Work has been underway for the last few weeks at Hebgen dam, in the Madison-Missouri system, to determine the cause of the higher discharges and the falling lake levels. The utility established the problem was caused by stoplog failure at Bay 4 of the dam.
River flows had increased to 3400ft3/sec, which is well beyond the seasonal rate of up to 900ft3/sec but comparable to springtime flowrates in Madison river.
The utility is using the intake control gates as well as replacement stoplogs to regain flow control at the dam, and expects to have the flowrate back to normal levels within days.
Earlier efforts to control the flow involved using a specially fabricated steel bulkhead but the option was dropped due to installtion of the required guides being prevented by the water pressure and turbulence of the flow.
PPL Montana said that data from onsite monitoring equipment showed the dam to be safe and stable.
