Water breaches the Edenville Dam in Michigan on Wednesday, May 20. (CNN photo by Sarah Tilotta)
Thousands of Michigan residents were forced to flee their homes yesterday evening after heavy rains caused the Edenville Dam to fail. The hydroelectric dam unleashed so much water down the Tittabawassee River that the 2,600-acre Wixom Lake essentially drained itself and breached the nearby Sanford Dam as well, Paul Murphy reports for CNN. Most of Midland, a city of 41,000 downstream, was told to evacuate.

The incident highlights the poor condition of many of the nation's dams, and the difficulty of regulating privately owned dams. According to the National Inventory of Dams, the Edenville and Sanford dams are both privately owned by Boyce Hydro Power LLC ; both were built in 1925, and both were last inspected on June 26, 2018, and rated as a high hazard to the public if they fail.

About 64 percent of the nation's 91,000 dams are privately owned, which can make them trickier to regulate. It's difficult for "regulators to require improvements from operators who are unable or unwilling to pay the steep costs," David Lieb, Michael Casey, and Michelle Minkoff report for The Associated Press.

Another problem is that the nation's dams are aging: 69% were built before 1970, and 17.1% are high-hazard. The Association of State Dam Safety Officials estimates that it would take more than $70 billion to modernize the nation's dams, the AP reports.
Federal Emergency Management Agency map, based on U.S. Army Corps of Engineers data