Paper No. 10
Presentation Time: 10:35 AM
Wood Debris and Depositional Landforms
Wood debris is an important element in the sediment dynamics of many forest rivers. The deposition of wood debris in fluvial environments can create unique depositional landforms and increase the effective sediment storage within a river valley. Any river with riparian forests is subject to wood recruitment and the potential formation of channel altering logjams. Alluvial channels with large riparian trees are particularly susceptible to the formation of snags that become embedded in the substrate and go on to form trap other debris. Logjams create significant roughness and flow obstructions that alter patterns of flow and sedimentation in rivers, resulting in distinctive patterns in a channel's profile and planform. Surveys of natural logjams in Washington State indicate that logjams can elevate water elevations and the channel bed more than 4 meters, results that are similar to observations elsewhere in the world. Where logjams temporarily impound a channel they create depositional landforms with gradients less than the unobstructed river. Channel gradient adjusts again when the river finds a way around the obstruction. Overtime the result can be a floodplain mosaic of oblique, intersecting surfaces. Logjams also create patches of stable ground within otherwise dynamic channel migration zones. Radiocarbon and tree dating indicates that buried logjams in steep gravel bedded rivers can last for over a 1000 years and much longer in lower gradient systems. The persistence of buried logjams allows the overlying ground to continue to accumulate over-bank sediment while surrounding areas are reset by bank erosion. The result is patches of mature forest floodplain elevated above the primary floodplain. Channel and riparian clearing associated with human development has significantly diminished the widespread occurrence of logjams and their influence on the landscape. Recognizing the extent to which logjams can alter river morphology provides insight into interpreting current and geologic fluvial landscapes.