Reconstructing the natural hydrology of the San Francisco Bay-Delta watershed

Abstract

Abstract. The San Francisco Estuary, composed of San Francisco Bay and the Sacramento–San Joaquin River Delta, is the largest estuary along the Pacific coast of the United States. The tributary watersheds of California's Central Valley are the principal sources of freshwater flow into the San Francisco Bay-Delta estuary. The Delta serves as one of the principal hubs of California's water system, which delivers 45% of the water used statewide to 25 million residents and 16 000 km2 of farmland. The development of California, from small-scale human settlements that co-existed with an environment rich in native vegetation to the eighth largest economy in the world was facilitated by reconfiguring the state's water resources to serve new uses: agriculture, industry, and a burgeoning population. The redistribution of water from native vegetation to other uses was accompanied by significant declines in native aquatic species that rely on the San Francisco Bay-Delta system. These declines have been attributed to a variety of causes, including reduction in the amount of freshwater reaching the San Francisco Bay-Delta watershed (Delta outflow); decreased sediment loads; increased nutrient loads; changes in nutrient stoichiometry; contaminants; introduced species; habitat degradation and loss; and shifts in the ocean–atmosphere system, among others. Among these stressors, only the volume of Delta outflow has been regulated in an effort to address the decline in aquatic species. As native species evolved under natural landscape conditions, prior to European settlement in the mid-18th century, we evaluated the impact of landscape changes on the amount of Delta outflow. We reconstructed the natural landscape and used water balances to estimate the long-term annual average Delta outflow that would have occurred under natural landscape conditions if the climate from 1922 to 2009 were to repeat. These outflows are referred to as "natural" Delta outflows and are the first reported estimate of natural Delta outflow. We then compared these "natural" Delta outflows with current Delta outflows for the same climate and the existing landscape, including its re-engineered system of reservoirs, canals, aqueducts and pumping plants. This analysis shows that the long-term, annual average Delta outflow under natural landscape conditions is equal to current Delta outflow because the amount of water currently used by farms, cities, and others is about equal to the amount of water formerly used by native vegetation. The development of water resources in California's Central Valley transferred water formerly used by native vegetation to new beneficial uses without reducing the long-term annual average supply to the San Francisco Bay-Delta estuary. Thus, it is unlikely that reductions in annual average Delta outflow have caused the decline in native freshwater aquatic species.