The Columbia River, fourth-largest by volume in North America (annual average of 192 million acre-feet at the mouth) begins at Columbia Lake in the Rocky Mountain Trench of southeastern British Columbia at about 2,656 feet above sea level. The river flows north for some 200 miles and then turns south and flows for about 270 miles before crossing the border into Washington 749 miles inland from the Pacific Ocean.
For its first 150 miles in the United States, the Columbia forms the reservoir behind Grand Coulee Dam. The river then bends west, south, and east through central Washington, turns south and then west, and forms the border between Oregon and Washington to the Pacific Ocean. The mouth of the river is about 10 miles west of Astoria, Oregon. The total length of the river is about 1,243 miles. The drainage basin covers 259,000 square miles, and drains portions of seven states and British Columbia, and covers three degrees of latitude and nine degrees of longitude.
The Columbia mainstem has many large and small tributaries, including the Snake (1,078 miles), Kootenay (485 miles), Deschutes (252 miles), Yakima (214 miles), and Willamette (187 miles).
From its headwaters to its mouth, the river drops steadily at a rate of about two feet per mile, and most of its course is through rock-walled canyons, emptying an annual average of 192 million acre-feet into the Pacific; much of its volume originates in its middle and upper reaches.
The combination of high volume and stable canyons made the Columbia an ideal hydropower river. Today there are 14 dams on the mainstem Columbia, beginning with Bonneville at river mile 146 and ending with Mica at river mile 1,018. In addition, there are more than 450 dams throughout the basin. Dams on the Columbia and its major tributaries produce half of the electricity consumed in the Pacific Northwest.