Managers at the nation’s tallest dam made a critical mistake by allowing the lake behind it reach its highest level ever. That’s according to Bill Connolly, a Butte County supervisor in California whose district includes Oroville Dam. The Associated Press reported Wednesday that dam managers obtained an uncommon exemption from the Army Corps of Engineers to bypass a rule that would have required them to release huge amounts of water from the rapidly filling dam. The rising water in early February eventually topped a never-before-used spillway, which started breaking apart. That triggered an evacuation order for nearly 200,000 people. Connolly says that backup spillway was never intended to be used for flood control so “they never should have let the lake overflow.” He calls the dam poorly run.