Sources and mobility of arsenic in the upper Salt River (Arizona) were examined using a 6-year long mass balance for nine U.S. Geological Survey chemical monitoring stations, a more spatially intensive low-flow synoptic study, and laboratory leaching studies. An arsenic-rich lacustrine deposit (the Verde Formation) comprised only 4% of the watershed but contributed a third of the total watershed arsenic loading. Export from this part of the watershed was 10-20 times higher than export from most other parts of the watershed. Laboratory experiments confirmed the high As leaching potential for this formation. Although elevated levels of As were found in a geothermal spring and in several mine seeps, these sources of arsenic contributed little to the overall AS loading. About 15% of the AS loading from the upper Verde River watershed was retained by a two-reservoir system. Sedimentation and chemical precipitation appear to be likely mechanisms of AS retention in the reservoirs.