About 150 million years ago, the Rocky Mountains began to rise with an uplifting of sedimentary bedrock that formed the mountains present in the Flathead watershed today.

During the last Ice Age, glaciers sculpted the landscape. David Alt, a professor of geology at The University of Montana, and the author of Roadside Geology of Montana, explains how Flathead Lake was created in the following article.

A Freak Preserved by an Accident:
The Making of Flathead Lake

by David Alt

In one way or another, all lakes and ponds are freaks. Water naturally tends to run down through streams to the ocean. Lakes can exist only in places where something prevents water from running down hill. So every lake must have its reasons. The reasons for Flathead Lake's existence start back in the last ice age and extend far beyond the lakeshore.

Look at a map sometime to see what happens to the Flathead Valley north of the Canadian border. You will see that it extends all the way through British Columbia and into the southern Yukon as an almost perfectly straight trough with steep valley walls on either side. Geologists call it the Rocky Mountain trench and the Flathead and Mission Valleys are its southernmost end.

During the last ice age, and presumably during the earlier ones as well, an enormous glacier filled the Rocky Mountain trench. Ice poured off the neighboring mountains and down their valleys as smaller glaciers that met and coalesced in the Rocky Mountain trench. They filled it from wall to wall with a mighty river of ice thousands of feet thick and 20 or more miles wide.

As the ice age continued, the monstrous glacier slowly pushed south through the Flathead Valley where the north end of the Mission Range split it like a wedge diverting one branch into the Swan Valley and the other into the Mission Valley.

The branch that went down the Mission Valley made it to the vicinity of Ninepipes Reservoir, where the ice was so deep that it completely buried the Mission Range about as far south as Ronan. That explains why the range is so smoothly rounded north of the Ronan area and so rough and craggy south of there. The contrast is really quite striking.

Glaciers end when they reach a place with a climate warm enough to melt the front of the ice river back at the same rate it advances. We can think of glaciers as natural conveyor belts because they carry enormous loads of sediment of all sizes ranging from clay to enormous boulders. Where glaciers end, they dump much of that debris to form a ridge called a moraine.

The big moraine at Ninepipes is especially conspicuous because it contains hundreds of little lakes and ponds full of ducks. Each little lake and pond marks a place where a chunk of ice was once buried in the moraine and then later melted leaving a depression.

After the moraines had formed, the climate changed slightly and the ice front melted back a few miles. New moraines formed just south of Polson in the

Mission Valley. The Polson moraine is especially easy to see because it forms a prominent ridge across the valley. People driving north on Highway 93 get their first view of Flathead Lake just as they reach the crest of the Polson moraine. Nobody forgets it!

The large size of the Polson moraine suggests that the ice front must have remained at that position for quite a few years. During the summers of those years, glacial meltwater washed loads of sand and gravel through old stream channels across that moraine and spread them across the valley floor to the south. Those outwash deposits, as they are called, completely fill what would otherwise be a low area between the Polson and Ninepipes Moraines. Ronan and Pablo both stand on those glacial outwash deposits.

There is abundant evidence that the great ice age glaciers melted very rapidly, probably within a couple thousand years. As the glaciers thinned and began to shrivel they must also have slowed in their movement. Finally they stopped moving and became great sheets of stagnant ice lying upon the land.

I think Flathead Lake fills a basin that exists where a large mass of stagnant ice survived for a few centuries after the end of the last ice age. Had that ice not been there, the present lake basin would have filled with outwash sediments. The same explanation can be extended to most of the other large and small lakes that spangle the floor of the Rocky Mountain trench. But why does the lake basin remain unfilled and undrained?

In fact, Flathead Lake is partially filled. During the eight to ten thousand years of its existence, the Flathead River has built a large delta into the north end of the lake. The road between Bigfork and Somers goes right across the lower part of that delta. Given enough time, that delta will eventually fill the lake.

And Flathead Lake is also partially drained. People driving Highway 93 along the south side of the lake west of Polson often notice roadcuts in what looks like soft, white clay. Those deposits are old lake-bed sediments deposited when Flathead Lake was much deeper than it is now. In fact, the lake level must once have reached the crest of the Polson moraine, but it could not have stayed there long.

As soon as the lake rose to the lowest level on the crest of the Polson moraine, the water must have poured across and begun to cut a spillway. That overflow channel still exists as the course of the Flathead River through the Mission Valley. A large stream can erode glacial deposits quite easily so we can imagine that the newly-formed Flathead River must have sliced down through the moraine rather quickly. The river would have drained Flathead Lake long ago except for a curious accident.

For no reason except blind chance, the Flathead River happened to start flowing along a course that carried it directly across the top of a bedrock hill buried within the glacial sediments. It could as easily have started to flow on another course that would have taken it to one side or the other. When the rapidly eroding river cut down to the elevation of the buried hill, the hard bedrock slowed it down and so saved Flathead Lake. Since then the river has cut a narrow gorge through that buried bedrock hill. It is the site of Kerr Dam.

So Flathead Lake is a freak preserved by an accident. It formed in the first place because a big mass of stagnant ice didn't melt as fast as the rest of the Rocky Mountain trench glacier. And it still exists because the Flathead River just happened to erode into the top of a buried hill that it could as easily have missed.

This article first appeared in Montana Magazine.