Sacramento–San Joaquin River Delta

In its natural state, the Delta was a large freshwater marsh, consisting of many shallow channels and sloughs surrounding low islands of peat and tule.

[8] Nearby cities include Lodi and Stockton to the east, Tracy and Manteca to the south, Brentwood to the southwest, and Pittsburg and Antioch to the west.

[citation needed] "The virgin California Delta was so vast, wild and confusing – its sloughs meandered everywhere and led nowhere – that John C. Frémont lost a whole regiment in there for several days, and some who ventured in just disappeared."

The Delta was formerly located at the bottom of a large inland sea in the Central Valley, which formed as the uplift of the California Coast Ranges blocked off drainage from the Sierra Nevada to the Pacific.

The drainage of all the water through this narrow gap formed a bottleneck in the Central Valley's outflow; this constriction is essential to provoke the slowing of river current and the resulting sediment deposits that now make up the Delta.

The early delta was composed of shifting channels, sand dunes, alluvial fans and floodplains that underwent constant fluctuation because of rapidly rising seas – one inch (25 mm) per year.

[11] Immediately before large-scale human development, most of the Delta islands had saucer-like cross sections, with low natural levees flanking a marshy interior "bowl" that flooded intermittently with the seasons and tides.

A malaria epidemic in 1833 decimated local native populations; this was probably exacerbated by the marshy geography of the area, which bred large amounts of mosquitoes.

[17] Because of the flat terrain coupled with year-round availability of fresh water, irrigation here was cheaper and simpler than other fertile regions of California.

As a result, the Delta remained California's richest farming region (alongside Los Angeles County) until the 1940s, when massive federal water projects finally made possible the full-scale irrigation of the main body of the Central Valley.

[18] In 1850, Congress passed the Arkansas or Swampland Act, which allowed for the transfer of title for wetlands to private owners on the conditions that the land would be reclaimed.

[18] Major crops grown in the Delta include corn, grain, hay, sugar beets, alfalfa, tomatoes, asparagus, and safflower; various fruits are also raised here, as well as some livestock.

[5] Crops in the Delta cannot be irrigated using conventional methods due to the highly absorbent quality of the peat soil and irregularities caused by land subsidence.

[8] Intrusion of brackish water into the Delta is a recurring natural phenomenon; however, it became a serious issue after the development of agriculture in the upper Sacramento and San Joaquin valley reduced inflows.

The growing Delta water quality issue provided the initial impetus for building dams on Central Valley rivers to boost dry-season freshwater flows.

The Port Chicago Naval Magazine National Memorial offers visitors a chance to learn about the disaster that represented the largest domestic loss of life in the U.S. in World War II, and the impact it had on race relations in the U.S. military.

The California Department of Water Resources has experimented with re-flooding areas for wetland restoration, in order to sequester carbon and rebuild soil levels.

However, regulation provided by dams helps boost freshwater flows during dry summers and autumns, reducing the risk of salinity intrusion in these months.

This has caused multiple environmental issues, such as the disruption of fish migration and salinity buildup in the eastern Delta, where salts can no longer be flushed to the sea by natural river flows.

[40] This study concluded that the Delta levee system is now relatively robust but should be improved to effectively eliminate the risk of failure in extreme floods and earthquakes.

[citation needed] A 2019 article states that "a catastrophic levee failure, defined as 20 islands flooding at once, has a 62 percent chance of occurring in the next two decades if subsidence isn’t addressed.

Over thousands of years, the growth and decay of the various swampland plants formed a layer of peat 50 feet (15 m) deep in places, hence the extremely high fertility of Delta soils.

[45] The Delta was formerly populated by large herds of deer and tule elk; their trails were so vast that early Spanish explorers supposed the area was inhabited by cattle.

[47] Large mammals in the Delta have not fared so well; with most of their habitat converted to agriculture, remaining populations were highly susceptible to human impacts and natural disasters.

[49] The California Office of Environmental Health Hazard Assessment has issued a safe eating advisory for any fish caught in Central and South parts of the Delta due to elevated levels of mercury and PCBs.

[51] Delta fish have also been found to have high levels of a toxin called methylmercury which originates from old gold rush era mines in the Sierras.

State officials are concerned that they will harm the infrastructure that sends water to San Joaquin Valley farms and urban areas.

The high prevalence of nutrient concentrations in the Delta also plays a significant part in the increased frequency of Microcystis aeruginosa blooms.

[59] Microcystis benefits greatly from anthropogenic inputs of nitrogen which allows it to out-compete other primary producers and dominate the lower trophic levels.

[66] In addition, large numbers of Delta smelt and other endangered species are killed by the pumping plants that provide water for the aqueducts.

Northern California's Sacramento–San Joaquin River Delta. The Sacramento River flows into the delta from the north and the San Joaquin River from the south through Stockton .
Part of the Delta viewed from above Sherman Island , with the Sacramento River above and San Joaquin River below
The Old River, a former channel of the San Joaquin River located in the southwestern Delta
An 1850 drawing of the Sacramento River in the Delta, showing the lush foliage that made up much of the virgin Delta.
An 1873 map showing the river systems of the Central Valley, with wetlands marked in gray. The Delta, at roughly left center, was the largest area of contiguous, perennial wetlands in pre-development California.
The Delta (located south of Sacramento, middle left, at the confluence of the Sacramento and San Joaquin rivers) is central to California's water supply system.
Land subsidence in the Delta as of 1995
Sacramento–San Joaquin Delta at flood stage, spring 2009. Photo by Doc Searls .
The tiny Delta smelt is an Endangered Species Act threatened species that has caused controversy over the amount of water that can be sustainably pumped from the Delta.
Water tower in Isleton, California
The launch of 100 floating sensors into the Sacramento River for the Floating Sensor Network project