CHANNEL MORPHOLOGY AND WOODY DEBRIS ARCHITECTURE IN MOUNTAIN STREAMS REFLECT DIFFERENCES IN HYDROLOGIC REGIME

The amount and arrangement of wood in river channels reflect the bioclimatic setting and flow history of the channel. Yet the degree to which differences in the flow regime, which refers to the temporal pattern in streamflow over time, lead to differences in the spatial arrangement of woody debris is unclear. To better understand how the flow regime controls the type and architecture of woody debris, and to understand how wood arrangement relates to channel morphology, we collected detailed wood and geomorphic surveys in mountain rivers with similar drainage areas but different flow regimes, including both spring-fed and snowmelt-runoff streams. Flows in spring-dominated streams are near constant and vary by less than a factor of 2 over the course of the year, rarely exceeding bankfull. In contrast, flow in highly variable runoff-dominated streams can vary by as much as two orders of magnitude. Study sites were located in the Oregon Cascades, and had comparable riparian forest compositions, logging histories, climates, and geologies.

We found that the flow regime does control the arrangement of woody debris in mountain stream channels, and can lead to important differences in channel morphology. Wood in stable, spring-fed streams is largely immobile (83%), found as a single piece (67%) or as part of an open-fabric log jam (33%), and is near perpendicular to the channel, indicating little movement since emplacement. In a real sense, wood becomes part of the channel and represents the dominant structural feature, accounting for a large fraction of total flow resistance and form drag. Consequently, spring-fed streams tend to have higher width to depth ratios and finer streambeds. In contrast, the majority of wood in runoff-dominated streams is mobile (83%), has stripped bark (86%), occurs in densely-packed log jams (84%), and is found parallel or sub-parallel to the channel, all indicators of frequent fluvial transport. In these systems, wood moves through the channel quickly and is a much smaller component of total resistance. Our findings suggest that, in some streams, a change in the presence or frequency of wood-mobilizing flow, due to land use or climate change, could have a significant impact on channel morphology and streambed stability.