Last Sunday, hundreds of hikers and members of the Havasupai tribe had to be evacuated from a remote canyon connected to the Grand Canyon. Early press reports cited a statement from the National Park Service attributing the surge of water to the failure of the “Redlands Dam,” an earthen dam allegedly over 50 miles upstream. Scarce information about the dam was available on the web. It’s not one of the major dams in the state operated by the Bureau of Reclamation, and it doesn’t appear on this comprehensive state list.
The Arizona state geologist reports its now thought the dam breach was a minor contributor to the flood.
The dam, seen here, was created by a ranch to retain water.
Why might it have failed? According to this geologic study, the area of the dam contains deposits of coarse gravel, which makes a particularly bad foundation for a dam since the water can leak through air spaces in the material. Since the wash drains a huge area of desert, any unusual rain event can cause flooding so it seems reasonable the dam failure played a minor role.
However, there are larger dams in the state worrying officials. The Arizona Department of Water Resources maintains a list of unsafe dams in the state. Four dams are rated in the most serious category, “Unsafe Dams with Elevated Risk of Failure,” which the state thinks could fail during a “100-year or smaller flood event.” The dams are Fredonia, Powerline, Magma, and Cook Dams.
Although dam failures are generally rare, Wikipedia contributors have compiled this list of U.S. accidents. One of the most spectacular was the 1976 collapse of Teton Dam in Idaho, which luckily only killed 11.