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o W 1-' ..c.. PART II. DESCRIPTION OF BASIN A. GeoloiW The upper or northern portion of the Colorado River Basin in wyoming and Colorado is a mountainous plateau 5,000 to 8,000 feet in elevation marked by broad, rolling valleys, deep canyons, and intersecting moun- tain ranges. Hundreds of peaks in these mountain chains rise to more than 13,000 feet above sea level and many exceed 14,000 feet in eleva- tion. Mountain lakes exist in considerable numbers. The southern por- tion of the Upper Basin is studded with rugged mountain peaks interspersed with broad, alluvial valleys and rolling plateaus. The main stream and its tributaries in Colorado generally flow in deep mountain canyons. The Green River, primary tributary of the Colorado River, flows in similar canyons in wyoming, Colorado, and Utah after rising in the Wind River Mountains. The San Juan River, a large tributary, emerges from the mountains of southwestern Colorado, flows through northwestern New Mex- ico, and then traverses the deep canyons of the San Juan in Utah before joining the Colorado River in Glen Canyon. The Glen Canyon section of the main stream and tributaries lies almost entirely in deep canyons. Rocks of all ages from those of the Archean age (the oldest known geological period) to the recent alluvial deposits, including igneous, sedimentary, and metamorphic types, are found in the Colorado River Basin. The high Rocky Mountains which dominate the topography of the upper re- gions are composed of granites, schists, gneisses, lava, and sharply folded sedimentary rocks of limestone, sandstone, and shale. Many peri- o~s of deposition, erosion, and upheaval have played a part in the pres- ,nt ~tructure of these mountains. In contrast to the folded rocks of the mountains which fringe the basin, the plateau country of southwestern Wyoming, eastern Utah, and northern Arizona is composed principally of horizontal strata of sedi- mentary rocks. Slow but constant elevation of the land area has allowed the Colorado River and its tributaries to cut narrow, deep canyons into the flat-topped mesas. This type of erosion reaches its culmination in the Grand Canyon where the Colorado River has cut through all of the sed- imentary rocks down to the oldest Archean granites. The Lower by low ranges. vial deposits. Basin is characterized by broad, flat valleys separated These valleys are filled by large accumulations of allu- Silt removed by constant erosion of the upper areas was deposited in Arizona, California, and Mexico and now forms the great delta of the Colorado River. 5