where did the columbia river begin?
British Columbia
entailment
 Because the Columbia was at the same latitude as the headwaters of the Missouri River , there was some speculation that Gray and Vancouver had discovered the long @-@ sought Northwest Passage . A 1798 British map showed a dotted line connecting the Columbia with the Missouri . When the American explorers Meriwether Lewis and William Clark charted the vast , unmapped lands of the American West in their overland expedition ( 1803 – 05 ) , they found no passage between the rivers . After crossing the Rocky Mountains , Lewis and Clark built dugout canoes and paddled down the Snake River , reaching the Columbia near the present @-@ day Tri @-@ Cities , Washington . They explored a few miles upriver , as far as Bateman Island , before heading down the Columbia , concluding their journey at the river 's mouth and establishing Fort Clatsop , a short @-@ lived establishment that was occupied for less than three months . 

entailment
 The Columbia enters eastern Washington flowing south and turning to the west at the Spokane River confluence . It marks the southern and eastern borders of the Colville Indian Reservation and the western border of the Spokane Indian Reservation . The river turns south after the Okanogan River confluence , then southeasterly near the confluence with the Wenatchee River in central Washington . This C ‑ shaped segment of the river is also known as the " Big Bend " . During the Missoula Floods 10 @,@ 000 to 15 @,@ 000 years ago , much of the floodwater took a more direct route south , forming the ancient river bed known as the Grand Coulee . After the floods , the river found its present course , and the Grand Coulee was left dry . The construction of the Grand Coulee Dam in the mid @-@ 20th century impounded the river , forming Lake Roosevelt , from which water was pumped into the dry coulee , forming the reservoir of Banks Lake . 

contradiction
 The Columbia begins its 1 @,@ 243 @-@ mile ( 2 @,@ 000 km ) journey in the southern Rocky Mountain Trench in British Columbia ( BC ) . Columbia Lake – 2 @,@ 690 feet ( 820 m ) above sea level – and the adjoining Columbia Wetlands form the river 's headwaters . The trench is a broad , deep , and long glacial valley between the Canadian Rockies and the Columbia Mountains in BC . For its first 200 miles ( 320 km ) , the Columbia flows northwest along the trench through Windermere Lake and the town of Invermere , a region known in British Columbia as the Columbia Valley , then northwest to Golden and into Kinbasket Lake . Rounding the northern end of the Selkirk Mountains , the river turns sharply south through a region known as the Big Bend Country , passing through Revelstoke Lake and the Arrow Lakes . Revelstoke , the Big Bend , and the Columbia Valley combined are referred to in BC parlance as the Columbia Country . Below the Arrow Lakes , the Columbia passes the cities of Castlegar , located at the Columbia 's confluence with the Kootenay River , and Trail , two major population centers of the West Kootenay region . The Pend Oreille River joins the Columbia about 2 miles ( 3 km ) north of the US – Canada border . 

entailment
 When the rifting of Pangea , due to the process of plate tectonics , pushed North America away from Europe and Africa and into the Panthalassic Ocean ( ancestor to the modern Pacific Ocean ) , the Pacific Northwest was not part of the continent . As the North American continent moved westward , the Farallon Plate subducted under its western margin . As the plate subducted , it carried along island arcs which were accreted to the North American continent , resulting in the creation of the Pacific Northwest between 150 and 90 million years ago . The general outline of the Columbia Basin was not complete until between 60 and 40 million years ago , but it lay under a large inland sea later subject to uplift . Between 40 and 20 million years ago , in the Eocene and Miocene eras , tremendous volcanic eruptions frequently modified much of the landscape traversed by the Columbia . The lower reaches of the ancestral river passed through a valley near where Mount Hood later arose . Carrying sediments from erosion and erupting volcanoes , it built a 2 @-@ mile ( 3 @.@ 2 km ) thick delta that underlies the foothills on the east side of the Coast Range near Vernonia in northwestern Oregon . Between 17 million and 6 million years ago , huge outpourings of flood basalt lava covered the Columbia River Plateau and forced the lower Columbia into its present course . The Cascade Range began to uplift during the early Pleistocene era ( two million to 700 @,@ 000 years ago ) . Cutting through the uplifting mountains , the Columbia River created the Columbia River Gorge . 

entailment
 The Columbia River is the largest river in the Pacific Northwest region of North America . The river rises in the Rocky Mountains of British Columbia , Canada . It flows northwest and then south into the US state of Washington , then turns west to form most of the border between Washington and the state of Oregon before emptying into the Pacific Ocean . The river is 1 @,@ 243 miles ( 2 @,@ 000 km ) long , and its largest tributary is the Snake River . Its drainage basin is roughly the size of France and extends into seven US states and a Canadian province . 
