the Snake enters Hells Canyon, a steep, spectacular, rapid-strewn gorge that cuts through the Salmon River Mountains and Blue Mountains of Idaho and Oregon. Hells Canyon is one of the most rugged and treacherous portions of the course of the Snake River, posing a major obstacle for 19th-century American explorers. Here the Snake is also impounded by Hells Canyon, Oxbow, and Brownlee Dams, which together make up the Hells Canyon Hydroelectric Project. At the halfway point in Hells Canyon, in one of the most remote and inaccessible sections of its course, the Snake River is joined from the east by its largest tributary, the Salmon River. From there, the Snake begins to form the Washington–Idaho border, receiving the Grande Ronde River from the west before receiving the Clearwater River from the east at Lewiston, which marks the head of navigation on the Snake. The river leaves Hells Canyon and turns west, winding through the Palouse Hills of eastern Washington. The Lower Snake River Project's four dams and navigation locks have transformed this part of the Snake River into a series of reservoirs. The confluence of the Snake and Columbia rivers at Burbank, Washington is part of Lake Wallula, the reservoir of McNary Dam. The Columbia River flows about farther west to the Pacific Ocean near Astoria, Oregon. Geology Most of western North America was drowned by shallow seas off and on from around 800–200 million years ago. This was the accumulation phase of the mountain building cycle, where mostly sedimentary rocks accumulate as the seafloor subsides. The western edge of North America was a passive continental margin and therefore was not near a plate boundary. 180 million years ago, the tectonic setting of western North America changed to an active continental margin, and a convergent plate boundary formed, which crumpled the deposits from the accumulation phase into fold-thrust mountain ranges. Plate convergence gave rise to the Coastal Ranges and then to the Rocky Mountains due to friction between the Farallon Plate sinking beneath the North American Plate. Another tectonic player is the Yellowstone hotspot, which is a large plume of partially molten material lodged in the mantle. As the North American Plate moved west–southwest over the hot spot, massive amounts of basaltic lava were extruded onto the crust. A load of these dense lava flows bent down the crust immediately below it, and the mountains formed earlier to make the