SALINITY AND AGRICULTURAL DRAINAGE

The Delta always has been at the mercy of river flows and tides. Before human intervention, salty ocean water from the Bay invaded the vast Delta marshes during dry summers when mountain runoff ebbed. Then, during the winter, heavy runoff from the mountains repelled seawater intrusion. The diaries of early Spanish explorers and more recent records illustrate that the salt line (referred to as X2 by scientists) moves according to the dryness of the year. A great flood in the 1860s resulted in a substaintially fresh water Bay. Conversely, salt water reached as far as Sacramento in the 1930s during one of the state's worst droughts.

The problem of seasonal water intrusion into the Delta was greatly alleviated by the year-round release of fresh water by upstream dams and reservoirs. However, salinity intrusion from the ocean or accumulation of minerals from farming discharged into Delta rivers remains a problem. The estuary generally becomes saltier during the summer and fall, but can be influenced in spring months when export pumps are running at full bore to capture runoff.

Historically, the need to keep the Bay salty water away from the rich Delta soils and local farms was seen as essential, and as early as 1880 the state proposed building a barrier between the Bay and the Delta. Hydraulic barriers, using upstream releases of fresh water to repel seawater (incorporated in today’s state water projects); physical barriers, such as low-level dams to separate fresh water from saline water with passageways from navigation and fish migration; alterations in existing channels to improve flow patterns; and construction of new channels, such as an "isolated facility" to isolate export water from brackish Delta waters are all means of reducing salt intrusion in the Delta and improving the transfer of water from the rivers to the export pumps.

In comparison to the rest of the Delta, the western Delta (roughly the area west of Isleton) suffers periodically from higher salt water content, presenting a negative affect on drinking water supplies for the 400,000 residents of eastern Contra Costa County. The Los Vaqueros Reservoir has improved the county’s drinking water quality.

Agricultural drainage also contributes to the Delta’s salinity problems. Because most Delta islands are below sea level, water from surrounding channels seeps through the levees onto the land. Farmers must pump this water from the lands while adding controlled amount of fresh water for productive agriculture. In the South Delta, where farmers rely primarily on the water of the San Joaquin River for their irrigation supply, the process of irrigation concentrates salts in drainage water, which is then pumped into nearby Delta channels. Sometimes there is no current to "flush" these salts through the Delta, creating localized salinity problems.

The salt content of the drainage water flowing down the San Joaquin River, primarily from the west side of the valley, is high, and sources of dilution water are limited. Most of the valley averages less than 10 inches of rain a year, and water from the sierra tributaries is now either exported or diverted for consumption uses. Flows in some stretches of the San Joaquin River during the summer irrigation season consist almost entirely of irrigation return flows. In turn, salty return flows increase the salt content of water used down-stream by Delta farmers and the amount of salty water flowing into the estuary.