California historically has been the largest user of the Colorado River among the seven states of the Colorado River Basin. And for many years the state’s use exceeded its apportioned share of the river’s water. That began changing in the 1990s.

Under the Law of the River — the series of treaties, compacts, decrees, statutes and other agreements that govern the river’s use — California has a basic apportionment of 4.4 million acre-feet of Colorado River water annually. (An acre-foot equals 325,851 gallons, enough to supply the average annual needs of roughly three California households, depending on their location.)

But the state is also allowed to divert “surplus” water in the Lower Basin. Because Arizona and Nevada, the two other states of the Lower Basin, weren’t using their full Colorado River apportionments, California beginning in the 1950s started using more than 4.4 million acre-feet. As populations in Arizona and Nevada grew and used more of their share of the river, California was pressured to cut back.

In 1996, Interior Secretary Bruce Babbitt began urging California to devise a plan to reduce its river consumption to its basic entitlement. The resulting Colorado River Water Use Plan, known colloquially as the 4.4 Plan, resembled a jigsaw puzzle of interlocking pieces that took almost a decade to assemble.

Watch a short video explaining the 4.4 plan

The basic plan was developed by the Colorado River Board of California and set a 2016 deadline for the state to pare down its consumption of Colorado River water to its 4.4 million acre-feet annual allocation. In order to allow California to live within its means, the plan set up ways to save up to 800,000 acre-feet of water per year through agricultural water conservation and transfers to Southern California’s urban areas, canal seepage recovery, groundwater banking and more flexible operation of Lake Mead.

A key part of the plan is the Quantification Settlement Agreement (QSA), which quantified the exact amount of Colorado River Water to which the San Diego County Water Authority (SDCWA), Coachella Valley Water District (CVWD), Imperial Irrigation District (IID) and the Metropolitan Water District of Southern California (Metropolitan) are respectively entitled.

The signing of the QSA, in 2003, made it possible for those agencies to transfer water among themselves. The biggest such transfer was a 35-year deal (with potential extensions to 75 years) of as much as 200,000 acre-feet a year from IID to SDCWA — the largest agricultural-to-urban water transfer in U.S. history. If the transfer is extended to 75 years, SDCWA would pay a total of an estimated $2 billion for water, which IID is using to modernize irrigation systems and mitigate economic impacts of the transfer.

Related deals include a transfer of 105,000 acre-feet annually between IID and Metropolitan, and a transfer of as much as 103,000 acre-feet annually between IID and CVWD.

The Salton Sea has played a key role in the Quantification Settlement Agreement.

The sea, which straddles Imperial and Riverside counties, provides important habitat for fish and migratory birds, attracting some 400 different bird species. However, it has been in a long decline due to salt build-up and diminished water supply. The receding shoreline and exposed lake bed have also created air quality problems from toxic dust.

Water transfer from farming in the Imperial Valley reduced the freshwater that runs off fields into the sea. This has hastened the sea’s shrinkage and increased its salinity.

In response, the Imperial Irrigation District and the San Diego County Water Authority agreed to send fresh water to the Salton Sea to replace Colorado River water diverted under the QSA. The deal expired in 2018.

In 2015, Gov. Jerry Brown created the Salton Sea Task Force, directing agencies to develop a comprehensive management plan for the sea to meet a short-term goal of building 9,000 to 12,000 acres of habitat and dust suppression projects, and a longer-term goal of up to 25,000 acres.

In 2017, the State Water Resources Control Board adopted a 10-year plan calling for 29,800 acres of ponds, wetlands and dust-suppression projects on portions of exposed lake bed. The plan includes annual targets for completion, ramping up to 4,200 acres in 2028. The agreement further commits the state to creating a long-term plan beyond the initial 10 years. In 2026, the state created the Salton Sea Conservancy to oversee restoration efforts at the sea.

Updated June 2026.