The watershed of California's Central Valley drains into San Francisco Bay, the largest estuary on the Pacific Coast. The water resources of the Valley have been intensively exploited, particularly in the last 50 years, with the construction of large-scale irrigation systems. About half the 35 km3 average annual freshwater inflow to the estuary is now diverted, with up to 85 per cent in dry years. These diversions are now having significant adverse effects on the estuarine ecosystem including substantial declines in the economically significant anadromous fishery. At the time the large dam projects in the watershed were designed, no consideration was given to impacts on the estuary. Growing recognition of this error has led the California state agency that governs water allocation to consider applying operating conditions on the upstream reservoirs in order to provide sufficient freshwater inflows in the estuary to maintain the estuarine ecosystem. A set of freshwater flow and salinity standards has been proposed and is described in this paper. These flow requirements are based on extensive research on the role of freshwater inflow on estuarine hydrodynamics and on the estuarine ecosystem. These requirements have three overlapping goals:. 1Maximizing phytoplankton production - the base of the estuarine foodchain. 2Protecting spawning and migration of anadromous fishery. 1Protecting brackish wetlands. .