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occasionally even diverting flow into inland basins such as the Salton Sea. The Colorado River Delta was further influenced by a remarkable tidal bore. Tides in the Sea of Cortez exceed 9.0 m at the river's mouth, among the highest in the world. Swift, destructive tidal currents moved with remarkable speed for more than 100 km upstream. Low Desert, Riverine Fish Communities Canyons of large desert rivers present two basic types of habitat. Most impressive are the reaches that are termed whitewater in the jargon of river runners, where bottom irregularities cause waves, whirlpools, and other turbulence. The second type is called flatwater, where deep, strong, unobstructed flow occurs, but with little surface roughness (Fig. 58). Figure 58. Looking downstream from a flatwater reach to a major rapid on the Colorado River in Grand Canyon, Arizona, 1970. If ---, The Colorado River is notorious for whitewater, especially in the Grand Canyon, and the Rio Yaqui flows through similarly constrained reaches through parts of its course. Wider places separating canyons tend to have different faunal compositions, covered below. Canyons of the Colorado River support a special fish community consisting mostly of humpback chub, speckled dace, bluehead mountain-sucker, and flannelmouth sucker. Of these, only the humpback chub (Fig. 59) is found in whitewater, where it lives in deep eddies associated with large boulders, indentations in canyon walls, or other protecting obstructions. This is one of the most bizarre minnow species in the world, with large, strong fins, leathery skin with deeply embedded scales, and a remarkable hump between the head and dorsal fin. The last reflects a powerful and compact musculature required for movement within its habitat and through rapids separating one eddying habitat from another. Despite years of study, little is actually known of the humpback chub's feeding and reproduction. We know that it lives a relatively long time, perhaps more than 30 years, and that it reproduces in spring/early summer. The fish is rare, and classed as endangered, so few have been sacrificed for biological examination. Speckled dace (Fig. 31) live along sand and gravel bars of flatwater reaches of the Colorado River. Flannelmouth suckers (Fig. 60) remain in deeper water except when feeding, when they move onto Figure 59. The largest known population of endangered humpback chub lives in the lowermost Little Colorado River, Arizona. This adult, 36.0 cm in total length, was photographed at Willow Beach National Fish Hatchery. 29