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Much of Washington is mountainous. Along the Pacific coast are the 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). About 100 mi (160 km) inward from the Pacific coast is the Cascade Range, extending northward from the Sierra Nevada in California. This chain, 50–100 mi (80–100 km) wide, has peaks generally 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.

Between the Coast and Cascade ranges lies a long, troughlike depression—the Western Corridor—where most of Washington's major cities are concentrated. 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). Of all the state's other major regions, only south-central Washington, forming part of the Columbia Plateau, is generally flat.

The Cascade volcanoes were dormant, for the most part, during the second half of the 19th century and most of the 20th. Early in 1980, however, Mt. St. Helen's began to show ominous signs of activity. 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. Tremendous plumes of steam and ash were thrust into the stratosphere, where prevailing winds carried volcanic dust thousands of miles eastward. The areas immediately surrounding Mt. St. Helen's were deluged with ash and mudflows, choking local streams and lakes, particularly Spirit Lake. About 150 sq mi (388 sq km) of trees and brush were destroyed; the ash fall also damaged crops in neighboring agricultural areas and made highway travel extremely hazardous. The eruption left 57 people dead or missing. Eruptions of lesser severity followed the main outburst; the mountain continued to pose a serious danger to life in the area as the estimated cost of the damage to property, crops, and livestock approached $3 billion. Another minor eruption, on 14 May 1984, shot ash 4 mi (6 km) high and caused a small mudflow down the mountain's flanks, but no injuries or other damage occurred. East of the Cascade Range, much of Washington is a plateau underlain by ancient basalt lava flows. In the northeast are the Okanogan Highlands; in the southeast, the Blue Mountains and the Palouse Hills. All these uplands form extensions of the Rocky Mountain system.

Among Washington's numerous rivers, the longest and most powerful is the Columbia, entering Washington from Canada in the northeast corner and flowing for more than 1,200 mi (1,900 km) across the heart of the state and then along the Oregon border to the Pacific. In average discharge, the Columbia ranks 2nd only to the Mississippi, with 262,000 cu ft (7,400 cu m) per second. 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, of which the largest 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).