60 Quaternary Geology/Geomorphology II

Tuesday, 6 November 2001
8:00 AM - 12:00 PM

AN EXAMPLE OF POST-CHANNELIZATION “SELF-RESTORATION:” CYPRESS CREEK, MCNAIRY CO., TN

Paper number: 60-14
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Cypress Creek was once a low-gradient (0.0001), sinuous (k=1.9), sand-bed river with a broad (2 km), forested floodplain and a drainage area of 380 sq. km. Beginning early in the 20th century, channelization and straightening of Cypress Creek and many of its tributaries allowed agricultural and urban encroachment on the Cypress Creek floodplain. It is likely that both incised tributaries and poor watershed management practices resulted in the voluminous sand load and large woody debris that has periodically blocked the Cypress Creek canal. New natural resource policies have eliminated large-scale canal dredging and clearing as a legal option for maintaining Cypress Creek canal, but little has been done to stem the excess sediment shed from the landscape. In 1999 a 2.1 km long debris jam (38,000 cubic meters) forced most of the flow from the canal onto the right floodplain through a well-developed crevasse splay system. After leaving the canal, water flowed approximately 750 m laterally across the floodplain in numerous distributary channels toward the topographic low of the valley. The water then turned to flow down the valley axis, alternately reoccupying the historic Cypress Creek meanders and flowing in new, naturally-formed channels.

Although there has been no human effort to restore this disturbed landscape, the new riverine system has naturally evolved to have many of the characteristics of a “restored” river. A key component of the functioning system is sediment management. The excess sand transported onto the right floodplain from the blocked canal is deposited in a large crevasse splay. The crevasse splay acts as a natural “sediment basin” that is likely significant for the stability of the natural channels located downstream. Some of the finer sediment settles out in relative calm water that ponds as the water turns from lateral flow to axial flow at the toe of the crevasse splay. The water exiting the 5.6 km long “self-restored” reach is free of sand and has improved clarity compared to the water entering the reach.

The head of the canal blockage migrates upstream an average 230 m/a. In the absence of appropriate sediment management in the watershed, the crevasse to the right floodplain will likely be blocked in the future, thus forcing the system to flow onto the left floodplain, abandoning the “self-restored” reach.