MEASURING LONG-TERM EROSION RATES IN THE NORTHERN FRONT RANGE—THE UNDERGRADUATE CONNECTION

Development of relief and evolution of Paleogene surfaces has long been a focus for studies of the post-orogenic southern Rocky Mountains. Undergraduate research has contributed to understanding erosion of the Front Range (since the late Pleistocene) through studies of landscape lowering and catchment-based erosion rates. Following the NSF-sponsored “WAMSIP” undergraduate research model pioneered in the early 1970s by Reinhard “Bud” Wobus, students from many institutions mapped and sampled in northern Colorado, supported by the Boulder Creek Critical Zone Observatory and the Keck Geology Consortium. Most returned to their respective institutions to perform additional analyses and to write extended reports—usually senior-year theses that contributed to presentations at national meetings and to published papers. Widespread application of three tools—GPS, cosmogenic geochronology using ¹⁰Be and ²⁶Al, and airborne lidar studies—transformed field-based geomorphic research in the decades after ~1990. Research by undergraduates suggests that modern, low relief landforms in the Front Range integrate tens of meters per million years of landscape lowering since they were first exposed. Since late Pleistocene time, landscape relief in Front Range granitic rocks has increased as upland glacial erosion and downstream canyon-cutting isolated scattered remnants of more slowly eroding, low relief surfaces. Lowering rates for bedrock outcrops calculated from cosmogenic nuclides range from 9 to 30 mm ky^-1 and average erosion rates for upland catchments are about 25 mm ky^-1 over the past 30 kyr, whereas downcutting of Boulder Canyon and glaciated valleys over the past 100 kyr occurred at rates >100 mm ky^-1. Field mapping and analysis of 1-m lidar digital elevation models show that during the ~18 kyr of the Pinedale glaciation sediment accumulation in the marginally glaciated Lake Devlin basin was driven by an overall basin erosion rate of ~55 mm ky^-1, and cirque erosion rates were ~70 to 180 mm ky^-1 . About half of the eroded Pinedale sediment accumulated in the ice-dammed lake; erosion in the past 14 ky removed 50% of that sediment. Collectively, these insights into the geomorphic evolution of the Front Range are a legacy of Wobus' dedication to and encouragement of undergraduate research.