600 Ancestral Pueblo and Hohokam Indians develop water distribution systems.

1500 Spanish explorers introduce livestock and ditch systems called acequias.

1847 Mormons arrive in the Salt Lake Valley; begin cultivating farmland.

1859 Oliver Wozencraft promotes idea of irrigating the Imperial Valley.

1865 Lower Colorado River lands begin to be set aside for American Indians.

1869-71 John Wesley Powell explores the Colorado River by boat; issues report in 1875.

1892 Colorado River water transported across Continental Divide into Eastern Colorado through the Grand Ditch – the river’s first transbasin diversion project.

1902 U.S. Reclamation Service (Bureau of Reclamation) established.

1905-07 Colorado River breaks through Imperial Valley Canal, creating the Salton Sea.

1908 U.S. Supreme Court rules that water be reserved as part of land for American Indians in Winters v. U.S.

1919 Grand Canyon National Park created.

1922 U.S. Supreme Court rules in Wyoming v. Colorado that appropriative water right doctrine applies regardless of state lines.

Interstate compact negotiated by Secretary of Commerce Herbert Hoover and seven states to divide use of the Colorado River.

1923 Except for Arizona, states ratify the Colorado River Compact.

1925 Metropolitan Water District of Southern California is formed; goal is to build the Colorado River Aqueduct. Legislature approves MWD in 1927.

Six-state Compact approved by California, Nevada and Upper Basin states.

1928 Congress approves Boulder Canyon Project Act and 1922 Compact.

1929 California passes California Limitation Act.

1930 California agrees to buy all power produced by Boulder Canyon project.

1930-36 Arizona files cases with U.S. Supreme Court trying to invalidate Boulder Canyon Project Act. Court refuses to hear cases.

1931 Construction of Hoover Dam begins.

1937 Colorado-Big Thompson project authorized.

1940 Reclamation completes All-American Canal.

1941 Metropolitan Water District of Southern California (MWD) completes 242-mile long Colorado River Aqueduct.

1944 U.S. and Mexico sign treaty to annually allocate 1.5 million acre-feet of Colorado River water to Mexico in normal years.

Arizona Legislature ratifies Colorado River Compact after 22 years of opposition.

1946-48 Upper Colorado River Compact apportions water among Upper Basin states, creates Upper Colorado River Commission and paves way for new water projects.

1947-49 Central Arizona Project plans released.

States and Congress ratify Upper Colorado River Basin Compact.

1951 Congress refuses to approve CAP until California and Arizona resolve differences.

1952 Federal bill for Upper Basin projects generates national opposition because one project, Echo Dam, would flood Dinosaur National Monument. Dam was removed from subsequent legislation signed in 1956.

1956 Colorado River Storage Project Act approves multiple projects in the Upper Basin including Flaming Gorge Dam and Glen Canyon Dam. Glen Canyon Dam is completed in 1963.

1964 U.S. Supreme Court Decree Arizona v. California holds California to 4.4 million, Arizona to 2.8 million and Nevada to 300,000 acre-feet annually in normal years as provided in the Boulder Canyon Project Act.

1968 CAP is included in Colorado River Basin Project Act. Proposed Grand Canyon dam removed after one of biggest environmental battles in U.S. history.

1973 Mexico and U.S. approve Minute 242 of the 1944 Water Treaty, establishing salinity standards for water delivered to Mexico.

Construction begins on CAP.

1974 Congress approves Colorado River Basin Salinity Control Act, authorizing desalination plant near Yuma, Arizona, and basinwide salinity control projects.

1979 U.S. Supreme Court appoints special master to review additional tribal Lower Basin Colorado River water rights.

1982-83 Master recommends additional tribal water claims be upheld.

Supreme Court rejects recommendation; refuses to reopen Arizona v. California awarding federally reserved water rights to five Lower Basin tribes.

1984 On-farm salinity control measures added to Salinity Control Act.

1985 CAP begins operation.

1988 Upper Basin states begin 15-year program (later extended) to protect four endangered fish.

MWD agrees to pay for water conservation measures in Imperial Valley in exchange for water conserved.

1991 Southern Nevada Water Authority formed.

1992 Grand Canyon Protection Act approved, requiring releases from Glen Canyon Dam to meet environmental, tribal, cultural and recreational interests.

Yuma desalination plant begins operation at one-third capacity.

Ten Tribes Colorado River Basin Partnership formed.

1993 CAP declared substantially complete, delivering water to Tucson, Phoenix, Arizona farmers and American Indian Tribes.

Yuma Desalting Plant suspends operation after 500-year flood on Gila River.

1995 Water transfer between Imperial Irrigation District (IID) and San Diego County Water Authority (SDCWA) proposed.

1996 Reclamation adopts new operating criteria that include periodic high-flow releases into the Grand Canyon to restore riparian habitat and improve fish habitat.

Interior Secretary orders California to implement plan to reduce its average use of 5.2 million acre-feet of Colorado River water to “live within” its 4.4 million acre-feet annual basic apportionment.

1997 Reclamation publishes proposed rule for off-stream storage and recovery of Colorado River water.

1998 IID and SDCWA boards agree to water transfer.

1999 Interstate banking rule allowing Lower Basin states to store water in Arizona aquifers is completed.

2000 States negotiate Interim Surplus Guidelines.

Environmental groups sue over Mexican Delta.

Upper Basin receives $100 million for endangered fish recovery programs.

ROD signed on Animas-La Plata project.

Multiyear drought begins on the Colorado River.

2001 Interim Surplus Guidelines ROD signed.

Arizona and Nevada agree on water banking.

2003 Quantification Settlement Agreement signed, enabling water transfer between IID and SDCWA and gradual reduction of California use to 4.4 million acre-feet.

Construction begins on Animas-La Plata.

2004 MWD-PVID sign 35-year deal to pay farmers to fallow and rotate crops, transferring saved water to urban Southern California.

2005 Lower Basin Multi-Species Conservation Program signed, a 50-year agreement to restore 8,100 acres of habitat between Hoover Dam and the U.S.-Mexico border.

Storage in Lake Powell drops to 33 percent of capacity.

Mexican business interests and two U.S. environmental groups sue to stop All-American Canal lining, saying seepage from unlined canal recharges Mexicali Valley groundwater.

2006 Congress passes legislation to waive environmental requirements and orders Interior to proceed with the canal lining and construction of Brock Reservoir in Imperial County.

2007 Seven States Agreement and federal ROD signed; includes Lower Basin shortage guidelines and rules to store conserved water in Lake Mead and agreement to “equalize” storage in Mead and Powell.

Yuma Desalting Plant operates at 10 percent capacity for 90-day test run.

2008 Third experimental high-flow release from Glen Canyon Dam to study effects on beaches and endangered species.

SNWA begins construction on third Lake Mead intake.

2009 All-American Canal lining completed.

2010 Colorado River Basin Supply and Demand Study initiated.

7.2 magnitude Baja California Earthquake occurs on Easter, damaging water infrastructure in the Mexicali area.

Mexico and U.S. subsequently sign Minute 318, an interim agreement that allows Mexico to store part of its allocation in Lake Mead while repairs are made to infrastructure damaged during the April 2010 earthquake.

Year-long Yuma Desalination Plant pilot run produces 30,000 acre-feet.

2011 Brock Reservoir completed.

Animas La-Plata completed.

2012 California Court of Appeal upholds QSA and Supreme Court leaves that decision standing.

U.S.-Mexico Minute 319 signed, creating binational framework to address shortages and allowing Mexico to store unused water in Lake Mead.

New protocol for high-flow releases from Glen Canyon released.

Colorado River Basin Supply and Demand Study released.

Colorado governor proposes a Colorado River Cooperative Agreement to resolve differences between Front Range and Western Slope entities.

2013 Federal officials establish working groups to implement Colorado River Basin Study.

Sacramento Superior Court validates QSA and 11 related agreements. The County of Imperial and Imperial County Air Pollution Control District filed an appeal.

2014 A pulse flow is released into the Colorado River Limitrophe (the 24-mile stretch that forms the U.S.-Mexico border) and Delta. Water flows to areas being restored by conservation groups and sets the stage for future management of what was once more than 2 million acres of riparian habitat and wetlands vital to birds and wildlife.

City of Phoenix creates the Colorado River Water Resiliency Fund, in which about $5 million annually pays for things such as well sharing and storing the city’s unused Colorado River water in underground recharge facilities. 

2015 The Moving Forward Phase 1 Report is released. It includes the recommendations from three workgroups focused on water use efficiency (urban and agricultural) and environmental and recreational flows.

2016  Major water suppliers in the Lower Basin begin begin work on a Drought Contingency Proposal that would ensure Arizona, California and Nevada are enrolled in what they agree is a shortage-sharing platform to avoid the undesirable aspects of Lake Mead falling to 1,025 feet above sea level – the lowest shortage trigger level contemplated in the 2007 Guidelines.

2017  Heavy winter snows in the Rocky Mountains provide enough water to the Colorado River to avoid any possibility of a shortage declaration in 2018, according to the Bureau of Reclamation.

United States and Mexican governments sign an agreement to the 1944 Water Treaty between the two countries called Minute 323. The Minute extends 2012’s Minute 319 that gave Mexico greater flexibility in managing its Colorado River allotment. The latest agreement provides mechanisms for increased conservation and water storage in Lake Mead to help offset the effects of drought and prevent a shortage from being triggered. Minute 323 dedicates 210,000 acre-feet of water over nine years for environmental restoration work in the Colorado River Delta.

2018 Bureau of Reclamation releases Tribal Water Study. It describes how tribal water use fits into the overall picture of Colorado River management, how future development of tribal water resources will alter river operations (including others using water to which a tribe may hold legal title) and how future development of tribal water rights will affect Basin operations.

2019 President Trump signs the Drought Contingency Plan. The DCP commits the seven Colorado River Basin states — California, Nevada, Arizona, Colorado, New Mexico, Utah and Wyoming — to a plan centered on the idea that all water users, not just those with junior water rights, have a stake in keeping the system whole by taking voluntary reductions on their Colorado River deliveries.

2021 With the water level in Lake Mead projected to reach 1,066 feet above sea level by Jan. 1, 2022, the first-ever shortage declaration was triggered, requiring Arizona, Nevada and the country of Mexico to reduce their take of the river in 2022. California, with the largest share of the river, was not required to cut back, but could face reductions if Lake Mead’s water level drops further.

2022 With the water levels in Lake Mead and Lake Powell continuing to drop, federal officials announce deeper water supply cuts for Arizona, Nevada and the country of Mexico in 2023 and further direct the seven states and tribes that rely on the Colorado River to develop plans to reduce consumption by 2 million to 4 million acre-feet a year.