Yazoo Basin

The combination of Pleistocene alluvial terraces and modern (Holocene) floodplain features and depositional patterns has resulted in distinctive landforms that have been mapped in considerable detail throughout the valley. Within the Yazoo Basin, these landforms are categorized as valley trains, backswamps, point bars, abandoned channels, abandoned courses, and natural levees (Kolb et al. 1968, Saucier 1994). Each of these landforms is discussed and illustrated in the following paragraphs.

a. Valley trains.
Valley trains are Pleistocene glacial outwash deposits from the Mississippi and Ohio Rivers, with surface features that reflect braided-stream depositional regimes. Although they make up about 54 percent of the Mississippi Alluvial Valley as a whole, they are of limited extent in the Yazoo Basin, where they have been largely eroded away by lateral channel migration or buried by deep sediments during recent (Holocene) times. The remnant valley train landscapes that occur in the northeastern and west-central part of the basin are evidently late Wisconsin in age, meaning they are at least 11,000 years old, but their precise age is not known. The topstratum of valley train deposits is a 1.5- to 3-m-thick layer of predominantly fine-grained material that forms a continuous blanket across the relict braided channels and interfluves but does not obscure their presence. The topstratum may include materials laid down during waning stages of glacial outwash deposition, loess, and slackwater overbank deposits from later Mississippi River meander belts. The buried channel systems on valley trains differ from abandoned channels within the Mississippi River meander belts in that the valley train channels tend to be filled with coarse sediments (massive sands) below the surface veneer of finegrained material, whereas more recent channels are typically filled with fine-grained material throughout.

b. Backswamps.
Backswamps are flat, poorly drained areas bounded by uplands and/or other features such as natural levees. In the Yazoo Basin, they are commonly found between the various past and present meander belts of the Mississippi River or adjacent to the valley wall. Backswamp environments are underlain by coarse glacial outwash, but surface deposits are fine-grained sediments that were slowly deposited in slackwater conditions. Thus, under unmodified conditions, backswamps characteristically have substrates of massive clays and are incompletely drained by small, sometimes anastomosing, streams. They may include large areas that do not fully drain through channel systems but remain ponded well into the growing season. In much of the Mississippi Alluvial Valley, backswamp deposits are 12 m thick or more, although they tend to be somewhat thinner in the south-central portion of the Yazoo Basin. Note that sites mapped as valley train and backswamp have essentially the same sequence of deep, coarse glacial outwash overlain by finegrained slackwater deposits. The basis for separating them as map units is the thickness of the fine-grained deposits; they are mapped as backswamp where the surface deposits are sufficiently thick to obscure the braided channel pattern on the valley train surface. On valley trains, surface deposits (other than those from historic erosion) are typically older and thinner and occupy better-drained landscape positions than similar fine-grained deposits of backswamps.

c. Point bars.
Point bar deposits predominate within the Yazoo Basin. They generally consist of relatively coarse-grained materials (silts and sands) laid down on the inside (convex) bend of a migrating stream channel. The rate at which point bar deposition occurs and the height and width of individual deposits vary with sediment supply, flood stage, and other factors. The result is a characteristic topography of low arcuate ridges separated by swales. Point bar swales range from narrow and shallow to broad and deep and usually are closed at each end to form depressions. The scale and depth of point bar swales depend on the depositional environment that formed the adjacent ridges and the degree of sedimentation within the swale since it formed.

d. Abandoned channels.
These features are the result of cutoffs, where a stream abandons a channel segment either because flood flows have scoured out a point bar swale and created a new main channel (chute cutoff) or because migrating bendways intersect and channel flow moves through the neck (neck cutoff). Chute cutoffs tend to be relatively small and to fill rapidly with sediment. They do not typically form lakes, but may persist as large depressions. The typical sequence of events following a neck cutoff (which is much more common than a chute cutoff) is that the upper and lower ends of the abandoned channel segment quickly fill with coarse sediments, creating an open oxbow lake. Usually, small connecting channels (batture channels) maintain a connection between the river and the lake, at least at high river stages; so river-borne fine-grained sediments gradually fill the abandoned channel segment. If this process is not interrupted, the lake eventually fills completely, the result being an arcuate swath of cohesive, impermeable clays within a better-drained point bar deposit. Often, however, the river migrates away from the channel segment and the hydraulic connection is lost, or the connection is interrupted by later deposition of point bar or natural levee deposits. In either case, the filling process is dramatically slowed, and abandoned channel segments may persist as open lakes or depressions of various depths and dimensions.

e. Abandoned courses.
An abandoned course is a stream channel segment left behind when a stream diverts flow to a new meander belt. Abandoned course segments can be hundreds of miles long, or only short segments may remain where the original course has been largely obliterated by subsequent stream activity. There are a variety of possible fates for abandoned courses. In some cases, they are captured by smaller streams, which meander within the former channel and develop their own point bars and other features. Within the Yazoo Basin, much of the Tallahatchie and Yazoo Rivers, and portions of many smaller streams, flow within abandoned courses of the Mississippi River. Where the stream course is abandoned gradually, the remnant stream may fill the former channel with point bar deposits even as its flow declines. Thus, while abandoned channels often become depressions with heavy soils, abandoned courses are more likely to be fairly continuous with the point bar deposits of the original stream or to become part of the meander belt of a smaller stream.

f. Natural levees.
A natural levee forms where overbank flows result in deposition of relatively coarse sediments (sand and silt) adjacent to the stream channel. The material is deposited as a continuous sheet that thins with distance from the stream, resulting in a relatively high ridge along the bankline and a gradual backslope that becomes progressively more fine-grained with distance from the channel. Along the modern Mississippi River, natural levees rise about 4.5 m above the elevation of the adjacent floodplain and may extend for several kilometers or more from the channel. Natural levees formed by smaller streams or over short periods of time tend to be proportionately smaller, but the dimensions and composition of natural levee deposits are the product of various factors, including sediment sources and the specific mode of deposition. Natural levees may be deposited in association with sheetflow or as a series of crevasse splays, which are deltaic deposits formed by small channels that breach the existing natural levee during high flows. A different type of crevasse splay occurs where man-made levees have been breached during major floods. These splays have an irregular, hummocky surface, and are composed of very coarse sediments, may be very extensive. They are the result of very high-velocity flows, because the initial levee break releases water that has a surface much higher than the adjacent land surface. Often, the point at which the levee failed is marked by a deep scour pool, commonly called a "blue hole."

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Web Date: October 1997
Updated: April 2008