Washington is mostly mountainous with Coast Ranges extending northward from Oregon and California. This chain forms two groups: the Olympic Mountains in the northwest, mainly on the Olympic Peninsula between the Pacific Ocean and Puget Sound, and the Willapa Hills in the southwest. The highest of the Olympic group is Mt. Olympus, at 7,965 ft (2,428 m) is the highest mountain in the Olympic group. The Cascade Range extends northward from the Sierra Nevada in California. This chain, 50–100 mi (80–100 km) wide, has peaks ranging up to 10,000 ft (3,000 m), except for such volcanic cones as Mt. Adams, Mt. Baker, Glacier Peak, Mt. St. Helen's, and Mt. Rainier, which at 14,410 ft (4,395 m) is the highest peak in the state.
Most of the major cities are concentrated in the Western Corridor which lies between the Coast and Cascade ranges. The northern section of this lowland is carved by Puget Sound, a complex, narrow arm of the Pacific wending southward for about 80 mi (130 km) and covering an area of 561 sq mi (1,453 sq km).
During the second half of the 19th century and most of the 20th most of the Cascade volcanoes were dormant. However, Mt St Helen began to show signs of activity towards the early 1980s. Finally on 18 May, the volcano exploded, blasting more than 1,300 ft (400 m) off a mountain crest that had been 9,677 ft (2,950 m) high.
Among the numerous rivers that flow through Washington, Columbia is the longest and most powerful. It enters Washington from Canada in the northeast corner and flows for more than 1,200 mi (1,900 km) across the heart of the state and then along the Oregon border to the Pacific. Washington's other major river, the Snake, enters the state from Idaho in the southeast and flows generally westward, meeting the Columbia River near Pasco.
Washington has numerous lakes, the largest of which is the artificial Franklin D. Roosevelt Lake, covering 123 sq mi (319 sq km). Washington has some 90 dams, providing water storage, flood control, and hydroelectric power. One of the largest and most famous dams in the US is Grand Coulee on the upper Columbia River, measuring 550 ft (168 m) high and 4,173 ft (1,272 m) long, with a storage capacity of more than 9.7 million acre-ft (11,960 cu m).