EVALUATING THE DEPENDENCE OF SEDIMENT CALIBER ON EROSION RATE IN A COASTAL MOUNTAIN RANGE, CENTRAL CALIFORNIA

In tectonically active mountain ranges, the adjustment of landscapes is driven by interactions among differential rock uplift, climatically modulated weathering and erosion, and the erodibility of bedrock. Although the grain size of sediment is understood to be a first-order control on hillslope transport and channel incision, the relative influences of climate and rock erodibility make it difficult to isolate the relationship between erosion rate and grain size of sediment. Here, we evaluate the relationships between hillslope morphology, grain-size distributions of sediment in first-order channels, and channel profile steepness in a region of uniform lithology and climate, but subject to differential rock uplift. A series of first-order watersheds draining the western flank of the Bolinas Ridge, in the Coast Ranges of central California, exhibit systematic spatial differences in landscape relief that are interpreted to reflect tilting and differential uplift along the ridge. Analysis of high-resolution elevation data reveals that both channel steepness and the curvature of ridgecrests along interfluves vary systematically along strike. These observations imply that both hillslopes and channels are adjusting to spatial differences in erosion rate. Measurements of the size distributions of sediment on the channel beds reveal that watersheds with steeper channels and higher curvature ridgelines are characterized by coarser median size fractions of bed sediment and wider variance in grain-size distributions. These observations confirm notional expectations of coarser sediment delivery to channels in regions of higher erosion rate. We are currently evaluating models of channel adjustment that incorporate a threshold for sediment transport to assess the impact of sediment-size on channel profile form.