Storing Sediment in a Coastal Plain Valley Plug: Obion Creek Stream Restoration

A primary assumption in the design of stream restorations is that the sediment supplied to the restoration reach should be transported out of the restoration reach. In watersheds that have been disturbed by human activity, however, the supply gravel and sand that form bedload-material load can be excessive, necessitating that the channel be designed to produce high boundary stresses sufficient to maintain transport. A more practical approach to restoration design would be the gross management of coarse and fine-grained sediments through either removal of the excess sediment sources or storage of the supplied sediment. This sediment management concept was employed in the restoration design for Obion Creek, located in the western Kentucky region of the Coastal Plain. Channelization of most streams in the Obion Creek watershed led to severe channel incision in most tributaries and aggradation of the Obion Creek channel bed at other locations. Upstream channels had incised, mobilizing the loess, sand, and gravel that composed the surrounding hills. The channelized reach eventually filled with sediment and debris, forcing water to flow overland in heavily braided patterns for nearly two miles before entering a recognizably active silt-bed channel. This existing valley plug was incorporated into the restoration design, allowing the channel to accumulate gravel and coarse sand and to splay fine sand and silt on a wide floodplain. To transport silt to the downstream channel, historic channels capable of transporting only silt were connected to the restored channel and reactivated. In the case of Obion Creek, prior modifications of the channel network and changes in hydrology had caused changes in the sediment regime. By incorporating sediment storage into the design, the restoration was able to manage the load while still supplying enough silt to maintain the downstream channel.