PPL Montana is reducing water level in Ennis Lake as the first step in a plan to assess and repair the damage caused when a large boulder, about the size of a bus, broke loose and fell onto the Madison Dam on Monday (8/30).

“Safety of the public and employees is the top priority as we begin the response effort,” said David Hoffman, director of External Affairs for PPL Montana. “The facility remains in a stable condition and there is no need for public action.

“We’re also sensitive to the importance of the fisheries, including angling and boating on this section of the Madison River, and to the local economy. We will work closely with local officials to limit any effects on recreation through the Labor Day weekend,” he added.

The immediate priorities of PPL Montana are to draw down Ennis Lake to reduce pressure on the dam while experts assess the status of a large section of rock remaining on the canyon wall above the dam.

“To safely remove the boulder that fell on the dam, we must first understand the condition of the rock remaining above the dam, and take appropriate action to remove or secure the rock,” Hoffman said.

PPL Montana has increased the water release rate on the lower Madison River to 3,300 cubic feet per second, which will speed the drawdown and have a limited effect on downstream recreational uses of the river.

To maintain recreational uses in the upper Madison River above Ennis Lake, PPL Montana proposes no flow reductions below Hebgen Reservoir at this time.

Local law enforcement officials have closed the road to the dam at Trail Creek trailhead, in effect closing the Madison River immediately downstream of the dam to recreational uses.

The current drawdown rate will reduce the level of Ennis Lake by about 5 feet in the next two days. PPL Montana will then hold the lake at that level while determining the best course of action with the rock remaining above the dam.

When those plans are in place, PPL Montana may draw down Ennis Lake further, to a level about 9 feet below normal, to conduct a full inspection of the dam, remove the boulder that fell Monday morning and make needed repairs.

The drawdowns will affect boat docks and public launch sites on Ennis Lake.

“PPL Montana engineers have conducted an initial inspection of the dam and concluded that the damage from the falling boulder is limited to some of the spill gates and equipment,” Hoffman said. “There is no noticeable deformation of the dam itself.”

Damaged equipment on the dam includes three spill gates, walkways and wooden flashboards that sit atop the dam and raise the level of water it is able to hold back.

The rock fall did result in a minimal spill of oil, which PPL Montana determined to be about 12 gallons, from hydroelectric plant equipment. Absorbing booms were placed in the river Monday as far as 12 miles downstream of the dam and collected most of the oil. Foot patrols downstream of the dam observed no effects from the minimal amount of oil that got into the river.

PPL Montana is coordinating its response plan with government officials from the local level up to the Federal Energy Regulatory Commission, which is responsible for oversight of hydroelectric facilities such as Madison Dam.

The company is meeting with representatives of government and regulatory agencies Tuesday morning (8/31) and members of the community Tuesday evening to update them on the response plans.

Madison Dam is a four-unit hydroelectric plant on the Madison River at the head of Bear Trap Canyon about 10 miles north of Ennis. The units have a total generating capacity of 9 megawatts. Madison Dam is 257 feet long and 35 feet high.

PPL has 500 employees in Montana who generate and sell electricity. PPL Montana operates coal-fired power plants at Colstrip and Billings, as well as 11 hydroelectric power plants along Rosebud Creek and the Missouri, Madison, Clark Fork and Flathead rivers. It has a combined generating capacity of about 1,200 megawatts and has offices in Billings, Butte and Helena. PPL EnergyPlus operates a trading floor in Butte that markets and sells power in the wholesale and retail energy market. PPL Montana and PPL EnergyPlus are subsidiaries of PPL Corporation (NYSE: PPL).