Mossback’s Northwest: How the Columbia River got its curves

But the river’s course was warped along the way. What bent the river? We spoke with Nick Zentner, host of Cascade PBS’ series Nick on the Rocks, where he explains Northwest geology. He suggested we meet on the river to understand what skewed the Columbia’s course.

Nick explains: “This is Chelan and the Columbia River is flowing north to south right by the town, and what’s cool is that both sides of the river are not matched geologically. So east of the river there’s this basalt lava 16 million years old. On the west side of the Columbia it’s not lava at all, it’s this migmatite, which is 160 million years old, from 20 miles below the surface of the Earth. So the idea is the Columbia has not always been here … the lava pushed the Columbia to this present location.”

Nick goes on: “Before 16 million years ago we’re quite confident that the Columbia River came out of British Columbia and flowed essentially straight to Tri-Cities in southern Washington. But here comes this molasses erupting out of a volcano in northeastern Oregon, and there’s three miles thick of this basalt lava that just buried the mountains and pushed the Columbia River right to this very spot.

But there’s still more to the story. “If volcanic fire shaped the landscape and changed the river, so too did ice,” adds Skip.

Over the millennia, ice ages came and went. The last Ice Age began to recede some 17,000 years ago. The vast Cordilleran Ice Sheet came down from the north and was thousands of feet thick. It had pressed south into the Puget Sound region in the west, and in the east it extended into the Okanogan, the Spokane area, northern Idaho and Montana. As ice advanced and receded, it altered the landscape. The ice sheet blocked and unblocked rivers, including the Columbia. Around 15,000 years ago, the massive ice sheet blocked the Clark Fork River in Idaho, and a lake — we call it Glacial Lake Missoula — formed behind a 2,000-foot-tall ice dam. The lake is believed to have had a water volume as great as that of Lake Ontario and Lake Erie combined. But the dam breached and sent a flood of almost inconceivable size across the land, sweeping away everything in its path.

It dug Eastern Washington’s Channeled Scablands — terrain that looks like it has been scoured raw by a galactic fire hose. It scoured out the Grand Coulee, it flooded the Columbia basin. In places new lakes formed where floodwaters backed up at choke points like Wallula Gap. The Columbia had taken millennia to burrow a gorge through the Cascade Mountains, but the flood waters now made the gorge wider and deeper. When the flood smashed through and spurted out the west end, it had a wall of water 500 feet high. It raced down Oregon’s Willamette Valley all the way to Eugene carrying enormous boulders stuck in icebergs, floated hundreds of miles from the nearest ice sheet, then dropped them on the landscape.