The Santa Monica Mountains of southern California have undergone moderate urbanization since 1960. Just a few decades ago, many of the riverine systems in the Santa Monica Mountains were intermittent, but they are now perennial. The transition to perennial streams has led many policy makers to conclude that urban runoff from landscape watering accounts for continuous flows during the lengthy dry season. The transition to perennial flows allows expansion of habitat for exotic and harmful species, raising arguments for controls on urban runoff during the dry season. Most communities in the Santa Monica Mountains depend entirely on State Project water imported from northern California. Stable isotopes of oxygen and hydrogen show that imported water is isotopically distinct from local precipitation, providing a useful tool for tagging source flows in Santa Monica Mountain streams. In this investigation, we perform a detailed analysis of dry weather flows in one of the Santa Monica Mountain's major streams, Las Virgenes Creek. We also present results of reconnaissance surveys of several other creeks. Las Virgenes Creek accumulates most of its initial flow along an urban part of its stream reach. Chloride and sulfate scatter plots indicate that Las Virgenes Creek is dominantly fed along the urban stream reach by locally sourced, groundwater base flow. Oxygen and deuterium isotopes show that Las Virgenes Creek and other creeks in the Santa Monica Mountains contain small percentages of imported waters (less than 10 percent), indicating little urban runoff during dry weather flows. The data suggest that perennial flow in urban streams results from the removal of riparian vegetation and deepening of channels rather than from urban runoff.