"Dams are considered "high hazard" if a failure would kill people. The number of those dams has grown to nearly 15,500 because of development and population growth below dams.
They are already failing. In the last two years, 80 failed in South Carolina alone due to heavy rains and a hurricane, causing millions of dollars in property damage.
"A lot of dams were built many years ago — in the '40s, '50s, '60s and even into the '70s," said Mark Ogden of the Association of State Dam Safety Officials. "They were built to the engineers' standards of the time. But a lot of those standards have changed and a lot of been learned about how dams perform."
The price tag for repairing and upgrading the nation's dams: $54 billion, Ogden's group estimates.
Climate change heightens dam risks, especially in California. The state has long had a boom-and-bust cycle of droughts followed by intense rain, as was illustrated by last month's flooding. Scientists warn that pattern will become more severe, with more precipitation falling as rain, rather than snow. That, plus rising temperatures melting the Sierra Nevada's snowpack more rapidly will intensify the strain on the state's 1,585 dams."