Effects of large woody debris removal on physical characteristics of a sand‐bed river

Abstract

Abstract1. Removal of large woody debris (LWD) is one of the most widely practised stream alterations, particularly in sand‐bed rivers of the south‐eastern USA. Selective removal of LWD has been proposed as an alternative to orthodox non‐selective clearing in order to conserve ecological resources, but methods for comparing hydraulic and environmental effects of selective and non‐selective removal have not been developed. Conservation of stream habitats requires quantification of LWD removal impacts on physical habitat.2. Physical characteristics of straightened, sand‐bed reaches of the South Fork Obion River in western Tennessee, USA that were rich in LWD were compared with those in similar reaches where debris had recently been removed using selective removal guidelines.3. The mean volume of LWD per unit water volume was 0.0545 in the uncleared reaches, but nearly 60% lower (0.0225) in the cleared reach.4. A simple technique for predicting hydraulic roughness in channels with varying amounts of LWD was developed. Hydraulic roughness, as measured by the Darcy‐Weisbach friction factor, was about 400% greater in uncleared reaches at base flow but declined to a level about 35% greater than for the cleared reaches at higher flows. Predicted friction factors were within 35% of measured friction factors at higher flows.5. Physical habitat diversity in this channelized sand‐bed stream was strongly related to the density of LWD. Flow conditions in the uncleared reaches were more heterogeneous than in the cleared reach, especially at low flow. At low flow, uncleared reaches tended to be shallower, have lower velocities, slightly finer bed material, and more heterogeneous conditions overall. Shannon indices based on depth and velocity were an average of 48% higher in uncleared reaches.6. Bed sediments underneath and immediately adjacent to LWD formations were finer and contained more organic matter than sediments distant from LWD. However, when all bed samples were considered, organic content was positively correlated with median grain size.