|
<br />
<br />5
<br />
<br />
<br />the interior basin consists of plateaus that range from 5,000 to
<br />8,000 feet in elevation. The entire basin averages about 6,700 feet
<br />in elevation. The portion of the basin above Glen Canyon Dam
<br />averages about.7,400 feet in elevation.
<br />
<br />The Colorado River and its principal tributaries flow mostly in deep
<br />canyons. The Green River, the largest tributary, flows through
<br />similar canyons in Wyoming, Colorado, and Utah. The Yampa, White,
<br />Duchesne, Price, and San Rafael Rivers, flow through canyons and
<br />rolling hills to the Green River. The Gunnison and Dolores Rivers
<br />drain much of western Colorado prior to joining the Colorado River.
<br />The San Juan River drains mountains and plateaus in southwestern
<br />Colorado, northwestern New Mexico, and northern Arizona, and flows
<br />through a large canyon in southern Utah to join the Colorado River
<br />in Lake Powell. Below Glen Canyon Dam, the Colorado River flows
<br />through Marble Canyon and Grand Canyon and then into Lake Mead. The
<br />Little Colorado River joins the Colorado River in this lower area.
<br />It flows through wide valleys and some canyon areas in its upper
<br />reaches and then enters a broad, low-walled, sandy channel in the
<br />middle reaches. In the lower reaches, it flows through a deep
<br />canyon until it reaches the Colorado River in Grand Canyon. The
<br />Virgin River combines with the Colorado River in Lake Mead.
<br />
<br />Many dams and reservoirs have been constructed in the basin over the
<br />years. The larger reservoirs are formed from water impounded by
<br />Fontenelle, Flaming Gorge, Blue Mesa, Morrow Point, Crystal, Dillon,
<br />Navajo, Glen Canyon, and Hoover Dams. Table 1.1 shows the drainage
<br />areas and reservoir capacities for these reservoirs.
<br />
<br />Rocks of all geological ages, from the Archean age to most recent
<br />alluvial deposits, are found in the Colorado basin. The Rocky
<br />Mountains in the basin are composed of granite schists, gneisses,
<br />lavas, and sharply folded sedimentary rocks. The high watershed
<br />mountains in Colorado (Rocky Mountains), Wyoming (Wind River
<br />Mountains), and Utah (Uinta and Wasatch Mountains) have all been
<br />glaciated. The geology of the plateau areas in the basins of
<br />southwestern Wyoming, eastern Utah, and northern Arizona is
<br />primarily horizontal sedimentary rock. There are also many
<br />formations of hard sandstone, limestone, and soft shale. Remnants
<br />of volcanic activity can also be found in many areas of the basin.
<br />The Colorado River and its tributaries have cut narrow, 'deep canyons
<br />into the flat-topped mesas of the basin. The broad Grand Canyon
<br />area was carved as the formations arched several thousand feet
<br />higher than the surrounding country, and the Colorado River cut
<br />through these formations to the ancient underlying granites.
<br />
<br />Soils of the basin consist of the remains of the underlying parent
<br />rocks found in the basin. The parent materials include limestones,
<br />sandy limestones, sandstones, shales, conglomerates, valley and
<br />river alluvium, terrace gravels, igneous and metamorphics, and
<br />volcanics. The soils include sands, silts, clays, and loams, and
<br />vary widely in areal extent and location. Most of the deeper soils
<br />are found in the higher elevations in both the lower and upper
<br />basins.
<br />
|