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Tidal action and Delta outflow work to create long and gradual salinity gradient from the Pacific Ocean into the Delta. Before Shasta Dam was built in 1943, the upper edge of this gradient (about 5 percent sea water) pushed far into the Delta in drier years. As shown on the 1921-1943 maximum salinity map, salinity reached as far as Stockton on the San Joaquin River and to beyond Courtland on the Sacramento River in 1931. Today Shasta, Folsom, and Oroville reservoirs help control salinity intrusion by providing fresh water releases during the drier parts of the year as shown on the 1944-1990 maximum salinity map.