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Saturday, September 21, 2019

Mission Reservoir in Western Montana - A Moraine-Dammed Lake

This is Mission Reservoir in western Montana - about 50 miles north of Missoula. St. Ignatius (pop. 842) can be seen in the distance. The reservoir is actually a moraine-dammed lake formed by a valley glacier during the last ice age. It has been modified to serve as a reservoir.

The “lateral moraines” were deposited along the sides of a valley glacier (a.k.a. "alpine glacier") during the last ice age. The moraines are the curved ridges along the sides of the reservoir. These forested ridges consist of rock material that the glacier removed from the mountains in the upper part of the drainage basin, high above the lake. Over the thousands of years since the moraine was formed, soil has formed on top and trees have taken root.

Right: Aerial photo of Mission Reservoir taken several years ago by Lawrence Dodge of Big Sky Magic Enterprises.

Below: This is similar to what the Mission Reservoir area probably looked like at the height of the last ice age ~20,000 years ago.

Rock material that has been transported and deposited by glaciers is called "till". As the glacier formed, rocks became stuck to its bottom and sides. Then as the ice flowed toward the valley floor, these rocks scoured away even more of the mountain’s surface. The glacial ice flowed to the position marked by the location of lake where it melted and dropped the rocks. For thousands of years, snowfall continued to replace the ice as it flowed away from the mountain tops. This "conveyor belt" took much of the mountain with it, forming the moraines. Much of the till deposited at the end of the glacier was washed away as the ice melted, so some of the original "end moraine” is missing. Since the end of the last ice age (10,000 years ago), soil has developed on the moraines and trees have taken root.

Geologists describe till as “unsorted” because it is made up of all sizes of rocks. This characteristic helps geologists distinguish materials deposited by glaciers from those deposited by running water, which tends to deposit different sizes of rocks in different areas.

Related Links . . .

1. CLICK HERE to access the blog post and photo tour of the hike I did to get the photo of Mission Reservoir.

2. CLICK HERE to watch a 3-minute drone video of the Mission Reservoir.

3. CLICK HERE to see a nice photo of Lake Wallowa in northeastern Oregona - another great example of a moraine-dammed lake.