HYDRAULIC GEOMETRY, MEDIAN GRAIN SIZE, AND STREAM POWER IN SMALL MOUNTAIN STREAMS

Few answers exist as to how hydraulic geometries, grain size, and stream power are related to one another in mountain streams. We examine these variables and their relationships using data obtained from 157 reaches in 31 mountain streams draining <10 km² and located in West Virginia, USA. The high gradient streams transport coarse sediment derived from resistant sandstones. We combine channel surveys, pebble counts, and drainage areas obtained from digital elevation models in a single dataset and analyze variables using log bin-averaging. As described by classic hydraulic geometry equations, channel widths increase as drainage area (A) increases and reach gradients (S) decrease with increasing drainage area. However, median grain size (D₅₀) fluctuates around a single value when plotted against drainage area. The lack of correlation between D₅₀ and drainage area stands in contrast to a positive, log-linear correlation between unit stream power and D₅₀. Fluctuations of unit stream power reflect systemic changes of hydraulic geometries, including gradient. The relationship of these variables to one another can be highlighted by using Hack's empirical formula, S = c'(D₅₀ / A)^C where c' and C are regression coefficients. Using our dataset, a strong log-linear relationship exists between gradient and D₅₀ / A, which we attribute to mutual adjustments of S and D₅₀. These adjustments are made independent of substrate resistance; unit stream power is not a function of underlying geologic formations or channel morphology. We did not observe evidence of debris flows. Collectively, our results support the hypothesis that D₅₀ and unit stream power are mutually adjusted within a system that is controlled by fluvial, as opposed to debris flow, processes. The power relationships between hydraulic geometry, D₅₀, and unit stream power suggest that modeling of mountainous, but stream-dominated landscapes may be possible using relatively simple rules.