 30 m deep. Below the leveed reach, the Atchafalaya bifurcates into acomplex network of anastomosed channels that locally discharge into shal-low lakes. Within this zone of anastomosing channels, dredging and chan-


FIG. 5.—Satellite image (LANDSAT TM data) showing major fluvial landforms, active and abandoned courses of the Mississippi and Red rivers, and selected geologicunits of the floodplain in the Old River area. Hal 5 Holocene alluvium, Qp 5 Quaternary/Pleistocene undifferentiated, Qpb 5 Quaternary/Pleistocene braided-streamterrace. Location of Figure 6 is shown.

nelization locally maintains navigation routes through the lower basin. At-chafalaya flow and sediment discharge into Atchafalaya Bay has construct-ed the historic Wax Lake Delta and the Atchafalaya Delta (van Heerdenand Roberts 1988; Roberts 1998).

The floodplain near Old River consists of a combination of active andabandoned courses of the Mississippi, Red, and Atchafalaya rivers (Fig. 5).The Mississippi flows within Meander Belt 1 along the eastern side of thevalley (Saucier 1994), and is connected to the Atchafalaya by the artificialOutflow Channel, which is located immediately downstream (west) of theOld River Control Structures. Several Holocene Mississippi River coursesrepresenting older Meander Belts 2 and 3 of Saucier (1994) are locatedwest and northwest of the Atchafalaya River. The Red River flows north-east through Moncla Gap before joining the Atchafalaya River at Old River.Several Holocene courses of the Red River are present south of MonclaGap (Fig. 5).

Topographic maps, aerial photographs, and satellite imagery were ex-

amined to map major fluvial landforms including meander belts, point bars,natural levees, crevasse splays, and abandoned channels. On the basis ofthis mapping, a series of transects across the floodplain were selected alongwhich shallow cores (up to 13 m in length) were acquired using a Giddingshydraulic probe (Fig. 6). Approximately 30 cores totaling ; 300 m ofsection were examined. Cores were described and subsampled in the field.Logs of water wells and U.S. Army Corps of Engineers borings (up to 50m in length) were also used to characterize the regional alluvial stratigra-phy. Organic remains from core samples were archived, and five sampleswere submitted for radiocarbon dating (Table 1).

Topographic maps with five-foot (1.52 m) contour intervals were usedto calculate slope ratios (cross-valley to down-valley gradients). Cross-val-ley gradients were measured along the outer bend (west side) of majorMississippi River meanders in the vicinity of Old River. Gradients werecalculated by measuring from natural-levee crests to flood basin–naturallevee boundaries identified by decreases in slope and changes from silty to


FIG. 6.—Map showing the surficial geologynear Old River, Louisiana. MB 5 meander belt.Locations of cores and cross sections for Figures7–9 are shown.

TABLE 1.—Radiocarbon samples, dated material, 14C, and calibrated ages.

Lab No.Geomorphic

SettingMaterialSampled

SampleDepth(cm)

13C/12CRatio

14C Age1 Sigma

(14C yr B.P.)

Corrected andCalibrated Age

2 Sigma(cal yr B.P.) Interpretation

AA-264931

AA-264941

AA-264951

Beta-14735Beta-14736

Flood basinFlood basinNatural leveePoint barFlood basin

WoodWoodWoodWoodWood

260–280700720

1000260

225.6225.7228.6225.8227.7

845 6 502,165 6 55

845 6 50134.7 6 0.9% modern

110 6 40

910–6802340–2040910–680

0270–0

Min. age of Mississippi River meander belt 2Max. age of Mississippi River meander belt 2Max. age of Red River reoccupation of meander belt 2Modern Atchafalaya River point barModern Red River flood basin

1 Samples processed at the University of Arizona AMS Radiocarbon Lab. Other samples were analyzed by Beta Analytic Inc.

clayey soil types. Differential compaction of floodplain sediments was nottaken into account for the measurements of cross-valley gradient. Natural-levee or point-bar crest elevations were used to determine down-valley(channel-belt) gradients of the Mississippi and Atchafalaya rivers. Down-valley gradients are used as a proxy for the channel slope. Ratios of cross-valley to down-valley channel slope will be larger than the ratios of cross-valley to down-valley gradients used in this study because the MississippiRiver channel gradient is less than its valley gradient (i.e., its sinuosity isgreater than one).

ALLUVIAL ARCHITECTURE AND CHRONOLOGY

A series of cross sections across the floodplain south of Old River revealsfour major stratigraphic units (Figs. 6, 7). Brown silts and sands locatedalong banks of the Atchafalaya River represent historic point-bar and nat-ural-levee deposits. These deposits overlie or abut gray to blue-gray mottledmuds that range from 30 to 45 m in thickness. The fine-grained textureand stratigraphic position of these muds indicate that they represent Ho-

locene flood-basin deposits. A variety of sheet-like sand bodies are encasedin the flood-basin muds. Thin sheets consist of very fine to fine sand 2 to5 m in thickness, with a lateral extent in the range of hundreds of meters,based on the transects (Fig. 7) and studies north of Old River (Aslan andAutin 1999). Thick sheet sands are, by contrast, up to 2 km wide and 20m in thickness (W/T ; 100), and consist of fine to medium sand. TheAtchafalaya River incises thick sheet sands in the two southernmost crosssections described herein (Fig. 7B, C). On the basis of their sandy textureand geometry, the thin and thick sheet sands are interpreted as Holocenecrevasse-splay and channel-belt deposits, respectively. The basal unit con-sists of fine to medium sand and granule-size gravel, and underlies Holo-cene deposits throughout the LMV (Fisk 1944, 1947). Although the age ofthe unit is poorly known, it includes at least the late Pleistocene (Autin etal. 1991; Rittenour 2004).

Mississippi, Atchafalaya, and Red River deposits reveal important inter-fingering relationships (Fig. 8). Red River strata consist of red sands, silts,and clays that contrast strongly with the brown and gray deposits of the


FIG. 7.—A–C) Cross sections of the floodplainnear Old River showing downstream changes inalluvial stratigraphy and cross-sectional geometryof the Atchafalaya River south of MississippiRiver meander belt 2. Note the presence ofburied channel sands in (B and C) that areincised by the Atchafalaya River. Locations ofcross sections are shown in Figure 6. Data arefrom Fisk (1952).

Atchafalaya and Mississippi rivers. Red River deposits help define threestratigraphic units in the upper 15 m of the floodplain deposits: (1) historicAtchafalaya and Mississippi River sediments, (2) Red River deposits, and(3) basal late Holocene Mississippi River sediments.

Historic Atchafalaya and Mississippi River Sediments

The uppermost unit locally overlies Red River strata and consists of 0to 3 m of gray and brown silts and sands and gray muds (Fig. 8). Thenorthernmost transect shows that this unit is thickest within the flood basineast of the Atchafalaya River and thins towards natural levees of the At-chafalaya River and Mississippi River Meander Belt 2 (Fig. 8A). The unitthickens slightly to the southwest of Mississippi River Meander Belt 2 and

is several kilometers wide in the vicinity of the Atchafalaya River (Fig.8B). Geomorphic positions and textures suggest that these deposits repre-sent recent natural-levee, crevasse-splay, and flood-basin deposits of themodern Mississippi and Atchafalaya rivers.

Red River Deposits

The underlying unit consists of the red sands, silts, and clays of the RedRiver (Fig. 8). Red River deposits form a generally continuous, undulatinglayer that ranges from 0 to 3 m in thickness and extends laterally for upto 10 km. A Red River crevasse-splay complex is present at the floodplainsurface where the Atchafalaya River flows south from Mississippi River


FIG. 8.—A, B) Cross sections showing detailed floodplain stratigraphy in the Old River area. Historic Atchafalaya and Mississippi River deposits typically overlie RedRiver sediments. Red River deposits are underlain by Holocene Mississippi River channel, crevasse-splay, and flood-basin sediments that contain red mud interbeds ormottles of Red River origin. Locations of cross sections are shown in Figure 6.

Meander Belt 2 (Figs. 6, 8B). Two radiocarbon dates indicate that RedRiver strata here are younger than 900 14C yr B.P. (Figs. 8B, 9A).

Late Holocene Mississippi River Sediments

The basal unit is up to 10 m thick and consists of interfingering graysand and silt, and gray and blue mud with occasional red mud interbeds

or mottles (Fig. 8). The muds are interpreted as flood-basin sediments.Sheet sands that are 1 to 2 m thick and at least 1 km wide are crevasse-splay deposits. In some instances, these thin sheets pass laterally into len-ticular sands and silty sands that represent minor crevasse-splay channels(Fig. 8B). Major crevasse-splay channels are represented by sands that arelaterally restricted (, 2 km wide) and at least 5 m thick (Fig. 8B). Natural-levee deposits are wedge-shaped accumulations of silty sand and sandy silt


FIG. 9.—A) Stratigraphic section showingMississippi River meander belt 1 natural-leveesilts and sands overlying Red and MississippiRiver flood-basin muds. A radiocarbon date onplant remains indicates that Red Riverreoccupation of Mississippi River meander belt 2occurred after ; 900 14C yr B.P. Location ofcore shown in Figure 6. B) Stratigraphic sectionfrom the Moncla Gap area showing Red Rivernatural-levee sediment overlying Red andMississippi River flood-basin deposits. Aradiocarbon date on plant remains indicates thatthe Red River flowed through Moncla Gap nolater than ; 100 14C yr B.P. Holocenesediments overlie a paleosol and channel sand ofprobable Pleistocene age.

that are 1 to 4 m thick, and occur adjacent to Mississippi River meanderbelts. A single radiocarbon-age date from wood in flood-basin mud under-lying a natural levee of Mississippi River Meander Belt 2 suggests that thismeander belt is younger than ; 2,000 14C yr B.P. (Fig. 8A).

DOWN-VALLEY CHANGES IN AVULSION-DEPOSIT GEOMETRY

Stratigraphic data show important down-valley changes in the geometryand abundance of avulsion deposits related to historic development of theAtchafalaya River and the Mississippi–Atchafalaya diversion. Near OldRiver, historic Atchafalaya River sediments represent the incipient stage ofMississippi avulsion. The sediments are up to 3 m thick, consist of grayand brown silt and sand, and are confined to a trough , 2 km wide locatedbetween Mississippi River Meander Belt 3 (west) and crevasse-splay com-plexes of Meander Belts 1 and 2 (east) (Fig. 8B). Correlative depositslocated ;100 km down valley in the Atchafalaya Basin occupy ; 8,000km2, and overlie Red River sediments (Fig. 10). These historic Atchafalaya

River deposits are up to 60 km wide and 6 m thick, and they thin awayfrom the margins of anastomosed channels (Fig. 10A). Sediments depositedduring the incipient avulsion also thicken where they partially fill lakes.Atchafalaya Basin lakes were larger prior to the avulsion and have under-gone substantial filling since the 1880s (Tye and Coleman 1989; Roberts1998). These down-valley changes demonstrate the importance of preex-isting topography on avulsion-deposit geometry. Where alluvial ridges areclosely spaced and flood basins are shallow, avulsion deposits are thin andconstitute a small proportion of the floodplain deposits. In contrast, wherealluvial ridges are spaced far apart and flood basins are large, avulsiondeposits are thick and widespread.

FLOODPLAIN EVOLUTION

Geomorphic and stratigraphic data provide a basis for reconstructing thefloodplain history over the past 5 ky (Fig. 11). Initially, the MississippiRiver flowed along the western edge of the valley within Meander Belt 3


FIG. 10.—A) Map showing historicsedimentation (1932–1950) in the AtchafalayaBasin downstream of the Atchafalaya–Mississippi River diversion. Rapid enlargementof the Atchafalaya River during this timeresulted in significant floodplain sedimentaccumulation over an area of ; 4000 km2.Location of figure is shown in Figure 1. B)Cross section across the lower Atchafalaya Basinshowing the extent and thickness of historicAtchafalaya River avulsion deposits overlyingpre-Atchafalaya, Red and Mississippi Riversediments. Data compiled from Fisk (1952).

and the Red River joined the Mississippi south of Old River (Fig. 11A).Meander Belt 3 is thought to have originated ; 5000 yr B.P. (Saucier1981; Debusschere et al. 1989; Aslan and Autin 1999). By 2000 yr B.P.,the Mississippi River avulsed to the eastern side of the valley south of 318N and began to form Meander Belt 2 (Fig. 11B). Surficial mapping showsthat the avulsion site was just north of Old River (Saucier and Snead 1989).The Red River flowed in abandoned channels of Meander Belt 3 and fol-lowed a separate course to the Gulf of Mexico. By 900 yr B.P., the eastwardshift of the Mississippi River was completed by an avulsion near Vicks-burg, Mississippi, which led to the initial development of Meander Belt 1(Fig. 11C). Eastward shift of the Mississippi caused the Red River to avulsenortheastward through a small gap in the Pleistocene uplands. The RedRiver reoccupied an abandoned course of Mississippi River Meander Belt2 and joined the Mississippi immediately south of Old River. This RedRiver avulsion led to accumulation of red floodplain deposits near Old

River and the development of landforms such as the crevasse splay locatedsouth of Mississippi River Meander Belt 2 (Figs. 6, 11C). By ca. 1800A.D., the Red River avulsed again, this time flowing north through MonclaGap and joining the Mississippi River in Turnbull Bend (Fig. 11D). Oc-cupation of Moncla Gap by the Red River occurred recently, although theexact timing of this avulsion is controversial (Pearson 1986) (Fig. 9B). TheAtchafalaya River formed near this time. From Turnbull Bend, the At-chafalaya River flowed west through an abandoned channel of MississippiRiver Meander Belt 2, and then south through the abandoned Red Rivercrevasse-splay complex.

AVULSION THRESHOLD CONDITIONS AND TRIGGERS

Conditions for avulsion commonly include (1) rapid aggradation andproduction of cross-valley relief or (2) a decrease in channel capacity (Mak-


FIG. 12.—Longitudinal profiles of the Mississippi, Red, and Atchafalaya River floodplains and continental shelves. Note that the regional slope of the Atchafalaya Riveris slightly less than twice that of the Mississippi River.

←

FIG. 11.—Maps (A–D) showing floodplain development and avulsion history in the Old River area over the past ; 5,000 years. A) ; 5,000 14C yr B.P. The MississippiRiver flowed along the western margin of the valley within Meander Belt 3. The Red River joined the Mississippi near 308 309 N. B) ; 2,000 14C yr B.P. The MississippiRiver avulsed eastward below 318 159 N and established Meander Belt 2 while abandoning Meander Belt 3 south of Old River. The Red River continued to flow to theGulf of Mexico via the abandoned course of Meander Belt 3. C) ; 900 14C yr B.P. Mississippi River completed its shift to the eastern side of the valley by avulsingsouth of Vicksburg, Mississippi. This avulsion marks the inception of Meander Belt 1. Further south, southwestward progradation of the Lafourche subdelta across BayouTeche (abandoned course of Meander Belt 3) caused the Red River to avulse northeast and join the Mississippi River ; 318 009 N. The Red River reoccupied an abandonedchannel of Mississippi River Meander Belt 2 and a major Red River crevasse splay developed ; 318 009 N. D) ; 1800 A.D. The Red River avulsed again and flowednortheast through Moncla Gap, reoccupied segments of Meander Belts 2 and 3, and joined the Mississippi River in Turnbull Bend. These final changes set the stage forthe development of the Atchafalaya River.

aske 2001). In addition to these two major threshold conditions, there areother factors that influence avulsion, including neotectonics, subsidence,substrate composition, discharge variations, sinuosity changes, and humanactivities (Fisk 1952; Schumann 1989; Schumm et al. 1996; Jones andHarper 1998; N.D. Smith et al. 1998; Stouthamer and Berendsen 2000;Berendsen and Stouthamer 2002; Makaske et al. 2002). Analysis of theMississippi–Atchafalaya diversion suggests that Mississippi avulsion isstrongly influenced by (1) aggradation rate and gradient advantages, (2)substrate composition, and (3) active and abandoned floodplain channels.The latter two factors appear to be especially important in the case of theMississippi–Atchafalaya diversion.

Aggradation Rate and Gradient Advantages

Channel-belt aggradation and construction of local floodplain relief isthought to strongly influence avulsion frequency (Bridge and Leeder 1979;Bridge and Mackey 1993; Mackey and Bridge 1995; Mohrig et al. 2000;Tornqvist and Bridge 2002). Channel-belt aggradation can lead to avulsionby creating cross-valley slopes that exceed down-valley slopes. Modelingindicates that cross-valley to down-valley slope ratios of . 8 (Slingerlandand Smith 1998) or 3 to 5 (Tornqvist and Bridge 2002) are potentiallysignificant thresholds for avulsion occurrence. Local cross-valley gradientadvantages diminish because after a short distance water tends to flow downvalley with the dominant large-scale slope. Flow diversions lead to avulsiononly where an outlet for diverted flow exists (Tornqvist 1994; Stouthamerand Berendsen 2000; Stouthamer 2001; Berendsen and Stouthamer 2002).Where flow ponds such as in a closed flood-basin depression, water surface

slopes and any gradient advantages are reduced and avulsive channels can-not develop.

Regional floodplain gradients are steeper in the Atchafalaya Basin thanthose associated with the modern Mississippi River (Fig. 12). However,local gradient advantages, expressed by the slope ratio (ratio of cross-valleyto down-valley slopes), are highly variable in the vicinity of Old River,and range from ; 16 to 110 (Table 2, Fig. 1). Upstream of Old River,slope ratios are large (. 58) primarily because natural levees are generallynarrow (widths 5 1–2 km; Aslan and Autin 1998, 1999). At Lower OldRiver, the slope ratio is ; 35, which is generally similar to values down-valley (Table 2). The major exception is Bayou Latenache, which has aslope ratio of ; 16 because of the presence of a crevasse-splay complexand broad natural levee (; 2.9 km wide). All the Mississippi River sloperatios are generally greater than 30, and are substantially larger than thresh-old values presented by Slingerland and Smith (1998) and Tornqvist andBridge (2002).

Results of this gradient analysis demonstrate that significant local gra-dient advantages exist along the outer bend of virtually every meander ofthe modern meander belt, and yet Mississippi avulsions are rare. For ex-ample, three major crevasse-splay channels (bayous Latenache, Fordoche,and Grosse Tete and Blue) originate along the apex of Mississippi Rivermeanders located down-valley of Old River (Fig. 13). The channels flowsouthwest into the Atchafalaya Basin, and local cross-valley gradients andslope ratios are generally similar to those near Old River (Table 2). How-ever, none of these crevasse channels led to Mississippi avulsion. Giventhe rarity of Mississippi River avulsion, these observations suggest that


FIG. 13.—Map showing the distribution ofmajor crevasse channels south of Old River andthe Atchafalaya–Mississippi River diversion.Modified from L.M. Smith et al. (1986).

TABLE 2.—Summary of cross-valley and down-valley gradients.

Location Latitude

LeveeHeight

(m)

LeveeWidth

(m)

Cross-valleySlope(Scv)1

Down-valleySlope(Sdv)2

SlopeRatio

(Scv/Sdv)3

Deer ParkBayou CocodrieBlack HawkLower Old RiverBayou Latenache

318259318159318089318009308529

2.14.03.02.41.8

1006969930

19002896

0.0020870.0041280.0032260.0012630.000622

0.000035740.000037530.000038640.000036330.00003781

58.41109.99

83.4834.7716.44

Bayou FordocheBayou Grosse Tete & BlueSterling Bayou CrevasseBayou PlaquemineBayou Goula

308459308389308379308179308139

3.02.14.64.63.0

31611850411544012841

0.0009490.0011350.0011180.0010450.001056

0.000030860.000032370.000033070.000027500.00002315

30.7535.0733.8038.0145.61

McCallBayou LafourcheVacherieHymelia CrevasseDavis Crevasse

308069308059308029308019298559

3.03.03.03.02.1

43342457360321851667

0.0006920.0012210.0008330.0013730.001260

0.000024510.000031250.000021780.000024370.00001817

28.2439.0738.2356.3469.33

1 Scv 5 Cross-valley slope.2 Sdv 5 Down-valley slope. This value represents the channel-belt slope.3 Scv/Sdv 5 Ratio of cross-valley slope to the down-valley slope.

factors other than gradient advantages must play more important roles inavulsion.

Substrate Composition

Another critical factor that influences avulsion is substrate composition(Fisk 1952; Stouthamer and Berendsen 2000; Makaske et al. 2002). Win-keley (1977) provides several excellent examples of the resistance of flood-basin mud to channel scour and the difficulty of completing MississippiRiver articial cutoffs within fine-grained floodplain deposits. Winkeley(1977) also notes that dredged channels floored by resistant flood-basinmuds typically filled with sediment during low-water conditions, whichhelps explain why an avulsion is so difficult in areas underlain by fine-grained sediments. Major crevasse channels located south of Old River,

such as Latenache and Fordoche, did not capture the Mississippi River,probably because of the presence of widespread and thick flood-basin mud.Substrates such as peat can also inhibit avulsion. For example, abundantpeat in the lower delta plain of the Rhine–Meuse system and Orinoco Deltaresists fluvial erosion and avulsive channel development (Stouthamer andBerendsen 2000; Aslan et al. 2003).

Stratigraphic relationships in the Old River area suggest that the presenceof erodible subsurface sands facilitated the development of the AtchafalayaRiver (Figs. 6, 7). The Atchafalaya River is remarkably straight where itexits the abandoned channel of Mississippi River Meander Belt 2 and flowssouth (Figs. 6). This straight reach of the river probably reflects local steepgradients and the presence of flood-basin mud (Fig. 7A). Downstream,however, the river begins to incise shallow sands and flows within severallarge meanders (Figs. 6, 7B, C). Erodible sand likely contributed to rapidenlargement of the Atchafalaya River and increased its capacity to divertflow from the Mississippi. N.D. Smith et al. (1998) noted that reoccupationof channels with sandy fills facilitated the most recent avulsion of theSaskatchewan River. Recognition of stacked sand bodies both in modernand ancient fluvial settings provides further evidence that erodible sandysubstrates play a major role in avulsion (Maizels 1990; Kraus 1996; Krausand Gwinn 1997; Makaske et al. 2002).

Floodplain Channels

Another poorly documented factor that influences avulsion are the effectsof active and abandoned floodplain channels. In the case of the Mississippi–Atchafalaya diversion, the Red River played a pivotal role in the growthof the Atchafalaya River (Fisk 1952). Capture of the Red River allowedthe Atchafalaya River to enlarge its channel and maintain seasonal low-water flow and sediment transport.

The other key impact of floodplain channels is that they provide aneffective and ready-made means for transporting water and sediment di-verted from the mainstem channel and developing avulsive channels. The


significance of abandoned channels is readily apparent in the Mississippi–Atchafalaya diversion. The incipient Atchafalaya River flowed westthrough an abandoned segment of Mississippi River Meander Belt 2, whichitself had been reoccupied by the Red River prior to the development ofthe Atchafalaya River (Figs. 5, 6). The Atchafalaya River then exited theabandoned channel of Meander Belt 2 and flowed south through an aban-doned crevasse-splay complex of the Red River.

The Red River also avulsed by reoccupying abandoned floodplain chan-nels (Fig. 5). Avulsion of the Red River northeast from the mouth of theRed River Valley was accomplished by reoccupation of Mississippi RiverMeander Belt 2. Reoccupation and subsequent overbank deposition by theRed River produced a blanket of red-colored natural-levee sediments insetagainst older gray and brown Mississippi River deposits (USDA 1986).The most recent avulsion of the Red River also involved reoccupation ofseveral channel segments of older Mississippi River channel belts includingMoncla Gap, a probable late Pleistocene Mississippi River course (Fig. 5).

Avulsion by channel reoccupation has been noted widely (Brizga andFinlayson 1990; N.D. Smith et al. 1998; Aslan and Blum 1999; Morozovaand Smith 1999, 2000; Stouthamer 2001; Aslan et al. 2003). However, thesignificance of channel reoccupation to avulsion may be greater than gen-erally recognized. Incipient avulsion by the Mississippi River into the At-chafalaya Basin is primarily an example of avulsion by diversion into aflood basin (sensu Aslan and Blum 1999 and Morozova and Smith 1999).However, the uppermost reaches of the Atchafalaya River developedthrough channel reoccupation. The observation that a single Mississippiavulsion can involve different processes and avulsion styles is strikinglysimilar to the most recent Saskatchewan River avulsion (N.D. Smith et al.1998). In their example, the avulsion originated with a crevasse-splay chan-nel reoccupying an abandoned Saskatchewan channel filled with significantquantities of erodible sand. The avulsive channel enlarged and flow con-tinued downstream within an abandoned channel of a Saskatchewan trib-utary before eventually bifurcating into a series of crevasse-splay com-plexes (N.D. Smith et al. 1998).

SUMMARY

The widely held view that avulsion is controlled primarily by gradientadvantages is misleading. Mississippi River floodplain data show that gra-dient advantages exist at many locations and yet avulsions are rare. Whilegradient advantages are necessary for avulsion, this study suggests thatother factors play critical roles. Analysis of the Mississippi–Atchafalayadiversion demonstrates that erodible substrate and active and abandonedfloodplain channels were key to the success of this incipient avulsion, andsuggests that these factors may play more important roles in avulsion thangenerally recognized. While it is also clear that human activities such asdredging impacted growth of the Atchafalaya River, we conclude that avul-sion would have occurred regardless of human intervention.

It is widely known that channel belts in fluvial–deltaic settings are ele-vated. While we have not completed the type of gradient analysis presentedhere for other systems, we speculate that similar local gradient advantagesare pervasive in fluvial–deltaic systems of continental margins as well asin other areas where elevated channel belts have been described (e.g., Mak-aske et al. 2002). Furthermore, the gradient advantages along the lowerMississippi River floodplain significantly exceed threshold values for avul-sion based on numerical models. This observation raises the possibility thatthe Mississippi River, as well as rivers from similar fluvial–deltaic systems,is continuously poised for avulsion. If this condition is true, a critical ques-tion concerning avulsion is: why do rivers such as the Mississippi notavulse more often? We suggest that future field and modeling studies in-corporate additional information on pre-avulsion floodplain topography anddrainage network configurations to evaluate avulsion causes. Channel-sand-stone stacking patterns in ancient fluvial deposits have been used to identifyavulsion events (e.g., Kraus and Gwinn 1997; Makaske et al. 2002), and

avulsion by channel-belt reoccupation could explain many examples ofmulti-storied sheet sandstones. Future stratigraphic studies in ancient flood-plain settings should be conducted to further evaluate the significance offloodplain substrate and channels on avulsion.

CONCLUSIONS

1. Over the past 5 ky, the Mississippi River avulsed four times and theRed River avulsed twice in the southern LMV. Relocation of the Missis-sippi River eastward shifted local base level and led to Red River avulsion.

2. Mississippi and Red River avulsion occurred primarily through chan-nel reoccupation. Incipient avulsion by the Mississippi River into the At-chafalaya Basin involved channel reoccupation in its uppermost reachesand crevasse-splay and lacustrine-delta progradation downvalley. Accu-mulation of avulsion deposits in the lower Atchafalaya Basin over the past; 200 years has resulted in a sedimentary unit that is up to 6 m thick andcovers ; 8,000 km2.

3. Floodplain stratigraphy shows that crevassing occurred for severalthousand years prior to development of the Atchafalaya River. This long-term crevassing probably contributed to avulsion by depositing erodiblesand and forming abandoned channels. Similar observations have beenmade for historic avulsion of the Niobrara River (Ethridge et al. 1999). Inthis sense, prehistoric crevasse-splay events can be viewed as the earlieststage of the Mississippi–Atchafalaya diversion, which implies that condi-tions leading to an avulsion may develop over thousands of years.

4. While gradient advantages are required for avulsion, they do not nec-essarily lead to frequent avulsion. Gradient advantages along the Missis-sippi River floodplain are widespread. Ratios of cross-valley to down-valleyslope range from ; 16 to 100 and are ; 35 at Old River. These valuesexceed avulsion threshold values suggested by numerical models, yet avul-sion is rare in the LMV. This observation suggests that numerical modelsneed to place less emphasis on gradient advantages and focus on otherfactors such as floodplain topography and stratigraphy.

5. The Mississippi–Atchafalaya diversion was strongly influenced byshallow erodible sand, the presence of abandoned floodplain channels, dis-charge from the Red River, and human activities. Future studies need totake a broader approach to studying avulsion and should include moreinformation on the effects of substrate composition, abandoned-channeldistributions, and floodplain channel networks.

ACKNOWLEDGMENTS

The authors thank the Louisiana Geological Survey and the U.S. Geological Sur-vey Statemap (New Roads, Jackson–Natchez) mapping projects. Reviews and help-ful suggestions by Henk Berendsen, Simon Fagan, and JSR editor Colin North sig-nificantly contributed to the content and clarity of the manuscript. The PetroleumResearch Fund of the American Chemical Society funded this research.

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Received 27 May 2004; accepted 16 December 2004.

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