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Show about 70 inches per year. The resulting loss from Lake Powell approximates 500,000 acre- feet per year. Maximum ground- water recharge occurs during the winter months, when rainfall usually exceeds evaporation. In all, less than 1 inch of the total annual precipitation is accounted for by surface runoff ( USDI 1984). II. B. 3. Geology. Glen Canyon NRA is in a highly dissected plateau landform. The broad upwarped surface is transected by two upfolds, the Waterpocket Fold and the Echo Monocline. The area is characterized by a maze of deep canyons with nearly vertical walls. Flat- topped mesas and rock platforms rise in large tiers from the main drainages to the upland regions of the Colorado Plateau. Elevations in the recreation area range from 3,100 feet at Lees Ferry to 6,200 feet in the Orange Cliffs. Surface stratigraphy includes outcrops of rock that range in age from Pennsylvanian to Cretaceous. Pennsylvanian and Permian rocks are exposed in the Cataract and San Juan Canyons. Cretaceous rocks are present in the eastern part of the Kaiparowits Plateau, between Rock Creek Canyon and Navajo Point ( USGS 1975). Marine deposition occurred during the Permian, Triassic, Jurassic, and Cretaceous Periods. The Navajo and Wingate Sandstones that are found extensively around Lake Powell originated from wind- blown sands deposited between the second and third occurrences of the inland seas. I I. C. Ground- Water Resources II. C. 1. Aquifers. Ground water typically occurs in the older rocks at or below the canyon floors, although perched bodies of water also occur on and beneath most mesas ( Fig. 3). The aquifers are composed of beds of sandstone that lie between nearly impermeable layers of siltstone and mudstone. The main fresh- water aquifers are in the Coconino Sandstone, Wingate Sandstone, Navajo Sandstone, Saltwash member of the Morrison Formation, and alluvium; but all other units yield some water locally to wells and springs ( USGS 1975). Recharge from rainfall or snowmelt near the aquifer outcrops moves vertically to a saturated zone, then downgradient along the regional dip of the geologic strata. Secondary structural features such as faults and fractures affect movement locally by enhancing the permeability of the rocks along their alignments. The annual volume of recharge is a small percentage of both annual precipitation and the total ground water in storage. Hydrogeologic conditions suggest that most recharge occurs on sandy mesa tops and along canyon floors ( USDI 1984), and probably occurs as direct infiltration during snowmelt and rainstorms. In the canyons, some of the ephemeral surface flow infiltrates the alluvium and sandstone bedrock of the stream channels. Although little data is available on total recharge in the area, recharge is estimated to be in the range of a few hundred acre- feet per year in excess of what is absorbed 9 |