Relationships between hydraulic variables and bedload transport in a subalpine channel, Colorado Rocky Mountains, U.S.A.

This study used three cross-sections along a subalpine channel to examine associations between flow and sand and gravel bedload transport in a cobble-bed channel. At each section, velocity was measured at 0.2, 0.4, 0.6, and 0.8 of flow depth for ten increments across the section. At each increment, velocity measurements were immediately followed by 10-min bedload samples with a 7.6 cm Helley-Smith sampler. Linear regressions between bedload and flow variables suggest that grain shear stress, stream power, near-bed velocity, average velocity, and the Froude number are the most relevant indicators of bedload transport, particularly when regressed against the Dmax or D84 of the bedload grain-size distribution. No significant relationships, however, were constant at all three cross-sections. Inconsistency in relationships between bedload discharge and hydraulic variables is at least in part the result of the extreme temporal and spatial variability present in East St. Louis Creek. Photographs and observations made throughout the field season suggest the importance of random events, such as tree-fall and log-jam formation, in regulating bedload discharge. The data from East St. Louis Creek suggest equal mobility for sand and gravel transport.