The bus-sized boulder that crunched the top of Madison Dam about a month ago still sits on top of the dam, but officials expect the work to remove the boulder and fix the dam to begin in earnest in the coming weeks.

The boulder sloughed off the cliff above the west side of the dam in the early hours of Aug. 30. The accident led to the draw down of Ennis Lake as PPL engineers and crews worked with large sand bags to stem the water flowing around the boulder and over the dam.

However, from the beginning the concern has been with the remaining boulders on the rock face above the dam, said David Hoffman, spokesman with PPL, the company that owns and operates the dam.

The next step is to put a barge into Ennis Lake and navigate it down to the top of the dam, which could happen in the next week or two. The barge will be big enough to hold a crane that will allow crews to safely secure the remaining boulders along the rock face, Hoffman said.

“Once that rock wall is secured, then we will begin to build the coffer dam to isolate the rock that’s on the dam,” he said.

The coffer dam will allow crews to begin removing the boulder and fixing that part of the dam, Hoffman said.

“We’re hoping to have that done by the end of the year,” he said.

The Ennis Lake and lower Madison River levels are down to where they are for winter and will stay there until spring, he said.

The road down to the dam will be open when possible, Hoffman said. However, the road will continue to be closed when needed by crews and heavy equipment.

“We’ll have staff at the site to direct the public and provide information,” he said.

UPDATED Sept. 7: Large sandbags have slowed the flow of water around the boulder that still rests on the crest of Madison Dam on Ennis Lake.

The sandbags were part of an effort to control water going over the dam around the sides of the boulder, said PPL spokesman David Hoffman.

The bags were set into place on Tuesday and slowed the flow of water around the rock from about 500 cubic feet per second to about 190 cfs, Hoffman said.

“It’s pretty well got the flows stemmed,” he said. “It’s helping a lot.”

The plan now is to open the road to the dam on Friday at sunrise and keep it open through the weekend at least, Hoffman said.

The west side of the Madison Dam was pounded Aug. 30 by a huge boulder that sloughed off the cliff face over hanging the dam. The boulder is about 30 feet long and 12 feet wide and the majority of it is positioned up stream of the dam and resting on sediments on the bottom of the reservoir.

The dam is owned and operated by PPL Montana.

A large boulder still clings to the cliff above the dam and in the 1970s engineers anchored it into the cliff face.

A Thursday investigation of the Madison Dam and the rock face eased concerns PPL officials had about the structural integrity of the dam and the stability of the rocks above it.

Part of Thursday’s inspection focused on remaining rocks and portions of the cliff that over hang the west side of the dam. That investigation continued Tuesday as geologists were taken by helicopter to the top of the overhanging cliff to inspect the rocks that remain, Hoffman said.

“The next step is going to be securing the overhanging rocks with drilling and large rock bolts and cables,” he said.

That will probably take a couple of weeks and then PPL will begin work on a cofferdam to isolate the boulder on the dam and begin the process of breaking the huge rock apart and removing it.

The tentative time schedule is to have the cofferdam built in October and the boulder removed and the dam fixed by the end of the year, Hoffman said.

Ennis Lake has been drawn down about five feet below full levels and officials are keeping that level stable for now.

“We don’t plan to bring it down anymore, as a matter fact we’ll probably start to inch it back up here in a few days,” he said.

Despite concerns over fish problems in the lake, Hoffman said the draw down was slow enough that fish weren’t stranded on the flats at the south end of Ennis Lake.

“The way water came out of there so slowly, you just didn’t get the stranded fish,” he said.

The flows in the Madison River below the dam are holding at 1,400 cfs and should be pretty stable for now, Hoffman said.

Meanwhile the draw down on Ennis Lake continues through Thursday with the goal of being down five feet below full by Friday morning, Hoffman said.

“I don’t think you’ll see much of a change in that for a while,” he said.

UPDATE: Friday, Sept. 3

A Thursday investigation of the Madison Dam and the rock face eased concerns PPL officials had about the structural integrity of the dam and the stability of the rocks above it.

The road to the dam, which is also used to access Bear Trap Canyon will be opened Saturday for the Labor Day weekend.

The west side of the Madison Dam was pounded Monday by a huge boulder that sloughed off the face over hanging the dam. The boulder is about 30 feet long and 12 feet wide and the majority of it is positioned above the dam and resting on sediments on the bottom of the reservoir.

Part of Thursday’s inspection focused on remaining rocks and portions of the cliff that over hang that part of the dam, said David Hoffman, spokesman for PPL, which owns and operates the dam.

A large boulder clings to the cliff above the dam and in the 1970s it was anchored into the cliff face by engineers.

“The danger level of that over hanging rock doesn’t seem as quite as sever as we initially thought,” Hoffman said.

However, prior to any extensive work on the dam and the boulder, PPL will work to secure more rocks above the dam, he said.

“I suspect we’re looking at additional rock bolts and ties to secure that area,” Hoffman said.

Meanwhile the draw down on Ennis Lake continues through Thursday with the goal of being down five feet below full by Friday morning, Hoffman said.

The drawn down has been slow enough that turbidity in the lower river has been minimal despite the water levels being more than double of what they normally are this time of year. Thursday morning, the flows below the dam were 2,780 cubic feet per second.

The road to the dam will be closed again on Tuesday and be closed at least through the week while crews work on the dam, he said.

Next week crews will place composite bags around the boulder to try and stop the water flowing around it. Composite bags are essentially large sand bags that will be placed by heavy equipment, Hoffman said.

“If we can do that we can bring the lake back up to winter levels which are two feet below full,” he said.

A complete plan for fixing the dam isn’t complete yet, but Hoffman anticipates that some sort of cofferdam will eventually be used to isolate the boulder and allow the water level in the lake to be brought back up.

“I think right now we’ll probably see a cofferdam to isolate that boulder so ultimately it can be removed,” he said.

Hoffman also expects river flows below Madison Dam will be back to their seasonal normal – about 1,400 cfs – for the weekend.

For PPL’s official press release, click here.

UPDATE: Wednesday, Sept. 1

Damage to the Madison Dam on Ennis Lake appears to be isolated to the west end of the dam after a huge boulder fell on it Monday morning, said a PPL engineer Tuesday night at a public meeting in Ennis.

The boulder, which sloughed off the cliff above the dam, is still lodged in the hole it created, said Carrie Harris, hydro engineer with PPL.

The boulder is about 30 feet long and 12 feet wide and the majority of it is positioned above the dam and resting on sediments on the bottom of the reservoir, she said. It damaged about 20 feet of the dam.

The boulder fell on the dam in the early hours of Monday morning, setting off alarms and severing hydraulic lines used to operate the dam’s gates. However, the integrity of the dam appears to be intact, Harris said.

The main dam structure itself is about 35 feet high and the gate mechanism that spans the width of the dam is about nine feet high. Officials are lowering the water level in Ennis Lake to get a better look at the damage.

The plan initially is to only lower the water level about four feet, said David Hoffman, spokesman for PPL. This should be done by Thursday morning.

The flows below Ennis Lake on Wednesday morning were about 3,100 cubic feet per second. There’s about 500 cfs flowing around the rock, but that could decrease as the water is lowered, Hoffman said.

After the water is lowered four feet, the flows will be decreased to about 2,000 cfs below the dam, he said.

Safety of the public and workers at the dam are the first priority, said Jon Jourdonnais, biologist with PPL. However, the second priority is the fisheries of the Madison River above and below the dam.

“Our primary goal (from a resource standpoint) is to keep the upper Madison River unaffected,” Jourdonnais said.

PPL owns and operates both Madison River dams – Hebgen and Madison Dams. However, they aren’t planning on changing their regular operation of Hebgen Dam because of the problems at Madison Dam.

As part of their license with the Federal Energy Regulation Commission to operate the dams, PPL is supposed to keep at least 1,100 cfs going through the Madison Dam. That shouldn’t be a problem, he said.

Initially, PPL officials thought they would have to draw Ennis Lake down nine feet, but that may not be necessary, Hoffman said.

A decision on a plan of action for the repair of the dam will wait until more inspections are completed Thursday, Harris said.

“We just want to put eyes on it to ensure our gut and how we feel is really accurate,” she said.

Falling rocks have long been an issue at Madison Dam, she said.

One concern moving forward is other large rocks on a cliff face that overhangs the west side of the dam.

“Dealing with rocks and that wall at the Madison (Dam) is historically an issue,” Harris said.

The largest rock overhanging the dam was anchored into the cliff face in the 1970s, she said. Back then engineers drilled holes at a 45-degree angle into the rock and cliff and then filled them with concrete.

Before any work is done, Harris wants to make sure that rock and its anchors are still holding firm.

“We’re happy the rock’s anchored, but we really don’t have any warm and fuzzy feelings about it,” she said.

Unfortunately, the draw down of Ennis Lake is going to render it virtually useless from a recreation standpoint, Hoffman said.

Ennis is a very shallow lake with an average depth of about 10 feet. A few people at the meeting voiced concerns about fish and waterfowl using Ennis Lake.

The fish should move with the water level, said Pat Clancey, local Montana Fish, Wildlife and Parks fisheries biologist.

As the mud flats on the south end of the lake are exposed, Clancey and his crew will be out there trying to move fish that are stranded in isolated pools.

Right now, the hope is that the damage to the dam will be such that a cofferdam can be built around the boulder, which would allow the water level to be brought back up, Harris said.

A cofferdam could take a few weeks to build, she said.

“We’re going to get the lake back up as soon as we can,” Jourdonnais said.

PPL is holding off on making any firm repair plans until after the Thursday inspection, Hoffman said. After the inspection, it could take engineers a few days to determine the next course of action.

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