OROregon Geography

The lake in the hole where a mountain used to be

Roughly 7,700 years ago Mount Mazama, a Cascade volcano that stood somewhere near 12,000 feet, erupted hard enough to empty its own magma chamber and then collapsed into the space it had vacated. Ash from that single eruption has been identified across eight states and up into Canada. The caldera filled with snowmelt and rain, and what sits there now is Crater Lake, 1,943 feet deep, the deepest lake in the United States. It has no inlet and no outlet. Every drop arrived as precipitation and leaves by evaporation or seepage, which is why the water carries so little sediment and reads as an improbable blue. Wizard Island, the cinder cone breaking the surface near the western rim, erupted after the collapse. Congress protected the caldera as a national park in 1902, the fifth in the country, and it is still the only national park in Oregon.

One state, two climates, one wall between them

The Cascade Range runs the full length of Oregon from the Columbia to the California line and works as a weather dam. Pacific storms are forced up the western slope, cool, and unload; whatever crosses the crest has already been wrung out. The contrast is extreme. Parts of the Coast Range take well over 100 inches of rain a year, while the Alvord Desert in the southeast, lying in the shadow of Steens Mountain, gets around seven.

The volcanoes along the crest are the tallest ground in the state: Mount Hood at 11,249 feet, Mount Jefferson at 10,497, and South Sister at 10,358. Hood carries eleven named glaciers, which puts Oregon among the very few states outside Alaska and Washington with permanent ice. The Columbia River Gorge is the only break in the wall, the sole sea-level route through the Cascades, and the pressure difference between the wet and dry sides funnels wind through it hard enough to have made Hood River a windsurfing capital. Multnomah Falls drops 620 feet on the Oregon side of the gorge.

A treaty line, a river and the deepest gorge on the continent

Oregon's southern boundary is the 42nd parallel, and it is there because Spain and the United States agreed on it in the Adams-Onis Treaty of 1819 as the northern limit of Spanish claims. That line was drawn decades before anyone surveyed the ground, and it still separates Oregon from California and Nevada without a single bend in it.

The northern boundary is the Columbia River for almost its entire run, so Oregon and Washington share a working shipping channel instead of a fence. Portland lies about 100 river miles inland and still loads ocean vessels. On the east the Snake River does the same job against Idaho, and where it does it flows through Hells Canyon, the deepest river gorge in North America. From the top of He Devil Peak on the Idaho side down to the water is close to 8,000 feet, more than a mile and a half of relief.

The empty two thirds

Most people picture nothing east of the Cascades, but the ground there is more varied than the wet side. The Deschutes and Columbia plateaus are floored with basalt from the Columbia River flood basalts, lava that came out in sheets between roughly 17 and 6 million years ago and buried an area the size of a small country. The Blue and Wallowa Mountains rise in the northeast, the Wallowas granite and glacier-cut enough that people call them the Oregon Alps. Steens Mountain in Harney County is a single tilted fault block, 9,738 feet along its long western ramp and then a drop of about a vertical mile to the playa on the east side.

The John Day Fossil Beds hold 40 million years of plants and animals in banded claystone, and the Painted Hills get their red and gold stripes from ancient soils weathered under climates that no longer exist there. Oregon is also one of the states cut by a time zone line: most of Malheur County in the far southeast runs on Mountain Time, keeping step with Boise rather than Portland.

A coast that belongs to everybody, and a fault offshore

All 363 miles of Oregon shoreline are open to the public. Governor Oswald West had the beaches declared a state highway in 1913 specifically to keep them out of private hands, and the Beach Bill of 1967 confirmed public rights up to the vegetation line. No other state has done that for its entire ocean frontage. Between Florence and Coos Bay the Oregon Dunes pile as high as 500 feet above sea level across 40 miles of coast, the largest expanse of coastal sand dunes in North America.

The Willamette Valley, where most Oregonians live, owes its soil to catastrophe. When the ice dam holding glacial Lake Missoula failed and reformed repeatedly between roughly 15,000 and 13,000 years ago, the floodwater tore down the Columbia, backed up into the valley and left silt hundreds of feet deep. The Willamette itself is one of the few large rivers in North America that flows north, running from Eugene to Portland before it joins the Columbia. Offshore, the Cascadia Subduction Zone last ruptured on 26 January 1700, a date known precisely because the tsunami it sent across the Pacific was written down in Japan. Oregon's temperature extremes are 119 degrees, recorded at Pendleton and Prineville in 1898, and 54 below zero at Seneca on 10 February 1933.

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