SAN FRANCISCO • 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 earlier this week, in which nearly 190,000 residents were abruptly evacuated from a valley below the tallest dam in the United States, illustrates the safety risks of the Golden State's ageing infrastructure in increasingly populated areas.
Sixty-four California reservoirs, or around 5 per cent of the state's total, are restricted to holding less than their rated capacity owing to earthquake risks and other concerns.
At the same time, California's burgeoning population is putting increasing numbers of residents in the path of catastrophe if dams fail, said University of California, Berkeley engineering professor Nicholas Sitar.
"The number of people who live in the drainage area 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 added.
In Oroville, record rainfall pushed waters to near the top of the dam, and two spillways built to relieve pressure suffered damage and erosion. A giant hole opened in the dam's main spillway last week, forcing the authorities to activate an emergency overflow channel last Saturday for the first time. But it began eroding, threatening a rupture that would have sent water surging towards the valley below.
Danger from flooding at Oroville subsided on Monday, although helicopters were readied to drop rocks into eroded areas in the emergency spillway ahead of rain forecast for today and tomorrow. 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 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 added 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 report by the US Army Corps of Engineers.
California has not had a major earthquake in two decades, but seismic retrofitting is a constant worry in the Golden State's 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, Prof Sitar said, adding that it would cost up 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 Mr Adam Scow, California director for environmental group Food and Water Watch. He called for local, state and federal agencies to all pay up.
More than half of California's 1,585 dams would result in deaths if they failed, putting them in the category of "high hazard potential", according to the US Army Corps of Engineers. Oroville is part of that list, which does not refer to the integrity of the dam, but only the consequences of failure, according to the Federal Emergency Management Agency.
REUTERS, AGENCE FRANCE-PRESSE