Changes in woody debris and bed material texture in a gravel-bed channel

This study investigated the response of bed material texture under two distinct periods of log jam conditions in a 900-m study reach of Carnation Creek, Vancouver Island, BC. A field program extending over seven flood seasons, coupled with existing streamflow records, channel maps, and low-altitude aerial photography, provides observations of jam stability and permeability, surface and subsurface grain-size, as well as bed material fluxes, sediment supply calibre, streambed stability, and sediment exchange near jams. All jams functioned as partial dams throughout the field program, but the overall perturbation of sediment transfers decreased when two jams were breached. Bed material texture exhibited the largest differences over the study reach during a period when log jams exerted a stronger influence over the passage of water and sediment. At individual gravel bars, most adjustments in surface and subsurface grain-size constituted less than a 20% change in percentile values. Larger changes occurred primarily in subsurface sediment. Temporal patterns in bed material texture exhibited variation between bars, but downstream of breached jams, surface sediment conformed to the expected fining trend, while the subsurface sediment increased in size. Surface and subsurface sediments evolved in the same direction only 64% of the time, and the different magnitudes of adjustment caused local armor ratios to vary over time. Results highlight that the temporal dynamism of large woody debris (LWD) should be coupled with spatial considerations for effective stream restoration and management of streambed sediment. (C) 2003 Elsevier B.V. All rights reserved.