Boulder Creek drains 1160 km2 from the Continental Divide in the Front Range of Colorado to its confluence with South St. Vrain River, and spans elevations from 1480 to 4120 m (Fig 1). The upper half of the watershed is a mountainous landscape of crystalline rocks, while the lower half crosses the piedmont and is underlain by sedimentary rocks and lined with alluvial terraces. The project will focus on the mountain portion, carved in Precambrian (1.7 by) granodiorite and older biotite gneiss (Lovering & Goddard, 1950), where three distinct erosion regimes arise (Birkeland et al., 2003). At the crest, the watershed was glaciated. U-shaped valleys, cirques (a few still with ice), and rock-dominated valley floors are found. Cosmogenic radionuclide (CRN) exposure ages on glacially polished bedrock in the valley floor document that glacial erosion removed the previous cosmogenic production layer, and that glaciers retreated from their maximum position at ~17ka to the divide between 14-12 ka (Ward et al., 2006). East of the glacial limit is a broad, high (2500-2750 m) post-Laramide surface of low relief (Bradley, 1987), in which weathered rock profiles are up to 15 m thick (Isherwood & Street, 1976; Dethier & Lazarus, 2005). Still further east, renewed bedrock channel incision over the last 5 My has progressed headward from the plains, cutting deeply into the post-Laramide surface, forming the steep slopes and deep canyons of the range front (Anderson et al., 2006).
Award Abstract #0724960
PI: Suzanne Anderson