USE OF 10BE TO DEDUCE VARIATIONS IN SEDIMENT SUPPLY FROM THE FRONT RANGE TO THE HIGH PLAINS, WITH IMPLICATIONS FOR GENERATION OF FILL AND STRATH TERRACES

Sediment production on hillslopes affects the supply of sediment to the fluvial systems with which they are coupled. Aggradation or erosion of alluvium occurs when sediment supply is not equally matched with stream transport. In the Colorado Front Range, basin-averaged denudation rates, calculated from ¹⁰Be concentrations in stream sediments, indicate that small basins are producing sediment at a rate of ~2.5 cm/ka (Dethier and Lazarus, 2006). This is consistent with average sediment production rates of ~3.1 cm/ka that we calculate using ¹⁰Be from hillslope sites within the Boulder Creek CZO.

However, fluvial features indicate past sediment production in source basins of the Front Range was higher. Remnants of in-set fill terraces exist within steep mountain canyons, indicating alluvial sediments were deposited and evacuated multiple times as sediment supply fluctuated. ¹⁰Be data from LGM fill terraces in Boulder Creek indicate post-LGM alluvial evacuation rates of ~50 cm/ka (Schildgen et al., 2002). New OSL data from a 14 ka terrace in the adjacent, unglaciated Lefthand Creek also indicate rapid alluvial evacuation rates of LGM sediment of on the order of ~65 cm/ka. These data strongly suggest that rivers draining both glaciated and nonglaciated basins in the Front Range experienced large and rapid fluctuations in the sediment supply.

Outboard of the Front Range, broad gravel-capped strath terraces also suggest long periods of lateral planation associated ample sediment supply, punctuated by brief periods of vertical incision and strath abandonment. A suite of strath terrace ages and elevations above the Lefthand Creek (95 ka at 74 m; ~91 ka at 24 m, and preliminary age of <50 ka at 15 m) indicate rapid bedrock incision from terrace to terrace. ¹⁰Be inheritance in depth-profiles on these surfaces suggests that basin-averaged paleodenudation rates at the time of deposition were higher than modern rates, up to 5.5 cm/ka. Modern denudation rates from canyon mouths of several Front Range streams are in progress, as well as OSL and CRN dating of the lowest terrace in this suite. We argue that these data support a model in which basin-specific oscillations in sediment supply drive both fluvial aggradation and evacuation in the canyons, and also associated cycles of strath cutting and abandonment on the High Plains.