CLIMATE CONTROLS ON HILLSLOPE MORPHOLOGY IN THE COLORADO FRONT RANGE

The role of climate in shaping unglaciated hillslope topography remains unclear, but it is important because it impacts 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 strong altitudinal gradient in climate in the Northern Colorado Front Range, which spans nearly 2 km of relief, to evaluate the impact of climate-related gradients in temperature, precipitation magnitude and phase, and vegetation 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 an ~1 m resolution lidar-derived digital elevation model of the Poudre River basin in Northern CO, we use existing objective methods to define stream channel heads, map drainage divides, extract hillslope metrics of length, relief, and mean gradient, and calculate hilltop curvature. Results show that hillslope length and relief systematically increase, and mean hillslope gradient and hilltop curvature systematically decrease from low to high elevation. Because existing ¹⁰Be-derived erosion rates are consistently between ~15 – 20 m/Myr, spatial changes in erosion rates do not appear to relate to observed differences in hillslope morphology. To explain the variation in hillslope morphology as a function of elevation, we place our results in a non-dimensional framework that shows that the hillslope sediment transport efficiency systematically increases with increasing altitude, which explains the longer, rounder hillslopes at higher elevations. Furthermore, our results show that higher elevation hilltops are further from steady-state predictions compared to lower elevation hilltops. This result suggests that late Cenozoic climate change might be forcing a transient adjustment of hillslope morphology by affecting regolith transport rates. Collectively, these results have important implications for understanding hydrology and sediment transport dynamics in the CO Front Range and highlight the role of spatial and temporal gradients in climate in shaping Earth’s topography.