PROCESSES AFFECTING SPATIAL VARIABILITY IN SEDIMENT SIZE AT CHALK CREEK, COLORADO

Spatial variability in fluvial sediment size is thought to be influenced by three processes: hillslope sediment supply, abrasion of sediment, and size-selective transport. However, isolating the effects of any one of these mechanisms in the field can be challenging. We take advantage of a natural experiment at Chalk Creek, Colorado, where hydrothermally altered granite is delivered to the Creek at a point source by debris flows. Characterizing the fluvial grain-size distribution at sites upstream and downstream of the debris flow-creek confluence, combined with size-distribution data from the debris-flow material itself, allows discrimination between the three mechanisms driving downstream changes in sediment size. We measured the intermediate axes of 200 grains at each of eight cross sections along the channel and on the debris flow fan. Upstream of the debris flow-creek confluence, the median and maximum grain sizes are 32 mm and 280 mm, respectively, compared with 64 mm and 320 mm downstream of the debris fan. The median and maximum grain sizes of the debris flow deposits are 9.5 mm and 380 mm, respectively. Our preliminary results indicate that highly heterogeneous grain sizes delivered by debris-flow in combination with size-selective transport control sediment size in the few tens of meters downstream of the confluence, whereas abrasion dominates further downstream. Using well-controlled natural experiments to constrain controls on sediment size is important for developing, calibrating, and evaluating models of long-term landscape evolution.