GLOBAL CONTROLS ON EROSION RATES: THE AHNERT CURVE, PRECIPITATION, AND LAND USE

The functional controls on erosion rates are central to understanding both controls on global sediment flux and the potential for feedback between tectonics, climate, and erosion in shaping topography. Frank Ahnert's seminal 1970 paper reported a strong relationship between erosion rate and mean local relief, and over the past decade the relationship between erosion rate and hillslope gradient has come to be considered highly non-linear in steep terrain, resulting in the development of threshold slopes that adjust to spatial (and temporal) variations in rock uplift rate through variability in landslide frequency rather than steepness. Analysis of a high-resolution (10-m grid) DEM transect across the convergent orogen of the Olympic Mountains reveals a non-linear relation between long-term erosion rates and mean slope, consistent with the concept of hillslope evolution by landsliding in steep terrain. The DEM data also reveal a relation between mean slope and mean local relief. Coarser-scale (1-km grid) global analysis of the relation between erosion rate and mean local relief reveals different trends for areas with low erosion rates and tectonically active mountain ranges, with the composite relation being well described by non-linear models. Superimposed on topographically-mediated influences on erosion rates are variations in precipitation rates strongly influence erosion rates. Lastly, variations in erodibility driven by land use account for greater variability in local erosion rates than do topography and climate. While interactions among all three factors (topography, climate, and land use) complicate assessing global patterns of erosion, the interactions provide a rich target for geomorphological study.