Freshwater Flow

The San Francisco Estuary connects California’s largest rivers to the Pacific Ocean. It provides spawning, nursery, and rearing habitat for native fishes and invertebrates; a migration corridor for salmon, steelhead, and sturgeon; and breeding and nesting habitat for waterfowl and shorebirds. The amount, timing, and pattern of freshwater inflows to the Estuary are key determinants for its ecological functioning and health.

Over the past 80 years, growing numbers of dams and increased water diversions in the Estuary’s watershed and the Delta have profoundly changed freshwater flow. Water storage in upstream reservoirs and diversions of water to supply agricultural and urban users have cut the amount of fresh water that flows into the Estuary by half. The Estuary’s historical pattern of high inflows during winter and spring, followed by declining flows as summer progressed to fall, has been flattened. Even the year-to-year variation of flows, which reflects California’s unpredictable sequences of wet and dry years, has been reduced. Today, because of these reductions and alterations in inflows, the Estuary experiences flow conditions in most years comparable to those that would have historically occurred only during an extreme drought.

The importance of freshwater flows to the health of the Estuary has been well-documented by decades of scientific research and monitoring. Since the 1990s, state and federal water resource managers have established various regulations for flows and water quality that were intended to improve, or at least stabilize inflows and estuarine habitat quality. Despite these efforts, Freshwater Flow conditions in the Estuary remain Poor and are still Declining. Ongoing and future demand for the Estuary’s water and changes in drought frequency and intensity will keep the pressure on, adding serious new challenges to management efforts to protect and restore the health of this vital Estuary.