Risky reservoirs: Montana counties where aging dams pose the greatest threat

Record rainfall pelted parts of Iowa for 12 hours straight in July 2010, sending 10 inches of rainwater draining toward Lake Delhi, Iowa, a popular recreation destination for decades since its construction in the 1920s.

Holding back 9 miles of lake water with more precipitation flowing in by the minute was the 80-year-old Lake Delhi Dam, constructed of hundreds of feet of earth, concrete, and steel. The water punched a 30-foot-wide hole in the dam wall, spewing floodwater into the surrounding region.

Emergency and state officials ord...