CLIMATE CONTROLS ON HILLSLOPE MORPHOLOGY IN THE POUDRE DRAINAGE BASIN, NORTHERN COLORADO

The role of climate in shaping unglaciated hillslope topography remains unclear, but it can impact the transport and routing of surface water and sediment through its influence on surface slope, drainage density, and sediment transport efficiency. In this study, we take advantage of the dramatic altitudinal climate gradient in the Poudre Drainage Basin in Northern Colorado to evaluate the impact of climate on hillslope morphology. We hypothesize systematic changes in hillslope morphology exist across this elevation gradient, driven by climate-dependent variation in hillslope sediment transport efficiency. Using a 1-m resolution lidar-derived digital elevation model, we use existing methods to define stream channel heads, map drainage divides, and quantify hillslope length, relief, gradient, and curvature. From low to high elevation, hillslopes lengthen >300 m, while relief increases only ~50 m. Mean hillslope gradient and hilltop curvature systematically decrease from low to high elevation. Topographic proxies for bedrock exposure (an inverse proxy for regolith depth) systematically decline with increasing elevation. Because existing ¹⁰Be-derived erosion rates are consistently between ~15 – 20 m/Myr, spatial changes in erosion rates cannot account for the differences in hillslope morphology. To explain the variation in hillslope morphology as a function of elevation, we place our results in a non-dimensional non-linear hillslope diffusion modeling framework. This analysis suggests that topography at higher elevations is further from steady-state predictions of hillslope morphology relative to lower elevations. We interpret these results as indicating that the hillslope sediment transport efficiency systematically increases with increasing altitude, which can explain the longer, rounder hillslopes at higher elevations. We infer that late Cenozoic climate change might be forcing a transient adjustment of hillslope morphology by elevating regolith production and transport rates through increased frost cracking intensity at high elevations. These elevation-dependent changes in hillslope characteristics likely affect hydrology and sediment transport and connectivity in ways that might relate to post-wildfire and rainfall-triggered slope stability hazard potential in Colorado.