Morphological indicators of downstream transitions in headwater channels
Chris Brummer
Ph.D., Department of Earth and Space Sciences
The transition from a dominance of hillslope processes to a dominance of fluvial processes is a fundamental transition in mountain channel networks. Although identification of this transition is important for understanding controls on the disturbance ecology of mountain streams, the routing of sediment through mountain channel networks, and ultimately landscape evolution, very few geomorphological studies have addressed the nature of this transition. Field surveys of headwater channel properties in a variety of physiographic and geological contexts are needed to address and differentiate the generalizable and location-specific controls on headwater stream morphology and processes, and to develop defensible criteria to delineate the transition from headwater colluvial channels to supply-limited reaches loosely termed "transport reaches."
Surveys of headwater channels are being conducted in watersheds located both on the west-slope of the northern Cascades and on the Hoh River of the Olympic Peninsula, where complementary data were collected in the 1990's. Results of field surveys will be used to investigate the relation of three morphological characteristics of headwaters channels to the downstream transition from colluvial to fluvial channels: (1) downstream coarsening; (2) the hydraulic geometry relation between channel width and drainage area; and (3) total and unit stream power.
Substantial debate occurs in the fluvial geomorphology community over the relative importance of selective transport and abrasion on patterns of downstream fining observed in most rivers. However, preliminary data from prior field surveys, and from other studies suggest systematic downstream coarsening in headwater channels that runs counter to conventional models of fluvial geomorphology. Hence, the transition from downstream coarsening to downstream fining can be hypothesized to represent the transition from hillslope dominance to fluvial dominance on channel form.
Downstream changes in channel width are widely considered to vary with the square root of drainage area in both alluvial and bedrock channels. However, some workers have reported that such relations do not necessarily hold for small headwater channels. The transition from an externally forced, local control on channel width to the more systematic downstream variation expected from classical hydraulic geometry relations can be hypothesized to represent the transition from hillslope to fluvial dominance.
Theoretical models of rivers-system evolution often assume stream power and/or unit stream power (stream power per unit channel width) to be uniform along a river profile. Although mountain channels do not exhibit the uniform drainage area-slope scaling expected based on this assumption, downstream patterns of stream power have not been systematically documented in mountain channel systems. Preliminary data from previous studies suggest that stream power increases downstream in headwater channels before attaining a decreasing trend in lower-gradient fluvial channels.
This project is investigating the hypothesis that all three of these characteristics are indicators of transition from hillslope-dominated to fluvial-dominated channels. For this hypothesis to be correct, all three of these characteristics should exhibit transitions at a similar drainage area, which is anticipated to be about one to several square kilometers.