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 (super 10) Be estimates of background erosion rates for ten large (10,000-100,000 km (super 2) ) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These (super 10) Be-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement ( approximately 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 ( approximately 950 m/m.y.) exceeded (super 10) Be-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 approximately 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.