Washington’s landforms are split in two by the Cascade Range, a north–south line of mountains and five active volcanoes whose highest summit, Mount Rainier, reaches 4,392 m (14,410 ft) by the long-standing National Park Service figure. West of the Cascades lie the Puget Sound lowland, a glacier-shaped arm of the sea, and the Olympic Mountains, topped by Mount Olympus at 2,432 m (7,980 ft); east of them the dry Columbia Plateau spreads across basalt lava flows that ice-age floods scoured into the Channeled Scablands, before giving way to the wind-built Palouse hills near Idaho. The state covers 184,830 km² (71,362 sq mi), and its lowest point is sea level along the Pacific coast. Six named features, taken one at a time, explain almost everything else about how the state looks.
Mount Rainier, the Most Glaciated Summit in the Lower 48
Rainier stands about 95 km south-southeast of Seattle, and on clear days it dominates the southeastern horizon across most of the Seattle–Tacoma area, so much that locals simply call it “the Mountain”. Its topographic prominence of 4,026 m (13,210 ft) is the greatest of any peak in the contiguous United States. The summit elevation itself is shifting: the National Park Service and USGS still quote 4,392 m, but surveyors working in the 2020s found that the Columbia Crest ice cap has melted so much that a point of bare rock on the southwest crater rim, at about 4,389 m (14,400 ft), is now the true top.
Ice is Rainier’s defining trait. Its 29 named glacial features, glaciers and perennial snow patches together, covered about 78.8 km² (30.4 sq mi) in 2015, making it the most heavily glaciated peak in the lower 48 states. The Carbon, Cowlitz, Nisqually, Puyallup and North Mowich rivers all begin at glaciers bearing their names, and in May 1970 the Nisqually Glacier was measured sliding up to 74 cm a day. Geothermal heat keeps parts of the two summit craters free of snow, and beneath the ice of the west crater lies a lake about 40 m long at 4,329 m, the highest crater lake in North America.
That same ice is why Rainier is on the list of 16 Decade Volcanoes, those judged most likely to cause loss of life if they erupt. Melting glaciers can turn into lahars, fast mudflows of rock and water. The USGS estimates that about 150,000 people live on old Rainier lahar deposits, and one engineering geologist has warned that a repeat of the Osceola Mudflow, which swept down about 5,000 years ago, would destroy Enumclaw, Orting, Auburn and Puyallup. Since 1998 a lahar warning system of sensors and sirens has watched the Puyallup River valley, where Orting sits just 32 km from the summit.
Mount St. Helens and the Crater of 18 May 1980
Eighty kilometres south-southwest of Rainier, Mount St. Helens shows what a Cascade volcano can do to a landscape in a single morning. At 8:32 a.m. on 18 May 1980, a magnitude 5.1 earthquake shook loose the bulging north face, and the whole slope collapsed in the largest landslide in recorded history. The debris avalanche filled the valley of the North Fork Toutle River up to 180 m deep over about 62 km², and the uncorked magma exploded sideways, flattening about 600 km² (230 sq mi) of forest. Fifty-seven people died.
The eruption rebuilt the mountain’s shape. The summit fell from 2,950 m (9,677 ft) to 2,549 m (8,363 ft), leaving a horseshoe-shaped crater 1.6 km wide that opens to the north. Lahars ran almost 80 km to the Columbia River, and ash fell in 11 states. Lava domes grew in the crater through 2008. In 1982 Congress set aside the area as the Mount St. Helens National Volcanic Monument, where the hummocky avalanche deposits, the new Spirit Lake shoreline and the blown-down slopes can still be read like a diagram of the event. St. Helens is also geologically young: the whole volcano formed within the past 40,000 years, a newcomer beside Mount Adams, whose cone-building began about 520,000 years ago.
The Olympic Mountains and the Hoh Valley
The Olympic Mountains are not part of the Cascades at all. They rise on the Olympic Peninsula as a roughly circular dome of steep peaks, with water on three sides: the Pacific to the west, the Strait of Juan de Fuca to the north and Hood Canal to the east. Mount Olympus, at 2,432 m (7,980 ft), is modest by western standards, but it climbs more than 2,100 m in 8.8 km from the Hoh River, and it carries seven named glaciers, the longest being the Hoh Glacier at 4.93 km.
Those glaciers exist because the western slopes are the wettest place in the contiguous United States. Storms off the Pacific hit the range first, and the Hoh Rainforest in the valley below Olympus averages about 3,600 mm (140 in) of rain a year. The valley itself is a glacial trough, carved thousands of years ago and now floored with moss-draped Sitka spruce and western hemlock. The range’s drainage is radial: the Quinault, Queets, Hoh and Sol Duc run west to the Pacific, the Elwha and Dungeness north to the strait, and the Dosewallips, Duckabush and Skokomish east into Hood Canal. The west peak of Mount Anderson is the point from which rivers run to all three.
Puget Sound and the Vashon Ice Sheet
Between the Olympics and the Cascades lies Puget Sound, a network of marine channels running about 160 km from Deception Pass south to Olympia. It averages 140 m deep and reaches 280 m off Jefferson Point, near Kingston, and only Chesapeake Bay is a larger estuary in the United States. Seattle, Tacoma, Everett and Olympia all sit on its shores.
The sound’s shape comes from the Vashon Glaciation. About 15,000 years ago the Puget Lobe of the Cordilleran Ice Sheet pushed south from Canada, burying the site of Seattle under roughly 910 m of ice and reaching past Olympia to near Tenino. As it melted back, beginning about 14,000 years ago, meltwater streamlined the ground into hundreds of aligned drumlins and deepened the long troughs now filled by Hood Canal, Lake Washington and Lake Sammamish. Temporary lakes such as Glacial Lake Russell filled the main trough before the sea flooded in.
Dry Falls and the Channeled Scablands
East of the Cascades the rain shadow takes over; parts of the Hanford Site receive only 150 to 180 mm of precipitation a year. Beneath the sagebrush lie the Columbia River basalts, flood lavas that erupted mostly between 17 and 14 million years ago and in places pile up more than 1.8 km thick. Across this plateau runs the strangest landscape in the state.
The Channeled Scablands are a maze of dry river channels, rock basins and bare basalt buttes, 30 to 230 m high. In the 1920s the geologist J Harlen Bretz argued that only immense, sudden floods could have cut them, and for decades his catastrophic floods met vehement opposition from other geologists. The source turned out to be Glacial Lake Missoula in Montana, dammed by ice on the Clark Fork River; each time the dam failed, the lake drained across eastern Washington. More than 40 such floods occurred during the last glacial maximum, the final ones between about 18,200 and 14,000 years ago. Bretz received the Penrose Medal in 1979, at the age of 96.
Dry Falls is the clearest single piece of evidence. At the head of the Lower Grand Coulee, a scalloped cliff 5.6 km long and 120 m high marks where floodwater once plunged over a cataract estimated at five times the width of Niagara Falls. Today small lakes sit in its plunge pools, and the Grand Coulee itself runs about 97 km from Grand Coulee Dam to Soap Lake, a riverbed with no river.
Steptoe Butte Above the Palouse Hills
South and east of the scablands, the floods left a different country untouched. The Palouse, shared with north-central Idaho, is built of loess, wind-blown silt that settled out of the air in layers over thousands of years; the steep, rolling hills look like dunes but lack the cross-bedding that moving sand leaves. The region is now a major farming district growing mainly wheat and legumes, and the landscape’s grassy origin may be preserved in its name, possibly from the French pelouse, meaning a lawn or land of short, thick grass. Pullman, home of Washington State University, averages about 518 mm (20.4 in) of precipitation a year.
Rising out of the fields 19 km north of Colfax, Steptoe Butte reaches 1,101 m (3,612 ft), about 300 m above the surrounding hills. It is made of quartzite more than 400 million years old, an island of ancient rock that the 15- to 7-million-year-old basalt flows lapped around but never buried. Geologists now use its name, “steptoe”, for any such summit poking through a lava field. From the top the view can stretch 110 to 160 km, with Mount Spokane, 110 km to the north, easily visible.
Put the six features side by side and the logic of the state appears: volcanoes and ice built the west, lava and floods shaped the east, and wind finished the southeast. The same pattern runs through the state’s rivers, climate and farming, which are traced county by county in our guide to Washington’s geography, while the volcanoes get closer treatment in the Mount Rainier volcano guide and the story of the Mount St. Helens eruption.