Geological Survey Bulletin 1191 Black Canyon of the Gunnison: Today and Yesterday

Since the early visit of Captain John William Gunnison in the middle of the last century, the Black Canyon of the Gunnison has stirred mixed apprehension and wonder in the hearts of its viewers. It ranks high among the more awesome gorges of North America.

Many great western canyons are as well remembered for their brightly colored walls as for their airy depths. Not so the Black Canyon. Though it is assuredly not black, the dark-gray tones of its walls and the hazy shadows of its gloomy depths join together to make its name well deserved. Its name conveys an impression, not a picture.

After the first emotional impact of the canyon, the same questions come to the minds of most reflective viewers and in about the following order: How deep is the Black Canyon, how wide, how does it compare with other canyons, what are the rocks, how did it form, and how long did it take? Several western canyons exceed the Black Canyon in overall size. Some are longer; some are deeper; some are narrower; and a few have walls as steep. But no other canyon in North American combines the depth, narrowness, sheerness, and somber countenance of the Black Canyon.

In many places the Black Canyon is as deep as it is wide. Between The Narrows and Chasm View in the Black Canyon of the Gunnison National Monument (fig. 15) it is much deeper than wide. Average depth in the monument is about 2,000 feet, ranging from a maximum of about 2,700 feet, north of Warner Point (which also is the greatest depth anywhere in the canyon), to a minimum of about 1,750 feet at The Narrows. The stretch of canyon between Pulpit Rock and Chasm View, including The Narrows, though the shallowest in the monument, is also the narrowest, has some of the steepest walls, and is, therefore, among the most impressive segments of the canyon (fig. 3).

Profiles of several well-known western canyons are shown in figure 1. Deepest of these by far is Hells Canyon of the Snake, on the Idaho-Oregon border. Clearly, it dwarfs the Black Canyon in the immensity of its void, though its flaring walls lack the alarming verticality of the Black Canyon. Arizona's Grand Canyon of the Colorado is acknowledged as the greatest of them all; it is not as deep as Hells Canyon, but it is wider, longer, more rugged, and far more colorful. Its depth is two to three times that of the Black Canyon. Zion Canyon, Utah, combines depth, sheerness, serenity, and color in a chasm that ranges from capacious to extremely narrow. Its Narrows have a depth-to-width ratio unmatched by any other major American canyon.

FIGURE 1.—Comparative profiles of several well-known American canyons. (click on image for an enlargement in a new window)

California's Yosemite Valley, in a setting of sylvan verdure, is unique among the gorges shown in profile in figure 1 in being the only glacial trough; its monolithic walls bear witness to the abrasive power of moving ice. Few cliffs in the world match the splendor of its El Capitan. Lodore Canyon, on the Green River in Dinosaur National Monument, Colorado, is best known, perhaps, for its noisy splashy rapids, first made famous by John Wesley Powell. Lodore Canyon also features towering cliffs of deep-red quartzite. Grand Canyon of the Yellowstone River, Wyoming, is noted for its great waterfalls, dashing river, and bright coloration. The Royal Gorge of the Arkansas River, Colorado, features the "world's highest suspension bridge."

The profiles shown in figure 1 afford some basis for comparing one canyon with another. They cannot abstract in two dimensions the overall impression that each canyon makes. Color, vegetation, outcrop habit, vantage point, season of year, length of visit—even the roar of the river or lack thereof—all contribute to this highly personal effect.

For a river of its size, the Gunnison has an unusually steep gradient through the Black Canyon. The river falls about 2,150 feet from the head of the canyon at Sapinero to the mouth at its junction with North Fork—a distance of about 50 miles and an average rate of fall of about 43 feet per mile. By comparison, the Green River flowing through Dinosaur National Monument—a comparable-sized stream in a comparable-sized canyon—has an average fall over a distance of 50 miles of about 12 feet per mile, and its maximum drop is near to the average drop of the Gunnison. The Colorado in Grand Canyon—a much larger stream— averages 7-1/2 feet per mile. To be sure, the Gunnison drops considerably less than 43 feet per mile through some stretches, but in the monument section it drops much more. The overall fall through the monument is about 95 feet per mile, the greatest drop being in the 2-mile stretch from Pulpit Rock to Chasm View. In that stretch the Gunnison falls nearly 480 feet and locally drops as much as 180 feet in half a mile.

So steep a gradient on a large stream is rare indeed, although notably steeper descents characterize a few rivers. The Yellowstone River in its Grand Canyon descends 1,600 feet in 20 miles at an average rate of 80 feet per mile, this rate including sheer drops of 140 feet and 330 feet at Upper and Lower Falls, respectively. And aside from these famous waterfalls, the Yellowstone still descends more than 56 feet per mile through its Grand Canyon.

Several geologic factors in combination helped create the Black Canyon. Appearances to the contrary, the canyon did not result from some great cataclysmic upheaval in the remote geologic past. It is a result, rather, of the slow tedious process of erosion, and though the day-to-day changes are all but imperceptible, the process is still very much at work. The more obvious manifestations include the turbidity of the river—swollen by floods—in early summer, a mud-laden wash after a heavy rain, an occasional rockfall from a high cliff, and the relentless creep of a landslide. The actual excavation of the canyon was done by the Gunnison River and its tributaries with a strong assist by various other agents of erosion. But the coincidental interplay of several other factors was necessary before the job of removing 25 cubic miles of rock was possible. These factors are discussed at length on the pages that follow.