Michigan has thousands of aging and under-maintained dams that could pose localized risks. The Michigan Department of Environmental Quality tells the Detroit Free Press that all but six of the state's 88 potential high-hazard dams are approaching 50 years old, the average engineered life span for a dam. The American Society of Civil Engineers gave Michigan a D grade on the condition of its dams in 2009, saying more than 90 percent of the state's nearly 2,600 dams would reach or exceed their design life by 2020.