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UPDATE: PPL Montana has begun to draw down Ennis Lake to allow engineers to inspect the damaged Madison River Dam.

A huge rock, measuring half the size of a house, fell from a cliff over the dam at about 3 a.m. Monday, crashing into the west side of the dam. The rock crushed one end of the dam and damaged at least two of the hydraulically operated gates releasing water from the lake.

About 12 to 15 gallons of hydraulic fluid used to operate the gates also leaked into the river. PPL Montana says it will drop the level of Ennis Lake by about five feet over the next few days, eventually lowering the lake about nine feet to allow dam repairs to begin.

Engineers say they don't know yet what caused the massive rock to fall on the dam.

"Speculation only, a lot of moisture in this region over the last several days, weeks even and water in the cracks of the rock. It's fractured rock anyway, beyond that no, no sign of any seismic activity or earthquake that would have caused it," PPL Director of External Affairs David Hoffman said.

Hoffman says until engineers get a closer look at the damage, it is impossible to predict how long repairs might take. He says lowering the lake will increase flows downstream on the Madison, but not enough to make boating or fishing dangerous.

A boulder broke loose and fell on Madison Dam near Ennis Monday, causing a small oil spill and impacting river levels.

PPL Montana reports in a news release that no one was injured and the facility is in stable condition.

"We've taken immediate action to start an emergency drawdown of Ennis Lake so we can identify where repairs need to be made and reduce pressure on the dam. There is leakage around the boulder in the spill gate section on the west side of the dam," said David Hoffman, director of External Affairs for PPL Montana.

The damage is causing river flows downstream of the Madison Dam to run about 2,000 cubic feet per second, which is about 500 cubic feet per second more than before the event.

"The lake will be lowered by about 9 feet, which will affect boat docks and public launch sites along the lake," he said. "We apologize for this inconvenience to our neighbors at Madison Dam, but the drawdown is required to make repairs to the dam."

PPL Montana engineers have inspected the dam and will develop plans to remove the boulder and make repairs, Hoffman said. Spill gates damaged by the falling boulder cannot be opened, but three undamaged spill gates are functioning. The hydroelectric plant continues to generate electricity.

Booms have been deployed in the Madison River to capture a small amount of oil - about 15 to 20 gallons - that spilled into the Madison River when the boulder severed a hydraulic line. No additional oil is leaking into the river. Flashboards were also damaged as a result of the falling boulder.

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.