Columbia River

The Columbia River is the largest river in the Pacific Northwest and, with a length of 1,953 kilometers (1,214 miles), is the 15th longest in North America. From its source at Columbia Lake at an elevation of 809 meters (2,650 feet) in Canada's Selkirk Mountains it first flows northwestward through eastern British Columbia, then turns southward toward the United States. It crosses the US-Canadian border north of Spokane, Washington, then flows southward across central Washington where it is joined by the Snake River, which drains southeastern Washington, eastern Oregon and southern Idaho. The Columbia then turns westward, forming the border between Washington and Oregon, flows through the Columbia River Gorge in the Cascade Mountains and on to its mouth at the Pacific Ocean near Astoria, Oregon.

The Columbia River ranks sixth in North America in terms of runoff after the Mississippi, MacKenzie, St Lawrence, Nelson, and Yukon rivers and is ranked 32nd among rivers of the world in area drained. The major tributaries to the Columbia are the Kootenai and Flathead/Pend Oreille rivers, which drain southeastern British Columbia, western Montana, and northern Idaho, the Snake River which drains western Wyoming, most of Idaho, eastern Oregon and southeastern Washington, and the Willamette River of western Oregon.

Columbia River Basin

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.

To the west of the Cascade Range, the Columbia River is joined by the Willamette drainage and begins a transition to the ocean tidal reaches. Salt water intrusion into the Columbia River estuary reaches about 23 miles upstream from the mouth. The effects of tides upon the flow rate and level of the river are felt up to Bonneville Dam, river mile 146.1. Ocean influences also dominate weather patterns along the western slope of the Cascade Range as the majority of precipitation in the western portion is in the form of rainfall during the winter months.

Most of the annual precipitation of the Columbia River Basin is concentrated in the winter months with the bulk of the precipitation falling in mountainous areas as snow to be stored in deep snowpacks awaiting the warmth of spring for its release. As a result, winter streamflows are generally low with high sustained runoff flows occurring in the spring and early summer. This runoff pattern of the Columbia River exemplifies a major seasonal maldistribution of flow with about 60 percent of the natural runoff of the Columbia occurring during the months of May, June, and July. The Columbia has an average annual runoff at the mouth of about 198,000,000 acre-feet (275,000 cfs) making it second only to the Missouri-Mississippi River System in the United States (611,000 cfs) in average annual runoff. The Canadian portion of the basin generally contributes about 50,200,000 acre-feet annually, or about 25 percent of the basin total.

During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earths surface engulfed about 63,000 square miles of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 6,000 feet. As the molten rock came to the surface, the earths crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin (Plateau). The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.

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; others 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.

Mt. St. Helens and the Columbia River

On May 18, 1980, Mount St. Helens violently erupted. During the eruption, a massive debris avalanche, moving down the north side of Mount St. Helens, was blasted into the North Fork Toutle River valley, depositing approximately 3 billion cubic yards of material in the upper 17 miles of the valley. Mudflows quickly developed in the South Fork Toutle River and in the Lewis River tributaries of Smith Creek, Muddy River, and Pine Creek. At least 11,000 acre-ft of water, mud, and debris were deposited in Swift Reservoir between 9 a.m. and noon on May 18. A massive mudflow originated on the debris pile in the North Fork Toutle River valley and caused widespread destruction as it moved downstream through the Toutle and Cowlitz Rivers. Considerable deposition occurred in these river channels and in the channel of the Columbia River, which was closed to shipping for about 1 week. Channel capacity of the Cowlitz River was reduced from 76,000 to 7,300 cubic feet per second (at the flood stage of 23.0 feet). Dredging of the shipping channel in Columbia River commenced shortly after May 18, and dredging was started in July in the lower reach of Toutle River and Cowlitz River.

Captain Robert Gray