Quantifying human impacts on rates of erosion and sediment transport at a landscape scale

Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact of human activities on Earth's surface. Here, we present Be-10 estimates of background erosion rates for ten large (10,000-100,000 km(2)) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These Be-10-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement (similar to 8 m/m.y.). Comparing these background rates to both rates of post-settlement hillslope erosion and to river sediment yields for the same basins, we find that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (similar to 950 m/m.y.) exceeded Be-10-determined background rates more than one-hundred fold. Although large-basin sediment yields during peak disturbance increased 5-10x above pre-settlement norms, rivers at the time were transporting only similar to 6% of the eroded material; work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial time scales, they can inform and enhance landscape-management strategies.