How many U.S. dams are at risk of a crisis like the one currently unfolding in California, as officials work to stave off disaster at the compromised Oroville Dam? The short answer is, we don't really know – but probably quite a few.
Until this month, Oroville's emergency spillway had never been used in its nearly 50-year history. After weeks of rain and a breach in the dam's main spillway, officials turned to the auxiliary one to help prevent an overflow from the dam's lake. The resulting hillside erosion threatened serious flooding and prompted the evacuation of some 188,000 people.
Environmental groups predicted the crisis in 2005 when they called for the emergency spillway to be reinforced with concrete, but state officials dismissed that request as unnecessary. Though cost wasn't publicly cited as a factor, critics can reasonably contend it was a consideration.
If so, "it's certainly not the only dam that put off upgrades because of upfront cost," says Jenny Rowland, public lands research and advocacy manager for the policy think tank Center for American Progress.
Oroville is just one of more than 90,000 dams across the country, many of which are aging and underfunded, prompting the American Society of Civil Engineers (ASCE) to give the nation's dams a "D" grade in its last infrastructure report. In Minnesota, for example, repairs to the 107-year-old Byllesby Dam have been postponed for lack of funding.
Byllesby is one of more than 15,000 dams federally classified as "high hazard potential," which says nothing about a dam's actual condition, but indicates that a failure would result in "probable loss of life." Missouri, North Carolina, and Texas have the highest concentrations of those dams, according to the infrastructure report – and there are at least a couple dozen in every state.