Fluvial geomorphological analysis of the recruitment of large woody debris in the Yalobusha River network, Central Mississippi, USA

The management of large woody debris (LWD) should be based on a rational assessment of its recruitment rate relative to its natural decay and removal. LWD recruitment may be controlled by ‘natural’ episodic terrestrial factors or by in-channel geomorphological controls related to the rate of bank erosion. The geomorphological controls are hard to quantify in laterally migrating channels, but in incising channels, a conceptual model may be developed based on the density of riparian trees relative to the knickpoint migration rate and bank stability analyses that predict the post-knickpoint width of the channel. The Yalobusha river network in Central Mississippi, USA, has twice been destabilised by channel straightening for flood defence and land drainage, most recently in 1967. System-wide rejuvenation has followed through a series of upstream migrating knickpoints several metres high that have caused mass failure of streambanks and the recruitment of large volumes of trees to the channel. LWD recruitment is maximised 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 between 2.2 and 5.4 m high and migrating at rates of 0–13.8 m year−1, based on 23–30 months of monitoring. 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 m3 m−2. Under rapid drawdown conditions, bank stability analyses suggest that the channels will widen in amounts ranging from 1.8 to 31.5 m. Combined with the knickpoint migration rates, riparian land losses are estimated to range from 8.0 to 433.8 m year−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 in situ 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. A longer-term model is also developed, based on ‘knickpoint severity’ and vegetation density in upstream and headwater riparian zones of each tributary. The 11 study sites are classified into groups with similar LWD management concerns based on these analyses. The models developed in this research provide the first precise quantification of LWD recruitment according to geomorphological controls and standing vegetation, and a rational assessment of its meaning, but further research is required to improve the accuracy of such estimates.