The Big Draining: Flathead Lake to Lake Pend Oreille

I've been east of the Washington State landscape. While not the primary purpose of my travels, I did see some features in Montana that played a huge role in the shaping a large swath of the Washington landscape and for that matter a good chunk of Oregon as well. 

 

 

Faint horizontal lines on the slope of the mostly grass covered hill in the center of the picture are wave cut terraces small beaches when the slope was on the shores of Lake Missoula. Many of the valleys of western Montana filled with water during the last glacial period when the Clark Fork River drainage was blocked by glacial ice at what is now Lake Pend Oreille in northern Idaho. The above and following images of the wave cuts terraces are in the southern Flathead River valley. 

 

The wave marks are very subtle in this image and show up as alignments of more brush vegetation

 

Besides the glacial ice blocking of the Clark Fork, there were plenty of other glaciers in the area. The Rocky Mountain Trench was filled with glacial ice. The Flathead valley located at the southern end of the Trench was the terminus of the great ice lobe that poured down the Trench. Flathead Lake, the largest fresh water lake west of the Mississippi River, formed behind the terminal moraine left behind by the Rocky Mountain Trench ice lobe.

Upon seeing the old lake shore wave marks it becomes apparent that terminal ice margin of the Rocky Mountain trench ice lobe likely extended into Glacial Lake Missoula. As the lake filled it would have reached the ice lobe. Perhaps the lower end of the ice lobe consisted of floating ice with ice bergs calving off into lake. The banks of the Flathead River just below Flathead Lake are lined with bluffs revealing glacial drift.

Predominantly silty glacial drift with scattered large angular boulders.

 

Floating ice would melt and rain silt onto the lake floor as well as occasional boulders. The exposure shown above could have been deposited when ice was floating on Lake Missoula, but in this case it may also have been an early version of Flathead Lake or some other localized temporary glacial lake. 

Regardless of the exact source of the drift shown above, it is easy to picture Lake Missoula within the Flathead valley having ice bergs carrying large boulders. When the ice dam on the Clark Fork broke some of those bergs and the boulders they contained would have begun their rapid journey to Washington State and in some cases even Oregon.

Erratic Rock Oregon State Natural Site (photo from Wikimedia Commons) 

 

Traveling back to Washington State we followed the Clark Fork valley to Lake Pend Oreille in Idaho and then on to Spokane. This was the route of the great floods. All the deep lake water that filled the valleys of western Montana that were not filled with ice poured down the Clark Fork matching the flow of all the rivers in world combined in a huge flood.

Montana Highway 200 follows the valley as does a railroad.

A train carrying Boeing fuselages passed during a stop.

 

Driving this section of the flood route, one gains an appreciation as to why Bretz was not certain where the massive flood waters that shaped the landscape of eastern Washington came from. Despite the scale of the flood, the features are subtle and in many cases obscured by post flood flows of water from the continental ice margin. The question of where the flood waters came from was settled by Joseph Pardee when he presented his findings of giant current ripples within areas formerly covered by Lake Missoula.