INTERACTION BETWEEN WOODY DEBRIS AND CHANNEL MORPHOLOGY: INSIGHTS FROM SPRING-FED STREAMS IN THE OREGON CASCADES

Woody debris in streams influences stream flow and channel morphology at both the local and reach scale. In order to develop a better understanding of the interactions between wood in rivers, channel hydraulics, and channel morphology we measured the characteristics of wood in a set of streams in the Oregon Cascades. We consider two classes of streams: runoff-dominated streams in which discharge varies by several orders of magnitude throughout the year, and spring-fed streams in which discharge is nearly constant. In our study we considered the relationship between 1) the size, shape, position and orientation of wood, 2) the characteristics of wood (relative amount of branches, presence of a root wad), 3) the geometry of the channel, and 4) the stream's discharge. Most of the characteristics of wood orientation can be understood by the ability of streams to orient and/or transport wood. For example, we find that as bankfull discharge increases isolated pieces of wood are more likely to be oriented parallel to the channel. Most streams exhibit a monotonically increasing width as bankfull discharge increases. For the subset of spring-fed streams we considered that have randomly-oriented large woody debris, width increases as discharge increases. Compared with runoff-dominated streams that have the same bankfull discharge, the constant discharge spring-fed streams have much wider channels because in runoff-dominated streams wood is oriented or transported by discharges much large than bankfull. The wider spring-fed channels can thus be explained by the stable wood in the river diverting flowing water around the wood, which in turn enhances bank erosion. The resulting increase in channel width will further stabilize the wood because as the channel width increases, water velocity decreases.

This hypothesis is consistent with our observation that the largest spring-fed streams are able to orient large woody debris, and their channels are not as wide as we would expect based on an extrapolation of the width-discharge relationship for smaller spring-fed streams. Because the mobility of wood and the channel shape are coupled, changes in woody debris loading (for example by fire, bark-beetle infestation, or the addition and removal of wood for habitat restoration) will affect both the dynamics of wood and the morphology of the stream.