The San Diego River watershed is the second largest hydrologic unit (HU) in San Diego County with a land area of 440 square miles. It has the highest population (~475,000) of the County’s watersheds and contains portions of the cities of San Diego, El Cajon, La Mesa, Poway, and Santee and several unincorporated communities. Important hydrologic resources in the watershed include five water storage reservoirs, a large groundwater aquifer, extensive riparian habitat, coastal wetlands, and tide pools.