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`-- and Pennsylvanian rocks are exposed in the Cataract and San Juan canyons. Cretaceous rocks are present in the eastern part of the Kaiparowits Plateau while marine deposition occurred during the Permian, Triassic, Jurassic, and Cretaceous Periods. The Navajo and Wingate sandstones that are extensively found around Lake Powell originated from wind blown sands deposited between the second and third occurrences of the inland seas. During Late Cenozoic period, volcanic activity created lava flows in the Grand Canyon that formed natural dams making temporary lakes in the Glen Canyon vicinity. Therefore, at some point during the evolution of the endemic Colorado River fishes, they may have survived and adapted to lake environments. 3. Climate: Glen Canyon NRA is classified as semi-arid to arid; annual rainfall ranges between 6 to 10 inches. High plateaus typically receive more rainfall than canyon bottoms. Summer thunderstorms are brief, intense and can cause flash flooding. March, August, and September are generally the wettest months, June is the driest. Evaporation greatly exceeds precipitation during all but winter months. The resulting loss from Lake Powell approximates 500,000 acre feet per year. 4. Water Resources: Prior to impoundment, the Colorado River flowing through Glen Canyon was a large, wide river prone to huge spring floods (80,000 to 300,000 cubic feet per second or cfs) which gave way to smaller, steady flows (3,000 cfs) in summer and winter. The smaller base flows were contained in the larger flood plain created by spring floods, giving the appearance of a shallow and turbid water course. The wide river was warmed rapidly by the sunnier sun, with peak temperatures nearing 80 F. Temperatures in the winter were near freezing, creating a wide annual temperature variance and making a harsh environment for wildlife living in the aquatic ecosystem. The river alternated between turbulent rapids cascading through narrow canyon sections with steep gradients, and long gentle runs and riffles in more open canyon sections (such as in Glen Canyon) with gentle gradients. Soils common to the area were constantly transported, being picked up by high flows and redistributed by lower flows. After completion of Glen Canyon Dam, water releases were dependent on power production needs. The law of the river, including compacts and treaties, requires the annual release of 8.23 million acre feet (maf) of water through the dam. Daily releases depend on power generation needs which are greater during the day than at night, and higher during seasons of high power use, such as wane summer days and cold winter nights. Interim flow restrictions call for operation of base flow near 8,000 cfs to maximum flow of 20,000 cfs. Post dam water management regime eliminated spring floods and increased summer and winter flows. Tributaries loin the Colorado River within Glen Canyon NRA. The largest tributary is the San Juan River. Average discharge between 1914 and 1986 was 2,517 cfs. Highest flow ever recorded was 70,000 cfs. Zero flow was recorded twice in the 1930's. The historic flow regime of the San Juan was similar to that of the Colorado, only on a smaller scale. Spring floods produced large volumes of water which then receded into modest base flows. Since the completion of Navaho Dam in 1962, there has been a fairly stable output, averaging 2,500 cfs, within the range of-500-12,000 cfs. 9