In the mountainous folds of California lie hundreds of dams that play 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 US dam, illustrates the safety risks of the Golden State’s ageing infrastructure in increasingly populated areas. Sixty-four California reservoirs, or around five per cent 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.

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 drought for years.

California’s dams are a lifeline to farms that supply fruits, vegetables and nuts to the nation, as well as water 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 US Army Corps of Engineers report.