where does the potomac river flow into?
Chesapeake Bay
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 By volume , the Columbia is the fourth @-@ largest river in the United States ; it has the greatest flow of any North American river draining into the Pacific . The river 's heavy flow and its relatively steep gradient gives it tremendous potential for the generation of electricity . The 14 hydroelectric dams on the Columbia 's main stem and many more on its tributaries produce more than 44 % of total U.S. hydroelectric generation - much more hydroelectric power than those of any other North American river . 

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 Originally created by plate tectonics about 35 million years ago and subsequently altered by volcanism and erosion , the river 's drainage basin was significantly modified by the Missoula Floods at the end of the most recent ice age . Humans began living in the watershed over 10 @,@ 000 years ago . There were once many tribal villages along the lower river and in the area around its mouth on the Columbia . Indigenous peoples lived throughout the upper reaches of the basin as well . 

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 The river is at its narrowest and most unpredictable in this basin . Channel flows are not apparent and are usually unmarked . The most efficient way to travel on this part of the river is by airboat . Approximately 3 @,@ 500 lakes lie within the overall St. Johns watershed ; all are shallow , with maximum depths between 3 and 10 feet ( 1 and 3 m ) . The river flows into many of the lakes , which further confuses navigation . Eight larger lakes and five smaller ones lie in the upper basin ; one of the first is named Lake Hell ' n Blazes ( sometimes polished to read as Lake Helen or Hellen Blazes ) , referencing oaths yelled by boatmen and fishermen in the early 19th century , frustrated when trying to navigate through floating islands of macrophytes , or muck and weeds , as the islands changed location with the creeping flow . Lakes Washington , Winder , and Poinsett — named for Joel Roberts Poinsett , a diplomat who brought the poinsettia to the United States — are located further along this stretch of the river . The northernmost points of the upper basin contain the Tosohatchee Wildlife Management Area , created in 1977 to assist with filtration of waters flowing into the larger St. Johns . 

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 The Columbia receives more than 60 significant tributaries . The four largest that empty directly into the Columbia ( measured either by discharge or by size of watershed ) are the Snake River ( mostly in Idaho ) , the Willamette River ( in northwest Oregon ) , the Kootenay River ( mostly in British Columbia ) , and the Pend Oreille River ( mostly in northern Washington and Idaho , also known as the lower part of the Clark Fork ) . Each of these four averages more than 20 @,@ 000 cubic feet per second ( 570 m3 / s ) and drains an area of more than 20 @,@ 000 square miles ( 52 @,@ 000 km2 ) . 

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 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 . 
