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Show THE HYDROLOGIC ENVIRONMENT 11. A. Introduction Glen Canyon National Recreation Area comprises a number of diverse water resources. The largest of these is Lake Powell, which was formed with the completion of Glen Canyon Dam on the Colorado River. As part of the extensive Colorado River Basin, Lake Powell receives water from sources in Wyoming, Colorado, New Mexico, Arizona, and Utah. II. B. Description of the Area II. B. 1. The watershed. Glen Canyon NRA is centrally located in the Colorado River Basin in southeastern Utah ( Fig. 2). The major tributary rivers flowing into Lake Powell are the San Juan, the Colorado, the Dirty Devil, and the Escalante. The watershed is extremely varied in ecosystem type and land use and therefore is influenced by many different factors. The total drainage area above Glen Canyon Dam is 111,700 square miles, which includes 3,959 square miles in the Great Divide Basin in southern Wyoming that are noncontributing to flow ( USGS 1984). A fifth major river, the Paria, enters the recreation area fifteen miles below Glen Canyon Dam, at the unit's boundary with Grand Canyon National Park. II. B. 2. Climate. Although the Colorado River drains some areas receiving more than 50 inches annual precipitation, the Glen Canyon region is classified as semi- arid to arid. Glen Canyon NRA receives an average of only six to seven inches of precipitation per year, although high plateaus typically receive several inches more and canyon bottoms several inches less. Annual precipitation extremes of four inches ( low) and ten inches ( high) have been recorded in the recreation area. Snowfall occurs in the area, but it normally remains on the ground only a few days below about 7,000 feet ( USDI 1984). Brief, intense thunderstorms produce virtually all of the moisture received during the summer. March, August, and September are generally the wettest months, and June is the driest. Summer thunderstorms that sweep through the area pose a dual threat to visitors. First, the brief but intense rains can cause flash floods in canyon bottoms, producing dangerous, fast- flowing water that carries large amounts of debris and in extreme cases extends to both walls in the canyons. Second, severe winds, often gusting to 70 miles per hour or more, have been known to capsize small craft on Lake Powell ( NPS 1979). Evapotranspiration in the Glen Canyon area greatly exceeds annual precipitation during all but the winter months, and is estimated to be 7 |