Channel geometry, median grain size, and stream power in small mountain streams

Small mountain streams are typically affected by the same variables as larger counterparts, but small headwater channels are also coupled with hillslopes. The added degree of system complexity limits extension of existing conceptual and quantitative models of stream response to small mountain streams. Our objective is to determine how the presence of multiple lithologies on channel beds and coarse sediments affect relationships between channel geometry and driving and resisting variables in small mountain streams when the variables are examined at the landscape-scale. We test for systematic scaling of channel geometry, stream power indices, and median bed grain size (D50) in headwater streams draining < 10 km2 and overlying multiple strata on a passive continental margin. Field data were collected from 157 reaches in 32 watersheds. Local strata can be divided into a thick sequence of resistant sandstones and a thick sequence of low resistance, calcareous siltstones and shales with interbedded limestones. Both stratigraphic units are present in every watershed and all streams carry abundant, coarse bed load derived from the resistant unit. Mean widths and gradients of the channels are correlated with upstream drainage area. Unit stream power is not statistically different as a function of rock unit, but channel gradients are lower atop the low resistance unit. D50 is not correlated with drainage area, but is positively correlated with unit stream power. The correlation between unit stream power and D50 is evidence for mutual adjustments among gradients, widths, and D50 as a function of the primary driving force in the drainage area, discharge. The small effect of bed lithology on stream processes and strong relationship between channel geometry, unit stream power, and D50 implies that available driving forces and grain size are the primary controls on stream processes in the study area. We conclude that future research into system response in this and similar settings should focus on sediment characteristics, production, and flux.