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SAN FRANCISCO (Reuters) - In the mountainous folds of California lie hundreds of dams that played a vital role in making it America's wealthiest and most populous state.
The Oroville Dam crisis this week, in which nearly 190,000 residents were abruptly evacuated from a valley below the tallest U.S. dam, illustrates the safety risks of the Golden State's ageing infrastructure in increasingly populated areas.
Sixty-four California reservoirs, or around 5 percent of the state's total, are restricted to holding less than their rated capacity due to earthquake risks and other concerns, a state dam safety official said on Monday.
At the same time, California's burgeoning population is putting increasing numbers of residents in the path of catastrophe if dams fail, said Nicholas Sitar, an engineering professor at the University of California, Berkeley.
"The number of people who live in the drainages below those dams has increased to the point where a significant release of water from those reservoirs can have a very significant impact on the population below," he said.
In Oroville, record rainfall had pushed waters to near the top of the dam and two spillways built to relieve pressure had suffered damage and erosion. Authorities believed the emergency spillway, which is earthen, was on the verge of collapse and issued swift, stern evacuation orders.
Danger from flooding at Oroville subsided on Monday, but Northern California is on track to mark its wettest winter on record, and the storm waters have created unexpected problems for a state that has struggled with years of drought.
California's dams are a lifeline to farms that supply fruits, vegetables and nuts to the nation, as well as to thirsty cities throughout the state, some of which do not see any rain in the summer months.
California Department of Water Resources engineer Eric Holland, in the Division of Safety of Dams, said restrictions on capacity affected 64 reservoirs out of the 1,250 dams overseen by the agency.
He said he was not allowed to identify specific dams, but that Oroville was not on the list. He did not describe what the state was doing to improve its dams, which are owned by private companies, local governments, the federal government, public utilities and the state.
Around 1,140 of California's dams were built before 1970, and only 52 have been built since 2000, according to a U.S. Army Corps of Engineers report.
California has not had a major earthquake in two decades, but seismic retrofitting is a constant worry in Golden State infrastructure.
"Most of the issues that I know of in California actually have to do with seismic deficiencies, mainly because our knowledge of seismicity has significantly evolved" in the last 20 years, Sitar said, adding that it would cost hundreds of millions to a billion dollars to fix the situation.
Environmentalists said the Oroville Dam crisis was a "wake-up call" for state leaders to fix existing water infrastructure instead of funding new projects, such as a tunnel to divert river water around the San Francisco Bay Delta to consumers in the south.
Billions of dollars are required to improve dam infrastructure in the state, said Adam Scow, California director for environmental group Food & Water Watch. He called for local, state and federal agencies to all pay up.
More than half of California's 1,585 dams would lead to deaths if they failed, putting them in the category of "high hazard potential," according to the U.S. Army Corps of Engineers.
Oroville is part of that list, which does not refer to the integrity of the dam, only the consequences of failure, according to the Federal Emergency Management Agency.
Reporting by Rory Carroll, editing by Peter Henderson and Mary Milliken