CONTROLS ON BEDLOAD MOVEMENT IN A SUB-ALPINE STREAM OF THE COLORADO ROCKY-MOUNTAINS, USA

East St. Louis Creek drains 8 km2 of alpine and subalpine terrain in the Colorado Rocky Mountains. Mean annual peak discharges of approximately 0.6 m3 s-1 occur during the summer, when the majority of the sand- to cobble-size bedload is transported along the steep channel. Repeat sampling of depth, velocity, and suspended and bedload sediment at 12 cross sections indicated significant correlations between both suspended and bedload sediment movement, and discharge, although sediment movement was highly variable at equivalent discharges. Correlations between sediment movement and velocity were not significant. Sediment entrainment relations suggest that present peak flows generate bed shear stress values close to critical threshold values for the D84 of the bed-surface layer. The majority of the sampled bedload was finer grained than the bed sediments. This may be explained by the presence of either a censored layer (which does not require mobilization of the coarse fabric), or a pavement, from which only a few coarse particles are entrained at any instant. It is hypothesized that East St. Louis Creek has a pavement. The lack of correlation between shear stress values and sediment movement indicates either that the use of hydraulic variables averaged across the entire cross section is insufficient to differentiate incipient motion, or that cross-sectional flow characteristics are not as important as reach-scale controls on sediment movement. Bedload appears to come primarily from valley-bottom and in-channel sources, particularly when the bed is disturbed by the movement of woody debris. Woody debris in the channel traps and stores bedload and acts as a major local control on temporal and spatial patterns of bedload movement.