Hundreds of volcanoes erupted in the Cascade mountain range. You can still see the 14 major peaks and hundreds of smaller peaks and cinder cones that form the range. Near Hood River, Oregon, you see dramatic views of Mount Adams and Mount Hood. Both are dormant volcanoes that could erupt within the next 50 years.
During this period, the Cascades began to uplift. As the mountains rose, the Columbia River carved out a deep gorge. This is the only near sea-level passage through the Cascades.
The Missoula Floods
16,000-14,000 years ago (Pleistocene)
Did you know that the largest floods to occur on the planet happened here? During the last ice age, ice sheets covered much of Canada. One lobe of ice grew southward, blocking the Clark Fork Valley in Idaho. This 2,000 foot (600 meters) high ice dam blocked the river, creating a lake that stretched for hundreds of miles. When the lake was full, it contained 600 cubic miles (2,500 cubic kilometers) of water. How much is that? Imagine a block of water a mile high (as high as the mountains around Bonneville Dam), a miles wide, and stretching from Bonneville Dam to San Francisco!
Eventually, water traveled under the ice dam. The water drained out of the lake in two or three days, flooding eastern Washington. The flood, moving up to sixty miles per hour, scoured out hundreds of miles of canyons called coulees, created the largest waterfall to ever exist, and left 300 foot (90 meter) high gravel bars. At Bonneville, the water crested at 650 feet (200 meters). If you look on the cliffs southeast of the dam, you will see a transmission tower (the one with three poles) that is 200 feet (60 meters) above the high water mark.
During a period of 2,500 years as many as 100 of these floods scoured the Gorge.
Sliding into History
500 years ago
Near Bonneville, the lava layers making up Table Mountain slid into the Gorge. This series of four landslides, covering five square miles, blocked the Columbia River. The Second Powerhouse butts against this landslide. If you look north of the dam, you can see cliffs exposed after the mountain gave way.
Original inhabitants of the area may have marveled at the 200 foot (60 meters) high landslide blocking the Columbia. They could have crossed on foot, possibly giving rise to a story about “The Bridge of the Gods”. This natural dam created a lake that stretched almost seventy miles (up to the present day John Day Dam). After a few months, the Columbia rose high enough to wash through the southern side of the landslide creating a flood of water that was 100 feet (30 meters) deep at Troutdale.
Things returned to normal, except the river was displaced a mile to the south and a set of rapids, the Cascades, had formed. In 1938, the rapids disappeared under water rising behind Bonneville Dam. The only hints of their existence are the remnants of a navigation lock at Cascades Locks built in 1896 to allow boats around the rapids.
The Gorge is still changing. In the winter of 1996, landslides similar to the Bridge of the Gods landslide destroyed homes in Warrendale. At milepost 35 on I-84 you can see this damage.
The Columbia River and its tributaries form the dominant water system in the Pacific Northwest Region. The mainstem of the Columbia rises in Columbia Lake on the west slope of the Rocky Mountain Range in Canada. After flowing a circuitous path for about 1200 miles, 415 miles of which are in Canada, it joins the Pacific Ocean near Astoria, Oregon. The river drains an area of approximately 219,000 square miles in the States of Washington, Oregon, Idaho, Montana, Wyoming, Nevada, and Utah. An additional 39,500 square mile portion of the basin, or about 15%, is within Canada.
The Columbia River pours more water into the Pacific Ocean than any other river in North or South America. In its 1,270 mile course to the Pacific Ocean, the Columbia flows through four mountain ranges — the Rockies, Selkirks, Cascades, and coastal mountains — and drains 258,000 square miles. Its largest tributary, the Snake, travels 1,038 miles from its source in Yellowstone National Park in Wyoming before joining the Columbia. This vast river basin was formed near the end of the last Ice Age, 12,000 to 19,000 years ago, by the Bretz Floods. Immense ice dams half a mile high held back melting ice, creating a huge lake in northwest Montana, called Lake Missoula. Each time the ice gave way, massive walls of water as high as four hundred feet hurled boulders and icebergs seaward with a great destructive force. These floods generally followed the route of the present day Columbia River and came at least 40 times.
The Columbia River Basin is bounded principally by the Rocky Mountain system on the east and north, the Cascade Range on the west, and the Great Basin on the south. The basin area includes 3,000 square miles of waterways and lakes, of which 2,500 miles are within the United States.
The Columbia River drops more than 735 meters from its headwaters in British Columbia, winding over 1,950 kilometers to the Pacific Ocean. Although the river itself flows from Canada through only two states, forming part of the Washington-Oregon border, the vast Interior Columbia River Basin is defined by the area drained by the river and its many tributaries. This 58-million-hectare area (about the size of France) extends roughly from the crest of the Cascade Mountains of Oregon and Washington east through Idaho to the Continental Divide in the Rocky Mountains of Montana and Wyoming, and from the headwaters of the Columbia River in Canada to the high desert of northern Nevada and northwestern Utah.
The Columbia River Basin is a complex tapestry of mountains, high plateaus, desert basins, river valleys, rolling uplands, and deep gorges woven together by the Columbia River and its tributaries.
During the early stages of the Columbia Basin formation, granite rock was slowly created by heat and pressure deep in the crust of the earth. Then the crust was uplifted, exposing the granite, creating mountains similar to the Okanogan Highlands north of Grand Coulee Dam. Forty to sixty million years ago the formation of the outline of the Columbia Basin was complete. The land had subsided below sea level, and a large inland sea had formed. The land was again uplifted and then, 10-15 million years ago, was flooded with volcanic lava. The boundaries of the flood lava were located in almost the same position as the former seashore. Many layers of lava were needed to build up to a 5,000 feet (1500 meter) thickness and form the smooth surfaced Columbia Plateau.
During the Ice Age, the old Cascade Mountains were also formed. Their outline still remains on the western slopes of the Cascades. The uplifting mountains were not able to block the flow of the Columbia River completely, and a deep Columbia River gorge was formed. Near the end of the Ice Age the volcanoes of the high Cascades rose to elevations of 14,000-15,000 feet (4000-4500 meters). Older volcanoes, such as Mt. Hood and Mt. Rainier, were sculpted by glaciers of the Ice Age; such as Mt. St. Helens remained unsculpted, retaining their original volcanic form.
Eighteen thousand years ago the Columbia Basin was nearly covered by floodwaters when an ice dam at Lake Missoula in western Montana broke. Large boulders were strewn near the outlet of the Lower Coulee (Lake Lenore). Other boulders were carried in icebergs as far as western Oregon. The floodwaters were 800 feet (250 meters) deep near Pasco and 400 feet (125 meters) deep at Portland. After the Ice Age, the Columbia River returned to its former channel. The channeled scab lands and large coulees that had been formed were left stranded 500-1600 feet (150-500 meters) above the present river floor and serve as a constant reminder of some of the most unusual episodes in geologic history. Learn more
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