COMPLEX CHANNEL EVOLUTION IN WEST TENNESSEE AND NORTHERN MISSISSIPPI

Watersheds of western Tennessee and northern Mississippi provide a significant opportunity to study a multi-decadal, large-scale watershed response to historic channelization. A recent inventory of drainage canals shows that at least 150 reaches in the region are characterized by extreme aggradation and a complex evolution not documented in channel evolution models (CEMs) published for the region. Valley plugs typically form when excess sediment from the upper watershed or channelized tributaries locally fills a low-gradient canal. Examples in Tennessee illustrating the variability of post-channelization evolution include Cypress Creek near Ramer, South Fork Obion River at Jarrell Bottoms, and Stokes Creek. Cypress Creek underwent channel blockage and avulsion followed by a complex channel evolution sequence including erosion of new channels, reoccupation of historic meanders, and continued occupation of non-occluded canal reaches. The head of the valley plug is currently migrating up-valley at approximately 30 m/yr. South Fork Obion underwent channel blockage, valley-wide ponding, valley aggradation, and valley-wide conversion from forest to emergent communities followed by a complex channel/floodplain evolution involving new channel incision, and reoccupation of old meanders and non-occluded canal reaches. The South Fork Obion plug was formed by sediment from a channelized tributary, whereas the head of the Cypress Creek plug is far from a tributary mouth. Stokes Creek avulsed where a relatively steep reach of the canal lost grade and capacity upon entering the nearly flat floodplain of a larger river. The Stokes Creek valley plug includes a network of small braided and anastamosing channels developed upon a sandy alluvial fan. The many aggradational reaches documented in the region share several characteristics, notably, the canal is filled or buried by sediment; the system locally develops multiple channels; and, in some cases, valley plugs foster the downstream development of stable channels by locally storing excess bedload via floodplain aggradation. Existing CEMs do not include plugs, nor predict the time at which sediment will fill an aggrading channel. Plugs can form at any time following channelization in response to myriad and stochastic causes.