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

This investigation explores the closely related ideas of low-flow at-a-station hydraulic geometry and resistance to flow of small, steep mountain streams. Thirteen reaches in five tributaries of Chilliwack River, British Columbia are examined. The data suggest that power functions well describe the relationships. Changes in the mean cross-section velocity account for the majority of the changing discharge; 31 of the 61 cross-sections have velocity exponents that are greater than the water-surface width and mean-depth exponents combined. The hydraulic geometry is highly variable between cross-sections in a reach, between reaches, and between streams. Resistance to flow in these mountainous settings is high and strongly dependent on stage. In the data, Darcy-Weisbach resistance factor varies over five orders of magnitude and Manning's n varies over three orders of magnitude. Despite this extreme range, both the power equations (at a cross-section) and Keulegan equation (for reach-averaged values) well describe the relationships.