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Show LAND FORMS AND GEOLOGY The Lower Colorado Region is composed of a complex of plateaus, mountains, canyons, deserts, and plains, with elevations ranging from 75 feet above sea level near Yuma, Arizona, to over 12,600 feet above sea level at Humphreys Peak near Flagstaff, Arizona, The topography takes in virtually every form and degree from level plains to precipi- tous moun-tains and canyons between these elevation extremes. Similarly, the geology of the Region includes a broad spectrum of sedimentary, metamorphic and igneous rocks which produce a wide variety of soils locally and along stream courses. In short, the principal physical characteristics of the Region are its great variety of land forms, topography, and geology. STREAM DEVELOPMENT Many of the streams tributary to the Colorado River head in narrow canyons and run through deeply cut gorges. The Colorado River became a through-flowing stream in the late Cenozoic time. Downcutting by the river and its tributaries resulted in deep entrenchment of the entire system creating spectacular canyons, most notable of which is the Grand Canyon of the Colorado. All runoff from the high plateaus and mountains in the Lower Colorado Region is tributary to the Colorado, Little Colorado, and Virgin Rivers. The Colorado and Virgin Rivers are perennial; all other streams are intermittent at their confluence with the Colorado River except for short reaches downstream from springs such as Blue Spring and those in Havasu Canyon and near St. Johns, Arizona. As measured in terms of fish habitat, there are approximately 2,500 miles of perennial streams in the Region, but much of the tribu- tary runoff never reaches the Colorado River. GROUND-WATER BASINS The storage and movement of ground water in sedimentary rocks are controlled by regional structure of the basins and upwarps. Storage and movement in alluvial basins are frequently controlled by valley plugs created by lava flows, tectonic movement, and nearly impervious alluvial deposits. The main areas of recharge are on the highlands, on the upwarps, and along the structural divides. Movement of ground water is downdip from the highlands toward the perennial streams. |