SEDIMENT AND GEOMORPHIC CHARACTERISTICS OF THE ANOMALOUS STRAIGHT REACH OF THE BIG BLACK RIVER, MISSISSIPPI

An anomalous straight reach (ASR) exists in the Big Black River (BBR) near Edwards, MS where the BBR makes an abrupt change from its general southwesterly flow to a west-northwest flow for about 6.5 km, before returning to its meandering southwesterly flow. Aerial photo, map and field reconnaissance reveal ridge and swale topography and multiple terrace surfaces along the ASR. Six hand augered holes were sampled to a depth of approximately 5 meters. Four holes were sampled south of the ASR along a bearing parallel to the river and two oxbows/meander scars were sampled north of the ASR. Optical Stimulated Luminescence and Radiocarbon dating confirm migration of the BBR along the ASR westward during the late Pleistocene and Holocene. Sediment textural characteristics show the older terrace dominated by mud-sized material with values ranging from 0.07 to 7.65 percent sand. The younger terrace varies between 9 and 73 percent sand. There is no variation in color between the terraces or the oxbows. Organic carbon reaches a maximum content of 6.5 percent in an oxbow sample and averages 1.6 percent for all samples. The younger terrace samples have the least variation in magnetic susceptibility. In the study area the three sample locations on the older terrace show more rapid sedimentation rates at higher elevations. Generally sedimentation rates become slower as the floodplain surface aggrades above an active channel. The irregularity in sedimentation rate may illustrate the complex interaction between fluvial and eolian processes in this loess dominated landscape.