RIVER RESTORATION STRATEGIES WITHIN THE REGIONALLY CHANNELIZED, LOW-GRADIENT LANDSCAPE OF WESTERN TENNESSEE, USA

Before European settlement, western TN was a landscape rich with sinuous rivers coursing down wide, hardwood-forested valley bottoms en route to the Mississippi River. This region is now one of the most hydraulically altered landscapes in North America with hundreds of km of drainage canals excavated in the last century to support farming and urban development. Legal proceedings against the Army Corps of Engineers catalyzed an end to the era of rapid, large-scale channelization in the 1970's, leaving a legacy of severely disturbed landscapes.

Many canals are not currently maintained. These canals are currently evolving in response to ambient hydrology, elevated sediment supply, and their landscape/geologic context. At least 150 reaches in the region are experiencing extreme aggradation and complex evolution forced by uncontrolled upland erosion and low valley gradient. These aggrading reaches (valley plugs) grow up-valley in response to the backwater they produce. In many areas the flow of water and sediment are further complicated by locally-constructed networks of ad hoc smaller levees and berms that criss-cross the wide floodplains. Further, broad, poorly drained railroad and highway grades locally cause both increased erosion in the central canal, and backwater conditions far up-valley. Lastly, in westernmost TN, some rivers have incised deep, steep-walled canyons through weakly consolidated, thick loess deposits.

It is in this challenging context that reach-scale, natural channel restoration is currently being attempted, chiefly to mitigate for wetland and river impairments elsewhere in the State. Although opportunities for successful reach restoration exist, the practice is risky in watersheds with high sediment yield and low gradient valleys.

An underappreciated alternative restoration strategy is the conservation of regions with valley plugs, where post-channelization self-restoration is taking place. The end result of extreme aggradation in the plugged valley is an occluded canal and avulsion into remnant sinuous channels or creation of new sinuous channels, within a floodplain that once again supports bottomland hardwood forest. The resulting channel/floodplain systems are self-formed and self-maintained. The State mitigation program currently has no mechanism to conserve land.