UTUtah Geography

Three physiographic provinces, one seam

Utah is cut into three pieces of continent that behave nothing like each other. The western third is Basin and Range, crust that has been pulled apart into parallel north south mountain blocks with flat valleys between them. The southeast is the Colorado Plateau, a slab of nearly horizontal sedimentary rock lifted high and then dissected by canyons. The northeast is Middle Rocky Mountains. What makes Utah unusual is that the seam has an address. The Wasatch Fault runs roughly 240 miles along the foot of the Wasatch Range, and Ogden, Salt Lake City and Provo are all built directly on the boundary between stretching crust and stable plateau. Roughly four out of five Utahns live in that narrow corridor.

The one big range that runs the wrong way

Almost every mountain range in the American West trends north to south. The Uintas do not. They run about 150 miles east to west, the largest range in the lower 48 with that orientation, arched up as a broad anticline cored by Uinta Mountain Group quartzite more than a billion years old. Kings Peak, at 13,528 feet, is the state's high point, and the range above 11,000 feet is less a set of summits than a rolling tundra plateau holding more than a thousand lakes scooped out by ice. Four important rivers start there, the Bear, the Weber, the Provo and the Duchesne. The Bear is the oddity: it runs about 500 miles through Utah, Wyoming and Idaho, doubles back, and empties into the Great Salt Lake roughly 90 miles from its own headwaters, making it the longest river in the hemisphere that never reaches an ocean.

A lake with no way out

The Great Salt Lake is terminal. Rivers pour in, nothing flows out, and water leaves only as vapour, so every mineral the rivers carry stays behind. It is the shrunken survivor of Lake Bonneville, which at its maximum covered something like 20,000 square miles and stood about a thousand feet deep over the site of Salt Lake City. When Bonneville rose high enough to spill north through Red Rock Pass in Idaho the outlet failed catastrophically, the lake surface dropped roughly 350 feet, and the flood tore down the Snake River. The old shorelines are still legible as horizontal benches cut into the Wasatch front above the freeways.

A rock causeway built for the railroad in 1959 divided the modern lake into two. The northern arm receives almost no fresh water, so it is far saltier and turns pink from halophilic archaea and Dunaliella algae, and the colour difference between the two halves is visible from orbit. West of the lake bed the Bonneville Salt Flats spread across roughly 30,000 acres of hard salt crust flat enough that land speed records have been chased there since the 1910s.

Where the rain goes, and where it stops

The Wasatch crest decides the fate of a raindrop. Anything falling east or south of it enters the Colorado River system, the Green cutting through Flaming Gorge and Desolation Canyon, the Colorado grinding through Cataract Canyon, both heading for Lake Powell and, in principle, the Gulf of California. Anything falling west of it enters the Great Basin, where the water has nowhere to go and simply evaporates. Utah averages about twelve inches of precipitation a year, second driest in the country after Nevada, yet the Wasatch canyons collect enormous lake effect snowfall as cold air crosses the unfrozen Great Salt Lake.

The state's low point sits in the far southwestern corner at Beaver Dam Wash, 2,180 feet, which is 11,348 feet below Kings Peak. That corner is also the only piece of Mojave Desert in Utah, complete with Joshua trees and desert tortoise, a few hours' drive from alpine tundra.

A rectangle with a bite taken out of it

Utah's outline is almost entirely arithmetic: the 37th parallel on the south, the 42nd on the north, and meridians at 109 degrees 3 minutes and 114 degrees 3 minutes west, both of which were originally specified as round numbers of degrees west of Washington rather than Greenwich. The notch in the northeast corner exists because Congress created Wyoming Territory in 1868 with straight sides and simply took the bite out of Utah. The southeastern corner is the Four Corners monument, the only spot in the country where four states meet. Modern measurement puts the marker a few hundred yards from where the stated coordinates fall, but the corner is legally where the 1870s surveyors set it, not where the mathematics says it should be.

The temperature range inside those lines is extreme. On 1 February 1985 a limestone sinkhole in the Bear River Range called Peter Sinks recorded 69 degrees below zero, the second coldest reading ever taken in the contiguous United States, because the bowl traps dense cold air on clear nights and cannot ventilate. Five months later, on 5 July of the same year, St. George at the opposite end of the state hit 117 degrees.

Geography of the states around Utah

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