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 t he Colorado Rocky Mountains. Mean annual peak discharges of approximat ely 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 suspen ded and bedload sediment movement, and discharge, although sediment mo vement was highly variable at equivalent discharges. Correlations betw een sediment movement and velocity were not significant. Sediment entr ainment relations suggest that present peak flows generate bed shear s tress 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 eithe r a censored layer (which does not require mobilization of the coarse fabric), or a pavement, from which only a few coarse particles are ent rained at any instant. It is hypothesized that East St. Louis Creek ha s 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 different iate incipient motion, or that cross-sectional flow characteristics ar e not as important as reach-scale controls on sediment movement. Bedlo ad appears to come primarily from valley-bottom and in-channel sources , particularly when the bed is disturbed by the movement of woody debr is. Woody debris in the channel traps and stores bedload and acts as a major local control on temporal and spatial patterns of bedload movem ent.