Imported Versus Local Water Sources? Origins of Stream Flows at the Wildlands/Urban Interface in Suburban Parts of Southern California

Abstract:

The Santa Monica Mountains of Southern California have undergone moderate urbanization since 1960. Many of the riverine systems in the Santa Monica Mountains were intermittent just a few decades ago, and many 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. Transition to perennially flows allows expansion of habitat for exotic and harmful species, raising arguments for controls on urban runoff during the dry season. Major segments of 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 previous isotope investigations from 2006 to 2009, we performed detailed analysis of dry weather flows in major streams in the Santa Monica Mountains. These creeks transition from open space, through city, and back through open space obtaining most of their flow along the urban reach. Our studies showed that most creeks are dominantly fed by locally sourced, groundwater baseflow during dry weather. Streams contain small percentages of imported waters (~15%) and consequently small percentages of urban runoff. Updated monitoring during serious drought conditions from 2012 to 2014 provide similar results. We postulate that perennial flow in urban streams is probably a result of removal of riparian vegetation and deepening of channels along urban corridors, and not as much a result of urban runoff. These findings have important ramifications on policy decisions regarding stream restoration and mandatory controls on urban runoff.