BASIN-SCALE CONTROLS ON CHANNEL MORPHOLOGY, DEBRIS FLOW DISTURBANCE, AND THE SPATIAL EXTENT OF SALMON IN STEEP MOUNTAIN STREAMS

Steepness and concavity indexes derived from the relationship between drainage area and channel slope provide a process-based characterization of river profiles and a useful context for identifying basins that express different reach morphologies, fish habitat capacity, and responses to episodic disturbance. Strongly concave profiles that develop in steep terrain indicate that almost all of the relief in the drainage network occurs in small headwater streams. In these basins a large proportion of the drainage network has low-gradient morphologies, such as pool-riffle sequences, which provide favorable rearing habitat for many salmonid species. Complex population structures can develop within these networks because fish distribution expands into the tributaries, allowing for a spatial spreading of risk that may enhance a population's ability to persist during adverse conditions for survival and growth. The severity of pulse disturbances is also reduced because debris flows typically form discrete deposits where steep tributaries abruptly encounter low-gradient mainstem channels at tributary junctions. In contrast, less concave profiles in steep terrain indicate that the spatial extent of high gradient reaches morphologies, such as step-pool and cascade sequences, are more extensive. The potential to develop complex population structures in these basins is diminished because most tributaries are too steep to provide habitat, confining fish to mainstem channels. Furthermore, the change in slope at tributary junctions is less pronounced and debris flows rarely form discrete deposits. Instead, these mass flows continue to travel down steep mainstem channels and alter aquatic and riparian habitats for long distances. The combined influence of a limited spatial distribution and the increased severity of debris flows may result in more extreme fluctuations in population abundance because they are less resilient to pulse disturbances.