Estimating Long Term Rates of Erosion and Sediment Generation Using Cosmogenic Isotope Abundance in Rock, Soil and Sediment

Abstract

Abstract : Currently, evaluating long-term rates of land surface erosion is a very difficult and uncertain task. Knowing such rates is imperative if land management decisions are to be made responsibly. Only if land managers can compare human-induced rates of surface change, with natural or background rates, can management decisions be made cost-effectively and with the best balance between preservation and use of the land. In order to further this goal, we determined the background or natural rate at which soil and rock surfaces are eroding at two sites: the Yuma Proving Ground (Yuma, AZ) and at Nahal Yael (an intensively studied, hyper-arid basin in the southern Negev of Israel). At Yuma Proving Ground, we determined that most sediment in upland drainage is derived from hillslopes and that rocky outcroppings are the most stable landscape features. On the main stem of Yuma Wash, we determined that much of the sediment transported by the Wash is actually derived not from erosion of the highlands but from reworking of the older valley filling alluvium. At Nahal Yael watershed, we determined that long-term sediment generation rates are substantially less than short term sediment yields as measured by trapping the last 30 years of sediment transported out of the basin behind a retention dam. These findings suggest that short-term measurements of sediment yield may be overestimates.