River restoration strategies in channelized, low-gradient landscapes of West Tennessee, USA

West Tennessee has a complex history of watershed disturbance, including agricultural erosion, channelization, accelerated valley sedimentation, and the removal and reestablishment of beaver. Watershed management has evolved from floodplain drainage via pervasive channelization to include local drainage canal maintenance and local river restoration. Many unmaintained canals are undergoing excessive aggradation and complex channel evolution driven by upland erosion and low valley gradient.

The locus of aggradation in fully occluded canals (valley plugs) moves up-valley as sediment continues to accumulate in the backwater behind the plug. Valley plugs that cause canal avulsion can lead to redevelopment of meandering channels in less disturbed areas of the floodplain, in a process of passive self-restoration. Some valley plugs have brought restored floodplain function, reoccupation of extant historic river channels, and formation of a “sediment shadow” that protects downstream reaches from excess sedimentation. Despite the presence of numerous opportunities, there is presently no mechanism for including valley plugs in mitigation projects.

In 1997 a survey of 14 reference reach cross sections documented relations between drainage area and bankfull geometry of relatively unmodified streams in West Tennessee. Reassessment of seven of those sites in 2007 showed that one had been dammed by beaver and that two sites could not be analyzed further because of significant vertical or lateral instability. In contrast to other regions of North America, the results suggest that stream channels in this region flood more frequently than once each year, and can remain out of banks for several weeks each year.