The Columbia Plateau is a wide lava plateau in the Pacific Northwest of the United States, spread across eastern Washington, north-eastern Oregon and western Idaho between the Cascade Range and the Rocky Mountains. The Columbia River and its largest tributary, the Snake, cut through it. The plateau is built from the Columbia River Basalt Group, flood basalts that buried about 163,700 km² (63,200 sq mi) of the region between roughly 17 and 6 million years ago, in places more than 1.8 km (5,900 ft) thick. On top of the basalt lie two very different landscapes: the bare, flood-scoured Channeled Scablands, cut by the ice-age Missoula floods, and the rolling wheat fields of the Palouse, built from wind-blown silt. Much of its best scenery is protected: Dry Falls lies in Sun Lakes-Dry Falls State Park, Steptoe and Kamiak Buttes have been a National Natural Landmark since 1965, and Wallula Gap on the Columbia carries the same federal designation.
Seventeen Million Years Ago: The Columbia River Flood Basalts
Before 17.5 million years ago, volcanism in the region came from the Western Cascade stratovolcanoes, which had erupted at intervals for more than 20 million years. Then the style of volcanism changed abruptly. Instead of cones, long fissures opened along an old fault line running from south-eastern Oregon towards western British Columbia, and very fluid basalt poured out of them and spread for great distances. The eruptions were most vigorous between 17 and 14 million years ago, when more than 99% of the basalt was released; smaller eruptions continued until about 6 million years ago.
Altogether the Columbia River Basalt Group contains more than 300 individual lava flows, averaging 500 to 600 km³ (120 to 140 cu mi) each, with a total volume estimated at 174,300 km³ (41,800 cu mi). One of the largest, the Roza flow in the Wanapum Basalt, covered 40,300 km² by itself and held about 1,300 km³ of rock. Geologists group the flows into formations including the Steens, Imnaha, Grande Ronde, Wanapum and Saddle Mountains basalts.
As the lava piled up, the crust beneath sagged into the space left by the erupting magma, leaving a broad, slightly depressed basin, the Columbia Basin. Lava advancing towards the north-west forced the ancient Columbia River into roughly its present course, and lava dams in side valleys created lakes whose beds now hold fossil leaves, petrified wood and insect remains. The heat source is still debated. The most widely accepted explanation is a mantle plume, often linked to the Yellowstone hotspot, because the youngest volcanic rocks in the wider region cluster near Yellowstone and the lavas get older to the west.
The Deccan Traps of India and the Siberian Traps dwarf it. Among the largest plateaus in the world it is modest in size, but it is one of the best preserved. As the lavas were erupting, the southern and western parts of the plateau also began to fold into long east-west ridges such as the Horse Heaven Hills, which later rivers would have to cut through.
Glacial Lake Missoula and the Floods J Harlen Bretz Proposed
Long after the eruptions ended, the ice ages rewrote the surface. Near the end of the last glaciation, a lobe of the Cordilleran ice sheet pushed into the Idaho Panhandle and blocked the Clark Fork River at the east end of Lake Pend Oreille. The ice dam stood up to about 600 m (2,000 ft) high and backed water roughly 320 km (200 miles) east into the valleys of western Montana. The resulting Glacial Lake Missoula covered about 7,770 km² (3,000 sq mi) and held about 2,100 km³ (500 cu mi) of water, half the volume of Lake Michigan.
On the usual explanation, the lake grew deep enough to float its ice dam, which then gave way. The lake emptied across eastern Washington in a matter of days, the ice re-formed, and the cycle began again, on average every 55 years. Floods occurred dozens of times, about 40 by one count, during the roughly 2,000 years between 15,000 and 13,000 years ago, and older deposits show similar floods more than 1.5 million years ago. USGS hydrologist Jim O'Connor and Gerardo Benito identified at least 25 massive floods, the largest discharging about 10 km³ an hour, some 13 times the flow of the Amazon, at speeds approaching 36 m/s (130 km/h).
The idea was resisted for decades. In the 1920s the geologist J Harlen Bretz argued that the scablands needed short, immense floods, but he could not say where the water came from, and most geologists of the day preferred slow, gradual erosion. In 1925 J. T. Pardee suggested to him that a draining glacial lake could supply the volume, and Pardee spent the next 30 years building the evidence. That evidence includes giant current ripples in Montana, 8 to 15 m (25 to 50 ft) high and 90 m (300 ft) long. The Geological Society of America finally awarded Bretz its Penrose Medal in 1979, when he was 96.
Dry Falls, the Grand Coulee and the Channeled Scablands
The floods poured south-west across the plateau, stripping away the soil and silt and exposing bare basalt. Bretz called the result scablands: a maze of channels, rock basins and cataracts that, in his words, "run uphill and downhill". Buttes left standing between the channels range from 30 to 230 m (98 to 755 ft) high, and rock basins run up to Rock Lake, 11 km (7 mi) long and 30 m deep. Planetary scientists treat the Channeled Scablands as perhaps the best Earth analogue for the outflow channels on Mars.
The showpiece of the scablands is Dry Falls, at the head of the Lower Grand Coulee. When the ice dammed the Columbia itself and formed Glacial Lake Columbia, the river was diverted through the Grand Coulee. Missoula floodwater then raced down the Upper Grand Coulee at an estimated 105 km/h (65 mph) and over a rock face 120 m (400 ft) high. The resulting cliff is 5.6 km (3.5 mi) long with four major alcoves. At its peak the waterfall was about five times the width of Niagara Falls, with an estimated ten times the flow of all the world's rivers today. When the ice melted the Columbia returned to its old course and left the falls dry.
Today the cliff and its plunge pools can be seen from the Dry Falls Interpretive Center in Sun Lakes-Dry Falls State Park, on Route 17 near Coulee City, while the Upper Grand Coulee above it is filled by Banks Lake. Farther south the floods backed up behind Wallula Gap, a narrow gap where the Columbia cuts through the Horse Heaven Hills. Less than a fifth of the roughly 800 km³ (190 cu mi) a day arriving could pass through, so the water ponded into a short-lived lake, Lake Lewis, over the Pasco Basin. The landforms of Washington guide sets these coulees beside the state's volcanoes and coast.
The Palouse Hills: Wind-Blown Silt on Basalt
East of the scablands the floods did not strip the hills, and the silt cover survives. The Palouse covers south-eastern Washington and north-central Idaho, traditionally the prairie north of the Snake and Clearwater rivers and centred on the Palouse River. Its hills look like dunes but are made of Palouse Loess, a wind-blown silt that covers more than 50,000 km² (19,000 sq mi) of the Columbia Plateau and reaches up to 75 m (246 ft) thick. The loess lies in layers separated by hard calcrete soils, showing that it settled out of the air over many glacial cycles rather than being shaped by moving currents. Much of the silt was itself blown from fine flood sediment that the Missoula floods had dumped around the Eureka Flats.
A few older summits poke through. Steptoe Butte, 1,101 m (3,612 ft), is a quartzite island more than 400 million years old standing above basalts dated to between 15 and 7 million years. It gave geologists the word "steptoe" for any older hill surrounded by lava. Where the scablands cut into the Palouse, the drop is abrupt: Palouse Falls drops 61 m (200 ft) into a canyon 115 m (377 ft) deep, about 6 km (4 mi) above the Palouse River's meeting with the Snake. The name probably comes from the Palus people, perhaps reshaped by French-Canadian traders into pelouse, "lawn" or short-grass land. The deep silt made the region a wheat country after the 1880s, and it is home to Washington State University in Pullman and the University of Idaho in Moscow, just 13 km (8 mi) apart.
The Columbia, the Snake and the Grand Coulee Dam Irrigation Scheme
Two large rivers cross the plateau. The Columbia, 2,000 km (1,243 mi) long, rises in the Rocky Mountains of British Columbia, flows south into Washington, then turns west to form most of the Washington-Oregon border on its way to the Pacific. It has the greatest flow of any river entering the eastern Pacific. The Snake, about 1,740 km (1,080 mi) long, begins in Yellowstone National Park, crosses southern Idaho, runs through Hells Canyon on the Idaho-Oregon border and the Palouse Hills, and joins the Columbia just below the Tri-Cities. Four navigation dams on its lower course made Lewiston, Idaho, the farthest-inland seaport on the West Coast.
The plateau's dryness and deep soils made it an obvious target for irrigation. After years of debate between a Spokane group favouring a gravity canal and a Wenatchee group favouring a high dam, the Grand Coulee Dam was built on the Columbia between 1933 and 1942. With a third powerhouse added by 1974 and later upgrades, its installed capacity of 6,809 MW makes it the largest power station in the United States. Its reservoir, Franklin Delano Roosevelt Lake, displaced more than 3,000 people, including Native Americans whose lands were flooded, and because neither it nor Chief Joseph Dam downstream has fish passage, no salmon now reach Grand Coulee.
Pumps lift water from the reservoir into the old flood channel of the Grand Coulee, now Banks Lake, from where the Columbia Basin Project distributes it. Irrigation water began arriving between 1948 and 1952. Today the project, the largest water reclamation project in the United States, irrigates more than 2,700 km² (670,000 acres) through 533 km (331 mi) of main canals and 2,155 km (1,339 mi) of laterals. The Snake's earlier course is traced in our Idaho geography guide, which follows it from its source near Yellowstone.