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Science writer Richard Hill gives a brief geologic history of the Columbia River Gorge
From its craggy black cliffs to its dazzling white waterfalls, the spectacular Columbia River Gorge took shape over millions of years of mayhem: massive lava flows from belching fissures, immense ice-age floods and colossal landslides. The powerful canyon-carving Columbia River survived the onslaught.
"Exposed rocks and the river help tell the story of how this region was formed by a combination of fire and water," says William Orr, a University of Oregon professor emeritus of geology.
Some 17 million years ago, molten rock spewed from long, wide cracks in eastern Oregon, eastern Washington and western Idaho. During the ensuing 11 million years, about 300 lava flows engulfed more than 63,000 square miles of the Northwest -- about the size of Washington state -- and accumulated to nearly three miles thick in spots.
At least 20 flows poured through the Columbia River channel, piling layer after layer of lava, which cooled into basalt up to 2,000 feet deep. Many flows surged all the way to the Pacific Ocean. Flows blocked its waters, creating large lakes, until the Columbia found an alternative path westward.
"Making a new channel was a never-ending chore for the Columbia," says Orr, co-author with his wife, Elizabeth Orr, of "Geology of the Pacific Northwest." "It kept cutting new canyons again and again and again. Some of the old Columbia's channels were in the Molalla-Estacada area. Every time it cut a new route, the river went farther and farther north, leaving a series of lava-filled channels to the south."
The gorge's most distinctive landmark, Crown Point -- a promontory 733 feet above the river -- represents a lava flow that filled an old Columbia channel about 14 million years ago. Rooster Rock, a 200-foot-high mass of basalt at the base of Crown Point, is part of a landslide from the steep cliff. By 2 million years ago, the brawny river sliced yet another gash for itself as the Cascade Range rose, resulting in the steep gorge of today.
After millennia of relative calm, the colossal Missoula Floods crashed through the gorge several times between 12,000 and 18,000 years ago. The source of the floods was the 2,000-foot-deep, 200-mile-wide Glacial Lake Missoula. Until the last ice age started to thaw, an ice sheet at the mouth of the Clark Fork River in northern Idaho and Montana blocked it. But slowly, melted water cut a channel into or under the ice, collapsing the dam and unleashing the lake's 500 cubic miles of water. It sped into the narrower confines of the gorge at 75 mph and submerged Crown Point. The ice dam repeatedly would reform, and the flood process would start again.
Recent studies by Jim O'Connor, a hydrologist with the U.S. Geological Survey in Portland, and Gerard Benito of the Center of Environmental Sciences in Spain, found evidence of at least 25 massive floods. They calculated the largest flood discharged roughly 2.6 billion gallons a second -- about 2,000 times larger than the Columbia's 1996 flood.
The narrows near Kalama, Wash., forced the water to back up into the Willamette Valley south to Eugene. Portland was under 400 feet of water -- only Rocky Butte, Mount Scott and similar high points poked above water. The floods brought massive sediment to the Willamette Valley and made the area agriculturally rich.
The one-two punch of lava flows and ice-age floods made the gorge perfect for more than 100 waterfalls, the largest concentration in the Northwest. The floods scoured the gorge of soil and vegetation, and streams careened over sheer cliffs of erosion-resistant basalt.
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