Stabilization of hazardous waste sites and radioactive uranium mill tailings requires that methods be developed to prevent erosion of the protective covering applied at each site to contain the wastes. In the case of uranium mill tailings a clay cap up to six feet thick is applied to prevent escape of radioactive radon gas. In arid areas where vegetation is difficult to establish, a rock mulch or gravel armor may be placed on the clay to prevent erosion by wind and rain. Natural talus slopes provide an excellent physical analog to this arrangement of materials and studies show the hydrological and erosional response of talus may depend upon the nature and distribution of fine material beneath the rock cover. The results (Sosedov, 1974; Yair and Lavee, 1976) indicate that talus can be either very stable or highly erodible due to removal of fine material by subsurface flow. It is suggested that this flow can occur along the interface between the rack cover and the underlying fine material.The proposed research is designed to investigate experimentally the nature of subsurface runoff generation and erosion processes on rock mulch and underlying surfaces. During Phase I, laboratory experiments will determine which characteristics of rock mulch are important in runoff generation and subsurface erosion. Phase II will extend the study to a field site, located on uranium mill tailings in western Colorado, where test plots of the best and worst cases, as determined by Phase I, will be examined using a portable rainfall simulator. The results will be directly applicable by those involved in stabilization of hazardous waste and uranium mill tailings sites.