Why are people concerned about dam safety?

Rivers are permanent; dams are not. Large dam-building technology is not that much older than nuclear power technology. Large dams have failed in the past, and will do so in the future. In 1983 Glen Canyon Dam nearly overtopped and experienced a near catastrophic failure of its spillway tunnels. This could have resulted in severe flooding downstream causing complete draining of the reservoir. The hydrologic conditions that brought this about were far from unique, and will occur again. Furthermore, the porous and erosive Navajo sandstone surrounding the dam may at some point give way itself. In fact, safety concerns about dams are a nationwide problem; three years ago, the American Society of Civil Engineers gave the US dam inventory a rating of "D" in terms of safety.