Richard H. Kesel

Channel migration and meander-bend curvature in the lower Mississippi River prior to major human modification

Channel migration and meander-bend morphology are examined for the lower Mississippi River between 1877 and 1924, prior to channel cutoffs, revetments, and change in sediment regime. The spatial pattern of meander-bend migration coincides with differences in flood-plain deposits. Migration of meander bends averaged 45.2 m/yr in the upper alluvial valley, where there are numerous clay plugs, but increased to 59.1 m/yr in the lower alluvial valley, where there are fewer clay plugs in contact with the channel. The highest migration rates occurred with meander bends having a curvature, r m / W m (ratio between meander-bend radius to channel width) between 1.0 and 2.0, which is a departure from previous models. Results from this study suggest that rivers with complex flood-plain deposits exhibit patterns and relationships that deviate from models derived in homogeneous flood-plain deposits.

The Decline in the Suspended Load of the Lower Mississippi River and its Influence on Adjacent Wetlands

Since 1850, there has been an overall decrease in excess of 70 percent in the suspended load transported by the Lower Mississippi River. A decrease of 25 percent between the earliest measurements and 1950 may be partly the result of a decline in discharge and partly the result of a change in land use practices. The largest decrease occurred in 1952–53 following construction of major main-stem reservoirs on the Missouri River. Similar construction on the Arkansas River has resulted in a further decrease in 1962–63. The decrease in suspended load, combined with the artificial levee construction program and the overall enhancement of the river channel for navigation has been accompanied by an accelerating decline in land area of the Louisiana coastal zone from 17 km2/yr in 1913 to 102 km2/yr in 1980.

Human modifications to the sediment regime of the Lower Mississippi River flood plain

The Mississippi River is one of the most regulated rivers in the world. Human modifications constructed mainly after 1920 include dams and reservoirs, artificial levees, dikes, concrete revetments and a series of channel cutoffs. This paper examines some of the effects of these modifications on the channel and sediment budget of the river. In particular, the changes to the thalweg profile and the size of channel bars are examined in detail. It is concluded, that prior to the 1930s, when major modifications were introduced, the Lower Mississippi River was an aggrading meandering river. The role of the flood plain has also changed. Prior to modifications, the flood plain was the major sediment source as the result of bank caving. Today the flood plain provides only a minor amount of sediment. It can be shown that major degradation to the channel including the growth of channel bars has occurred as a result of these engineered modifications. The data also indicates that the different geomorphic regions respond to modifications in different ways.

Lateral Erosion and Overbank Deposition on the Mississippi River in Louisiana Caused by 1973 Flooding

In 1973, the Mississippi River was out of its banks for more than 2 mo from April to June. The thickness and the texture of overbank sediments were determined for a number of depositional environments on the Mississippi River flood plain in Louisiana. Average sediment thickness ranged from 53 cm along the natural levee to 1.1 cm in the back-swamp. The texture of deposits varied from natural levee sediments with 68 percent sand to backswamp sediments with 97 percent silt and clay. The amount of lateral erosion of bordering Pleistocene bluffs increased 271 percent over the previous 9 yr of record because of the flood. Using the rate of backswamp deposition during the 1973 flood, it is suggested that backswamp deposits in this area required 10,000 to 11,000 yr to accumulate.

The role of the Mississippi River in wetland loss in southeastern Louisiana, U.S.A.

The suspended load of the Lower Mississippi River has decreased almost 80 percent since 1850. The long-term suspended sediment record can be loosely subdivided into three phases: a historic interval prior to 1900, a predam period (1930–1952) and a postdam period (1963–1982). The suspended load decreased 43 percent from the historic to the predam period and 51 percent from the predam to the postdam period. The decreases in suspended load after 1952 coincide with the construction of reservoirs and dams on the Missouri and Arkansas rivers. Earlier decreases may be the result of changes in land use measurement practices. The decrease in suspended load and the elimination of overbank flow by the construction of artificial levees are considered to be major causes of coastal wetland loss in southeastern Louisiana. During the historic period sediment accumulation of the marsh surface was greater than the rate of water level rise. During the pre and postdam periods, the rate of water level rise exceeded sediment accretion on the marsh surface. Although the elimination of overbank sediment clearly exacerbated the wetlands loss, an accelerated rate of water level rise during the past 25 years has been a dominant factor. Based on estimates of available overbank sediment, it is suggested that the most viable management strategy for the wetlands would be the diversion of sediment into selected areas where the land loss is most critical.

Some Effects of Human Modifications on Sand-Bed Channels in Southwestern Mississippi, U.S.A.

This study examines the effects of land-use changes and channel foreshortening on the sediment regime and channel morphology of two sand-bed rivers in southwestern Mississippi. Historical data including sequences of channel surveys at various bridge sites document the impact of these human modifications. Land-use changes caused an increase in point bar size, but this trend was found to be reversed as the result of conservation and reforestation practices. Channel foreshortening has produced a knickpoint that has migrated 50 km upriver. River response to this disturbance included 5 m of channel incision, a 4.5-fold increase in bankfull width, and a fourfold increase in point bar size.

Geomorphologic relationships and ages of soils on alluvial fans in the Rio General Valley, Costa Rica

Abstract Soil-geomorphic relationships on alluvial fans in the Rio General Valley, Costa Rica, show that the sequence of alluvial fan surfaces and soil development on them is related chiefly to uplift of the bordering mountain ranges and not significantly to climatic change. The oldest and most strongly developed soils are found only on the highest surfaces; successively younger and less-developed soils have lower positions. Oxisols characterize the oldest surfaces, Alfisols the intermediate ones, and Entisols the youngest surfaces. Minimum ages of these surfaces are estimated, from the extrapolation of sedimentation rates, solum development rates, and the rate of channel incision based on two radiocarbon dates, to range from about 45,000 to 65,000 years for the oldest, to approximately 100 years for the most recent surface. Pedons representative of the soils on the principal alluvial surfaces are described in terms of the physical, chemical, and mineralogic characteristic. We propose that the soils studied are a chrono-catena that represents soils development through time for this region, and can be used to establish a minimum time scale by which to guage stage and rate of soil development in a humid tropical environment.

BLUFF EROSION OF A MISSISSIPPI RIVER MEANDER AT PORT HUDSON, LOUISIANA

The rate of bluff erosion has been monitored and related processes examined on a Mississippi River meander bend at Port Hudson, Louisiana, from 1971 to mid-1980. The erosion data were analyzed using a stepwise multiple regression analysis to determine the conditions controlling the amount and distribution of bluff erosion. The results indicate a pattern of decreasing bluff erosion that is attributed to downstream migration of the zone of maximum basal scour. Bluff retreat in the period of active basal undercutting (1971 to mid-1975) was 18.9m/year and in the following period of decreased basal scour activity (mid-1975 to mid-1980) was 6.8m/year. Approximately, 61% of the erosion was associated with rising river levels. The data also indicated that 66% of the erosion occurred at bankfull discharge stage or above. Conditions that produce bluff erosion were found to vary between the two eriosion periods and also as river levels rose or fell in each of the periods. During the actively undercutting period, pre...

Tidal creek and pan formation in intertidal salt marshes, Nigg Bay, Scotland

Abstract Existing theories of creek and pan formation in intertidal salt marshes emphasise intertidal micro‐topography and resultant variation in vegetal colonisation as the basic factors controlling their pattern on the mature salt marsh. Morphological studies on certain Scottish marshes suggest that collapse of parts of the mature marsh surface results from sub‐surface piping phenomena. The existing theories of creek and pan formation are reviewed and an entirely new theory of formation is advanced.

Overbank sedimentation from the historic A.D. 2011 flood along the Lower Mississippi River, USA

In this study, we document sedimentary characteristics of overbank flood deposits associated with the epic A.D. 2011 flood along the Lower Mississippi River (southern USA) and directly compare the findings to sedimentation from a comparable flood event in 1973, with the general purpose of understanding how extreme floods contribute to floodplain depositional patterns and accretion rates of embanked fluvial systems. The thicknesses of the 2011 flood deposits averaged 138 mm along natural levee crests, 9 mm on meander scrolls, and 3 mm in backswamps. These thicknesses are considerably less than those documented for the 1973 flood, sampled at the same locations. We contend that less sedimentation in 2011 occurred because the flood was not supplied with much upstream sediment from the Missouri River. Further, the 2011 sediments are coarser than in 1973, indicating that the higher 2011 flood levels were associated with more energetic overbank flows that flushed fine-grained sediments downstream within the narrow embanked floodplain corridor. The largest recorded flood in North American history is only marginally preserved in the embanked floodplain stratigraphy of the alluvial valley of Earth’s third largest fluvial system.