65 Geomorphology of Stream Restoration and Natural Stream Design

Saturday, 27 March 2004
8:00 AM - 12:30 PM

CONTRASTING RIVER RESTORATION STRATEGIES IN WEST TENNESSEE: DECOMMISSIONING HUNDREDS OF KILOMETERS OF LARGE FAILING DRAINAGE CANALS

Paper number: 65-9
Program: THEME SESSIONS
Time: 10:20 AM – 10:40 AM

Authors

Abstract

Western TN rivers have a long, complex, and aggressive management history driven by the desire to maximize agricultural and urban land use via large flood-control canals. Between 1961 and 1977 the ACOE channelized nearly every river in western TN and northern Mississippi. Channelization was stopped in TN by court order against the federal government. Since that time, much of the channel system has gone without maintenance because permits are not forthcoming for environmentally unsound watershed management. Very high discharge of non-point source suspended sediment is one impact from the decaying canal system, with one study showing a channelized river with 15 times the annual area-normalized suspended sediment of a reference non-channelized river.

A State and Federal task force spent $2 million designing a 32 km long demonstration restoration project for the Middle Fork Forked Deer River. The design included a dual channel system with a large, straight, central canal crisscrossed several times by a diminutive meandering “bankfull” channel. This compromise design was promoted as a model that answers both environmental, and flood control, concerns. The design was abandoned following the very critical evaluation we submitted to the state in 1998. Our analysis exposed egregious geomorphic, hydrologic, hydraulic, and sediment-transport flaws. No further restoration projects have been implemented and the drainage canals have continued a complex, sometimes rapid, decay.

We have used west TN as a large-scale laboratory where we can observe and interpret the post-channelization evolution of the landscape. Cypress Creek (McNairy Co.) stands out as an example of river self-restoration. Cypress Creek is a valley plug where sediment and large woody debris are rapidly filling the drainage canal. Our data show that the head of the plug is migrating upstream at a rate of about 160 m/a. At the head, the canal avulses onto the floodplain leaving a broad crevasse splay of sand. Both low-flow and flood water flow down the floodplain for approximately 2.5 km before reentering the canal. In that span, self-restoration includes new channels, reoccupied old channels, and broad wetland development where suspended sediment is deposited. Over 150 other canals are filling and exhibit promising early signs of self-restoration.