Grand Canyon: A Transformed Sliver of the Mojave

The mile-deep gash of the Grand Canyon connects the drought-adapted Mojave Desert to the frost-adapted Great Basin Desert. As shown in the color insert on page C-2, the desert plants and animals in the bottom of the canyon have evolved to adapt to the odd combination of desert heat and rampaging river in the course of the canyon's five-million-year history, especially in the 10,000 years since the last ice age. However, no change has been as rapid or complex in its effects as the construction of a dam that has transformed the nature of the river as it twists and turns through the 277 miles (446 km) of the Grand Canyon.

That is especially true for the strange assemblage of desert fish that adapted to the moods of the warm, flood-prone river from the oncemassive delta in the Gulf of California to its cold, clear origins in the Rocky Mountains. Once upon a time, strange fish with whiskers, humps, fleshy lips, and giant tails ruled the capricious, silt-choked, flood-prone Colorado River. They thrived for millennia despite enormous floods, warm water, and silt so thick they often had little use for eyes. Then came the engineers, the dams, and remorseless invaders like trout, carp, and catfish. The world of the besieged native Colorado River fish changed abruptly, and the six-foot-long (2 m) squawfish, the pout-faced flannelmouth suckers, and the strangely shaped humpback chubs all but disappeared from the desert rivers they once dominated. On the Colorado, they held out mostly in the tributaries where conditions had remained largely unchanged since the waning of the Ice Age.

The Colorado River has been transformed by a series of dams that have eliminated the silt-laden spring floods that once increased the flow of the river 100-fold. Once, the river could carry 27 million tons of mud past a given point in a single day and was commonly described as too thick to drink and too thin to plow. Once, the river temperature varied dramatically from summer to winter. Now, however, water released from the bottom of Lake Powell keeps the river's temperature frigid and constant all year round. Moreover, nonnative fish adapted to the cold temperatures, lack of silt, and lack of floods now dominate the river.

The changes caused by the dam have altered the ecosystem in the Grand Canyon, creating hosts of winners and losers. Trout have thrived, squawfish have disappeared. Salt cedar has exploded, willows have retreated. Beavers have boomed, otters have vanished. On one hand, the clear water and the tamed floods have made the streamside ecosystem more productive by causing a population explosion at the base of the aquatic food chain. On the other hand, the surges of water released each day to generate electricity for faraway cities also threaten to strip sand from beaches, eroding the foundation of the whole system in the long term.

The most dramatic ecological changes took place in the water column, where the temperature plunged and the rush of light fostered sweeping changes. Bottom-growing forests of cladophora, a green alga, spread rapidly throughout the river. This ubiquitous green moss shelters microscopic, one-celled diatoms, which feed insects and invertebrates like the tiny, shrimplike Gammarus. These invertebrates were planted in the river to help the introduced trout, which love cold, clear water where they can hunt by sight. They gobble up Gammarus and black fly larvae, which cover rocks in clear, fast-moving water. All of which helps explain why the native fish have mostly disappeared. They cannot spawn in the cold water, and their young spawned on the tributaries cannot escape the hungry trout, carp, and catfish.

Other changes rippled through the ecosystem with the change in the river. For instance, elimination of the floods proved a boon for the resourceful salt cedar, also known as tamarisk. These thirsty, fast-growing imports quickly colonized the new, fluctuating waterline. The tamarisks exude chemicals that drive out other plants, root readily from debris washed downstream, and choke off the competition. Along most of the 277-mile (445 km) length of the canyon, the tamarisk have moved into the niche that might otherwise have gone to some combination of willows and cottonwoods.

The tamarisk invasion in its turn created a long list of winners and losers. Some suffered, including the willow-loving flycatchers, the mudloving swallows, and the cattail-loving yellowthroats. Others thrived— beetles, cicadas, and some birds. Most spectacular are the bald eagles, which flock to Nankoweap Creek in growing numbers each winter to eat their fill of the rainbow trout spawning in frenzied, mindless passion. The eagles probably didn't make much use of the canyon before the dam since they would have had a hard time finding fish in the opaque waters. Peregrine falcons also appear to be doing well, since the canyon apparently boasts the largest known population of the world's fastest fliers. They patrol the canyons, preying on the abundant swifts and bats, which in turn prey on the plentiful insects spawned by the enhanced productivity of the water and the streamside vegetation. Other birds have expanded their ranges into the canyon, moving along the ecological highway of the streamside vegetation, including sparrows, starlings, black-chinned hummingbirds, summer tanagers, hooded orioles, yellow warblers, and greattailed grackles.