Alluvial fans and fluvial terraces preserve evidence of system responses to sediment budget perturbations related to past climate change, and preserve evidence of processes that have contributed to the evolution of the Colorado Front Range. Cosmogenic 26Al, 10Be, 36Cl, and 14C dating of geomorphic surfaces within and around Boulder Canyon provide for a detailed temporal perspective on processes contributing to the evolution of the Colorado Front Range. This thesis contributes a series of new ages that help constrain the glacial chronology in the Front Range region, a detailed analysis of fluvial response to glaciation, and perspective on other processes that presently contribute to the evolution of steep Front Range canyons. These results, coupled with work performed in other regions, lead to a host of new information concerning the geomorphic history of Boulder Canyon:
- Boulders from late Pleistocene glacial moraines near Nederland in the Colorado Front Range and near Twin Lakes Reservoir have model cosmogenic 10Be, 26Al, and 36Cl exposure ages consistent with existing age estimates from sites in Wyoming, southwest Montana, and other regions of Colorado.
- Heights of Boulder Canyon terraces can be divided into intervals that correspond to Bull Lake, Pinedale, and Holocene events.
- No pre-Bull Lake deposits are preserved in Boulder Canyon.
- The height-age relationship of terraces reflects long-term net incision of Middle Boulder Creek with short-term fluctuations in base level during periods of rapidly changing stream load and power.
- Net river incision apparently occurred during transitions to interglacial periods.
- Middle to late Pleistocene incision rates represented by high terraces in Boulder Canyon are faster than early Pleistocene rates, possibly as a result of east to west migration of a knickpoint on Middle Boulder Creek.
- Extrapolation of incision rate based on the position and age of terraces suggests that canyon incision started ~ 2.5 Ma.
- Soil development, stratigraphic position, and age estimates suggests that terraces within Boulder Canyon can be correlated with alluvial surfaces on the High Plains east of the canyon.
- Low terraces (< 4 m above grade) and alluvial fans along Boulder Canyon between Nederland and Boulder record Holocene deposition resulting from forest fires.
- Forest fire occurrence in Boulder Canyon correlates with a dry late Holocene climate and with the warm and wet early Holocene climate.
These results provide new perspective on traditional work that has been performed both locally and regionally. Ages of geomorphic features from this thesis provide a basis for quantifying age estimates based on degree of soil development. Previous work in Boulder Canyon had overestimated the ages of many deposits, and had not recognized any Holocene record. A revised calibration for soil-based age estimates could lead to reinterpretation several other features in the region.
Future work in this region may entail a more thorough evaluation of the local glacial chronology using cosmogenic exposure age dating of several more glacial moraines. This could also help reveal whether or not there is evidence for multiple Pinedale advances, since the terrace record suggests at least two potential retreats. A more thorough investigation of terraces within the canyon and/or local history may offer insights on the low exposure ages derived from my soil profile at site T64, and could aid in supporting/refining the conclusions I have drawn from my data. Finally, results from this thesis would greatly benefit from corroboration with data from nearby catchments.
FOR MORE INFORMATION:
Contact Taylor Schildgen at tschildg@wso.williams.edu, or write 221 Richards Rd, Litchfield, CT 06759.