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Dredging altered the natural contours of rivers and disturbed the gravel beds used by salmon tor spawn- ing. Hydraulic and placer mining washed debris and sediments into rivers and streams that eventually clogged them and prevented some from discharg- ing into larger rivers. The population explosion that accompanied the Gold Rush also affected the terrestrial environment. Riparian forests were quickly cut down by new immigrants for fuel and construction materials. Native species of birds and mammals were hunted for food, clothing or export, disrupting the complex ecosystems that had evolved over previous millen- nia. Nonnative species were introduced, deliberately and accidentally, which disrupted the delicate balances of native ecosystems and sometimes led to extinction of California species. Early California settlers quickly recognized the potential of the Delta and the then-marshy Central Valley for farming. Land reclamation began in the Delta as early as the 1860s with the construction ot SUPPLYING WATER As California's population mushroomed after the Gold Rush, so did its water needs, Coastal areas such as San Francisco and Los Angeles were becoming urbanized. Between 1913 and 1923, Los Angeles, San Francisco and a group of East Bay cities built or planned reservoirs in the Sierra Nevada to deliver water by aqueduct to coastal urban areas. Conservationists such as John Muir objected to the projects, but their pleas to save the Sierra water sources tapped by the coastal cities were ignored. Meanwhile, support was growing for a statewide water plan that would address the needs at agricul- tural and urban water users as well as provide flood protection and of her benefits, In 1 933 California voters approved the Central Valley Project (CVP), but Depression-era financing problems caused the project to be turned over to the federal government in 1937. The start of CVP construction in the late 1930s marked the beginning of a massive replumbing of the state's water system that would transform the Delta and the river systems that supplied it. In 1951 the state Legislature authorized construc- tion of the Feather River Project, which later became known as the State Water Project, or SWP, Water that originates in the SWP's Oroville Reservoir is channeled through the Delta to the North Bay crude levees to drain wetlands, By 1930, about 313,000 acres of former tidal and nontidal wetlands in the Delta had been placed behind constructed levees, Today, more than 1,000 miles of levees guard some 57 man-made Delta islands, some of which have land surfaces that lie 20 feet below the surrounding water level because of subsidence, making them ditficult or Impossible to restore, Throughout the Central Valley watershed, farmers built hundreds of small dams and miles of levees on streams and rivers to divert water to irrigate their crops during the dry summer season and protect their land from flooding. The thirst for water to supply agricultural needs and the growing cities led to construction of even larger water-storage and delivery facilities. Today, about 7 million acres of land in the Central Valley are under cultivation, using about 26 million acre-feet of water per year for irrigation (an acre-foot of water, about 326,000 gallons, will cover a football field 1 foot deep, and meet the average annual indoor and outdoor needs of one to two urban households). Aqueduct and the California Aqueduct, which supply drinking water for parts of the Bay Area and southern California, and irrigation water for San Joaquin Valley agriculture. The federal CVP delivers about 7 million acre-feet of water per year, the SWP about 3 million acre-feet. The CVP and SWP provided enormous benefits to agriculture, helping to make California the leading farming state in the U,S, with about $20 billion in annual crop production by the end of the 20th century, The projects contributed to the post-World War II economic and population booms that made Califor- nia the nation's most populous state by the mid- 1 960s. They also helped stanch flooding that had periodically plagued urban areas such as Sacramento, and contributed much needed electric- ity to the state's power grid, But if harnessing the rivers brought agricultural bounty and protected California's growing popula- tion against floods, it also changed California's land- scape, particularly the Central Valley, by drying up wetlands, removing riparian areas, and altering habitat for birds, fish and mammals. By one estimate, less than 10 percent of historic riparian habitat in the Central Valley remains today, and about 80 percent of hisforic salmon spawning habifaf has been blocked by dams, s