Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2001
Publication Date: N/A
Interpretive Summary: Debris dams composed primarily of trees from failed banks have formed in downstream reaches of the Yalobusha River System several times in the last 100 years, causing severe flooding problems in the vicinity of Calhoun City, Mississippi. The most recent debris dam was formed following channel dredging in the 1960's. In upstream reaches, however, streambed erosion is occurring as a response to the 1960's channel work. As part of the Corps of Engineers plan to alleviate flooding problems there is concern that bank failures in eroding reaches will deliver additional trees to the river and either worsen the existing debris dam or initiate another. Results of research described in this paper provide estimates of the annual rate and volume of woody debris that can be expected to enter the river system as a result of failing banks and, therefore, points to critical erosion zones where mitigation measures would be useful.
Technical Abstract: The Yalobusha River network in central Mississippi has twice been destabilized by channel straightening for flood defense and land drainage, most recently in 1967. System-wide rejuvenation has followed through a series of upstream migrating knickpoints that have caused mass failure of streambanks and the recruitment of large volumes of trees to the channel. Large woody debris (LWD) recruitment is maximized at the transition between stage III and stage IV channels, focusing attention on 11 sites in the network. The sites are upstream of knickzones ranging from 2.2-5.4 m high and migrating at rates of 0-13.8 myr-1. Riparian conditions in 500 m2 plots on each bank upstream of the knickpoints range from treeless to forested, containing 0-98 trees with an average diameter at breast height of 0.18 m and average maximum height of 14.0 m. The average volume of wood on each bank is 0.02 m3m-2. Bank stability analyses suggest that the channels will widen in amounts ranging from 1.8- 31.5 m. Combined with the knickpoint migration rates, riparian land losses are estimated to range from 8.0-434 myr-1, resulting in the recruitment of almost 28 m3 of wood (or 100 trees) annually from the 11 sites. Assuming this LWD recruitment rate, a model is developed for the potential for debris dam initiation and growth, based on the ratio of tree height to channel width under current and post-knickpoint conditions, the annual delivery of 'large' trees and the annual total of LWD recruitment by volume.