CLIMATE AND TOPOGRAPHY CONTROL THE SIZE AND FLUX OF SEDIMENT PRODUCED ON STEEP MOUNTAIN SLOPES

Rivers set the pace of landscape erosion in their endless struggle to cut through mountains and reach the sea. Yet, flowing river water needs tools to cut into landscapes, and the erosion of surrounding slopes provides these tools in the form of sediment. Both the size and amount of sediment shed from mountain slopes help modulate river incision. Thus sediment production is central to the feedbacks between hillslope and fluvial processes that govern the interplay of climate and tectonics in landscapes. These feedbacks remain poorly understood, however, because there is no way to quantify how the size distributions of eroded sediment vary across landscapes. We overcome this limitation with a new method that uses cosmogenic nuclides and thermochronometric ages measured together in multiple sediment sizes. When applied to a steep catchment, our method reveals that higher-elevation slopes, which are colder, drier and steeper, also produce coarser sediment that erodes faster into the channel network. Sediment size and flux are evidently coupled with altitudinal variations in temperature, precipitation, vegetation and hillslope steepness across the catchment. We show that conventional analysis of just one sample from the creek would fail to detect the altitudinal gradients observed here and would underestimate the catchment’s spatially averaged erosion rate by more than a factor of two. This points to previously unrecognized sources of error in the interpretation of cosmogenic nuclides and detrital thermochronometry. Moreover, it suggests that a large body of previous studies will need to be re-evaluated and possibly revised in consideration of the sampling biases identified here. Our results suggest that climate and topography strongly regulate the chemical, physical and biological processes that break rock down and erode it into river networks. Altitudinal gradients in sediment production, such as the ones measured here, may help explain downstream variations in channel steepness, hillslope relief and aquatic habitats in mountain catchments that harbor altitudinal gradients in climate and topography.