STABLE ISOTOPES EXAMINE HYDROLOGIC INPUTS TO URBAN STREAMS, SANTA MONICA MOUNTAINS, CALIFORNIA

The Santa Monica Mountains of Southern California have undergone moderate urbanization during the past 40 years. Existing information indicates that many of the riverine systems in the Santa Monica Mountains were intermittent just a few decades ago. Many of these streams are now perennial. The transition from intermittent to perennially streams during the period of urbanization has led many policy makers to conclude that urban runoff from landscape watering and vehicle washing accounts for continuous flows during the lengthy dry season in Southern California. Transition to perennially flows allows migration and expansion of habitat for many exotic and harmful species, such as the New Zealand Mud Snail. This raises arguments for regulations and 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. Our studies using stable isotopes of oxygen and hydrogen show that imported water is isotopically distinct from local precipitation and locally derived groundwater, providing a useful tool for tagging source flows in Santa Monica Mountain streams. Our subsequent studies show that most of the urban streams in the Santa Monica Mountains are dominantly fed by locally sourced, groundwater baseflow during the dry season. Streams contain only small percentages of imported waters; and by association, small percentages of urban runoff (3% to 8%). Perennial flow in urban streams is probably more a result of removal of riparian vegetation and deepening of channels, and not so much a result of urban runoff. Channel modifications presumably reduce evapotranspiration rates and increase groundwater baseflow, establishing perennial flows in the urban streams we investigated.