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HELENA - Anyone who recreates at Canyon Ferry Reservoir knows it can be an incredibly dangerous body of water.

It's difficult to come by actual numbers because so many agencies have overlapping jurisdiction at the reservoir east of Helena. But according to anecdotal evidence, about 20 people have died in accidents at the popular camping, boating and fishing area. Most of those deaths occurred toward the southern half of the lake, and many were caused by gusty winds that produce unusually large waves for a Montana lake.

"Typically, on this lake I think people underestimate how quickly the waves can build," said Gil Alexander, who with his wife Marilyn has run the Montana Science Institute at Canyon Ferry for the past 19 years.

"From Cemetery Island to the dam, you rarely get waves that can't be handled by a 16-foot boat. But down by Goose Bay, along that part of the lake, you get some pretty treacherous areas out there."

The size and location of Canyon Ferry combine to create a recipe for disaster when the wind starts blowing - which it usually does between 3 and 5 p.m. every day. The reservoir is 23 miles long from the mouth of the Missouri River to Canyon Ferry dam, and lies at a northwest to southeast angle.

About 80 percent of the time, the wind blows from the north or northwest, which means that the wind crosses a large expanse of water as it moves from the northern dam area toward the southern edge of the lake near Townsend.

The farther across the water the wind blows, the larger the waves become, Alexander said. Wind creates ripples, and if it continues to blow the ripples build upon themselves, increasing the wavelength and height.

"By the time the wind gets down to the south end of the lake, that 10 mile per hour wind can produce 4 to 5 foot waves," he said. "So quite often when you may be having fairly quiet water around the dam, down around the Silos and Goose Bay you can be having a tremendously difficult time."

If the waves are high and the wind is strong, the crests of the waves can blow away, forming sheets of spray and whitecaps. One of the most dramatic cases of high wind was in December of 1995, according to Alexander, when this area experienced 60 to 100 mph winds. The waves were 5 feet tall and higher, and spray was lifted nearly 100 feet into the air.

On Sunday, the Alexanders were at their home along the east shore of Canyon Ferry. They noticed quite a bit of ice between Cemetery Island and the dam, and the wind was blowing hard enough that it moved the ice across the lake into piles on the shore.

"It was pushing up on shore and would fall over, then crest and break and pile up again on shore," Alexander said. "So there was a tremendous wind all afternoon Sunday. It wasn't gusting, it was just a continuous wind. I can't imagine anyone going out on the lake in those conditions."