NUMERICAL MODELING OF FILL-TERRACE FORMATION IN RESPONSE TO GLACIAL-INTERGLACIAL CLIMATE FLUCTUATIONS: A CASE STUDY FROM THE BOULDER CREEK CZO

We explore the effect of Milankovich-scale climate fluctuations on fluvial systems draining glaciated headwaters. Glacial-interglacial cycles are expected to produce notable fluctuations in bedload and water discharge into the rivers immediately downstream from glaciated catchments. While significant progress has been made in understanding the formation of strath terraces by glacial-interglacial fluctuations in sediment supply, little research has focused on the formation of alluvial terraces within steep bedrock canyons in the proglacial environment. We use a numerical fluvial sediment transport model coupled to a simple glacier model to improve our understanding of alluvial terrace formation. The fluctuating glacier footprint sets the hydrograph and sediment supply to the top of the fluvial system. We use this model to explore how Pleistocene glacial-cycle swings in sediment supply impact the longer-term upstream propagation of knickpoints in the Middle Boulder Creek catchment, CO, USA. Middle Boulder Creek is an ideal setting for this study because of (1) detailed documentation of terrace elevations above the modern river; (2) constraints on timing of terrace abandonment; (3) cosmogenic radionuclide-derived history of post-LGM deglaciation; and (4) long-term records of river-related variables.

This work furthers our understanding of the rate and timing of canyon incision and knickpoint propagation within the Laramide rages, and strath terrace formation outboard of the ranges. We find that resistant valley walls are more likely to accommodate swings in sediment supply by generating alluvial-fill terraces. In contrast, the more easily eroded rocks east of the Front Range promote the formation and abandonment of strath terraces recording the long-term Pleistocene incision of the High Plains.