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The Colorado River ecoregion supports a number of unique large-river fishes that have adapted to turbid, fast-flowing waters. The tributaries and springs of this ecoregion also support many unusual, endemic species. The Colorado River, which flows from its source in the Colorado Rockies to the Gulf of California, was historically one of the most powerful rivers in the world, with flows rivaling those of China’s Yangtze. Over geologic time, as it dropped more than two miles in elevation along its course, the Colorado carved such formations as the Grand Canyon. Just as the landscape is a striking reminder of the Colorado’s power to shape the Earth’s surface, the diverse species found in and along the river represent the river’s power to shape evolution.
The oddly streamlined forms of the Colorado’s fish are extraordinary examples of adaptations to an extreme and rapidly fluctuating flow of water. Today, these species struggle to survive in a severely altered environment, where the river’s flow is tamed by multiple dams on the main stem and its tributaries, and where water diversions and extractions have reduced the flow in dry years to a mere trickle along some parts of the river. Over millions of years of geographic isolation, 16 species of fish evolved in the turbulent Colorado River system. The humpback chub, with a dorsal hump that acts as a stabilizing keel, an overhanging snout that protects its mouth as it feeds, and reduced scales that minimize friction, is one of the river's declining species. Similarly, the endangered razorback sucker has a highly muscular dorsal keel and a flat head that help maintain its position on the river’s bottom as it feeds. The Colorado squawfish, yet another endangered species, is the largest North American minnow, attaining a length of up to 59 inches (150 cm) and weighing 44 to 77 pounds (20 to 35 kg). The springs and tributaries that feed the Colorado River support unusual species such as the Little Colorado spinedace, Kendall Warm Springs dace, desert pupfish, and springfish. Several native snails also live in these springs, including the Overton assiminea and the Grand Wash spring snail.
These and other endemic species are threatened by the Colorado River’s transformation into a regulated system. Human settlement and development of the Southwest since the turn of the century have required vast amounts of water. Beginning with the Hoover Dam in the 1930s, more than 20 major dams have been built on the Colorado main stem and its large tributaries, with serious consequences for native plants and animals. The dams have severely disrupted the river’s natural flow, replacing winter and spring floods with high summer flows to meet irrigation needs. The river temperature has also changed: The cold water that is regularly released from deep reservoirs has replaced the Colorado’s naturally warm waters. Within reservoirs, the silt and sediment that once gave the Colorado its red color settle out, which increases the transparency of downstream water. These changes, compounded by water withdrawals, have put native species at a disadvantage and have benefited exotic species, particularly coldwater gamefish, such as trout.
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