Variations in the concentrations of total and particulate organic matter were estimated from 1979 through 1988 in natural and simulated runoff events to improve estimates of reservoir gross organic loading from intermittent flows. Total organic matter concentrations in natural runoff from pinyon-juniper watersheds and particulate organic matter concentrations in runoff from creosote bush watersheds increased as a natural logarithmic function of discharge. Total organic matter and particulate organic matter discharge-weighted concentrations in natural runoff events were related to measurements of instantaneous discharge and total organic matter and particulate organic matter concentrations, and to estimates of the maximum discharge of total organic matter and particulate organic matter concentrations that occurred in the event. In rainfall simulations conducted in watersheds of creosote bush, desert grass, pinyon-juniper, ponderosa pine, and spruce meadow, only particulate organic matter concentrations in ponderosa pine differed significantly (P ≤ 0.05) from the other sites; there, concentrations were lower. The percentage of organic matter in the total suspended solids eroded during simulated runoff events decreased as vegetation cover decreased and bare soil area increased. Estimates of gross annual total organic matter loading revealed large annual variations at specific reservoirs and large variations of mean annual loading among reservoirs. These results suggest that intermittent runoff has a significant potential impact on reservoir trophic states in arid and semi-arid regions.