Summary: Today’s glacial topic is lakes again – but this time, it’s lakes that are gone. Glaciers were constantly melting, and the runoff, from on top of them, beneath them, along them, and in front of them had to go somewhere. Often, it just ran in rivers away from the glaciers, and the sediment carried by that flow created extensive deposits called outwash, glacially-derived sediment plains and mounds and more. But depending on the topography around a glacial front, you might get a lake pooling there, especially at times when glaciers were retreating. As I mentioned yesterday, the terminus of a stagnant glacier could result in a pile of sediment called a moraine, and combined with pre-existing topography, could easily form a lake. This happened repeatedly, and we recognize lakes by their typically thin-bedded sedimentary layers. In some cases, the layers represent annual sequences like tree rings – a winter layer of minimal sediment and a summer layer when more melting brought more sediment into the lake. Yosemite Valley, California, is one of the best examples of a glacially-carved valley anywhere. It has the characteristic U-shape, with steep walls ground away by glacial ice. Yosemite Valley contained more than 3,000 feet of ice at the peak of the most recent glaciation. As the transition to the present interglacial period happened, probably 11,000 to 9,000 years ago, the valley floor became the site of a lake about six miles long, with the terminal moraine at the valley mouth impounding it. The situation was much like that of the Finger Lakes in New York, except in high mountain country where the bedrock is the granitic rocks of the Sierra Nevada Batholith. So the floor of Yosemite Valley today is quite flat because of those lake sediments – it’s not really truly U-shaped. You don’t have to have a moraine to dam a river. A tongue of ice can do it, too. That’s what happened repeatedly in northern Idaho about 13,000 to 15,000 years ago, where an ice dam 2,500 feet high impounded the ancestral Clark Fork River. The resulting Glacial Lake Missoula was 2,000 feet deep and more than 225 miles long, in many branches into the mountain valleys of Montana. From time to time, with the changes in the climate and the build-up of pressure behind the ice dam, the ice would melt and erode until the lake began to empty catastrophically. The water volume is estimated at 60 times the discharge of the Amazon River, draining the entire lake in days to weeks. All that water poured out over eastern Washington state, where it carved the landscape into what’s called the Channeled Scablands today. There are features all over this country reflecting these events, from giant ripple marks at Camas Prairie, Montana, to dry waterfalls in Washington and wave-cut shorelines in the hills around Missoula, Montana that represent multiple lake levels. There were many fillings and emptyings of Glacial Lake Missoula, probably at least 40 times. There’s lots more to this fascinating story, of course, and lots to see in the area where it happened. I think your best source for detailed information about it is a book by David Alt titled Glacial Lake Missoula and its Humongous Floods (Mountain Press, 2001). Glacial and Pluvial Lakes (and route of Bonneville and Missoula Floods) Map by Fallschirmjäger, used under Creative Commons license There are a couple more big lakes that developed in the west. We call these pluvial lakes, meaning rain-derived, because they were not the direct result of snow and ice melt. Much of western Utah was covered by Lake Bonneville. The Great Salt Lake is a remnant of this lake, but Lake Bonneville covered more than 10 times the area of the Great Salt Lake. The Wasatch Mountains at Salt Lake City show two prominent horizontal lines which are shorelines of Lake Bonneville. It was more than 1,000 feet deep, and like Glacial Lake Missoula, it had a catastrophic emptying. The Bonneville Flood was about 14,500 year
