Our Northwest

Channeled scablands appear from the air

Saturday, March 15, 2014by  Jon Osterberg

This week while flying from Seattle to Spokane, I shot aerial photos bearing evidence of Washington's violent geologic past.
   Geology buffs know about Eastern Washington’s “channeled scablands” – scars from ancient floods caused by ice dams that impounded implausibly enormous Ice Age lakes, then burst. That cycle repeated itself many times over the eons.
   The short version: vast sheets of ice crept south from Canada and blocked Northwest rivers and streams. The Okanogan Lobe of the Cordilleran Ice Sheet dammed water to form Glacial Lake Columbia west of Spokane, but the monster reservoir was dammed by the Purcell Trench Lobe in north Idaho to form Glacial Lake Missoula – a whopping 2,000 feet deep spread over 500 miles.
   When the Ice Age climate thawed and Lake Missoula’s dam broke, a huge torrent gushed out in a matter of days and gouged coulees, trenches, and potholes out of basalt bedrock, leaving today’s channeled scablands.
   Natives and tourists know of Dry Falls, site of the largest waterfall in earth history. Water surged across the land near Coulee City at 60 mph and crashed 400 feet down into Grand Coulee.
   What’s not as obvious – until you fly overhead – is the scope and breadth of scars over the rest of Eastern Washington.
   Beyond the Cascades the Columbia River comes into view, clearly delineating the Columbia Plateau from the western half of the state. Lake Chelan appears at the top, a 55-mile-long remnant of glacial carving.

   Moses Coulee stretches more than 40 miles northeast from near Rock Island Dam (south of Wenatchee) to Jameson Lake, near Mansfield. Moses Coulee roughly parallels Grand Coulee, its neighbor to the east.
   Once the Columbia River became blocked by the icy Okanogan Lobe, it diverted east and discharged through Moses Coulee for a geologically short time until it too was blocked by advancing ice. Then the river rerouted east once again, into the larger (and better known) Grand Coulee, which it seized for a longer time. Look closely and you can see U.S. Highway 2 crossing Moses Coulee about mid-photo, 11 miles east of Waterville, by way of Armour Draw.

   An interesting feature caught my eye near Odessa. Furrows on a ridgeline next to basalt scabland suggest they were caused by rushing water, but I couldn’t find them noted on any map. Google gives an address for a nearby farmhouse (aren’t satellite maps great?!) as Zagelow Road North. Maybe someone familiar with that area can tell me about these furrows.
   Cormana Lake lies amid an ancient scoured path. It’s bordered by grain fields to the west and east, terrain high enough to have risen above the turbulent Ice Age waters. They retain their fertile loess soil.
   I landed in Spokane and flew home the next day. Again the Horizon Air Bombardier flew at a relatively low altitude, providing a great view for detailed photos.
   Coffeepot (left) and Deer lakes, like most channeled scabland features, run along a diagonal northeast-to-southwest line, the downhill direction for each Glacial Lake Missoula discharge. (This and subsequent photos look south.) And again, surrounding the scabland lies fertile, cultivated ground.
   Crab Creek looks like a snake from the air. Its name is deceptive. Though today Crab Creek is a small seasonal stream that runs from Reardan all the way to Beverly, south of Vantage, it was a major discharge channel for the Ice Age lakes. In Pleistocene times the diverted Columbia merged with Crab Creek at present-day Moses Lake before continuing southwest and intercepting the Columbia’s original channel at Beverly.
   Modern day Banks Lake (partially frozen here), a reservoir, occupies Grand Coulee. It terminates bluntly at Coulee City, south of which lies Dry Falls (center) and a series of lakes: Park, Blue, Alkali, Lenore, Soap, and to the east (left), Billy Clapp Lake. This was a major discharge channel for Glacial Lake Missoula. Today, those lakes are a trout fisherman’s paradise.
   Fishermen also covet Jameson Lake, near the north end of Moses Coulee. The scouring action of unleashed water is apparent here. Geologic evidence shows there was not just one ice-dam breach followed by a huge flood. The sequence played out dozens of times over thousands of years. The USGS writes, “When Glacial Lake Missoula burst through the ice dam and exploded downstream, it did so at a rate 10 times the combined flow of all the rivers of the world.”
   My plane flew over the Columbia and Wenatchee, then the Cascades, and soon the channeled scablands of Eastern Washington receded from view.
   For a great overview of the channeled scablands, check out this website.

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