Low-flow hydraulic geometry of small, steep mountain streams in southwest British Columbia

This investigation explores the at-a-station hydraulic geometry (AHG) of small, steep mountain streams at low discharge. Thirteen reaches in five tributaries of Chilliwack River, British Columbia, ranging in size from 12 to 77 km2 are examined. The resulting data set is composed of eight to twelve measurements of water-surface width, mean depth, and mean velocity at each of 61 cross sections or 625 unique combinations of the three variables. Mean velocity in a given cross section responds most rapidly to changing discharge, and 31 of the 61 cross sections have velocity exponents that are greater than the water-surface width and mean-depth exponents combined. The velocity exponent (m) averages 0.51, while the mean water-surface width exponent (b) and mean-depth exponent (f) average 0.20 and 0.29, respectively. Somewhat surprisingly, the AHG of steep mountain streams can be reasonably predicted from just a few measurements of the primary flow variables and stream discharge. While conditions at the cross section appear predictable from a few measurements, extrapolating the results from one cross section to another in the same reach involves large errors. The section-to-section variability of the exponents and coefficients, even when they are located in similar channel units such as riffles, prevents accurate extrapolation to unmeasured cross sections.