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<br />l'\) <br />W <br />N <br />00 <br /> <br />The geology of the Region generally consists of igneous and meta- <br />morphic rock boundaries (found on the mountainous perimeter) with <br />interior basins of sedimentary rocks. These sedimentary rocks were <br />formed in the Mesozoic era (a period of marine deposition) and are <br />richly supplied with carbons and hydrocarbons derived from ancient <br />vegetative sources. Thus, they form the basis of much of the Region's <br />mineral wealth. <br /> <br />These sedimentary rocks are also the source of actual or potential <br />degradation of water quality. Many of the rocks contain soluble minerals <br />(i.e., salts) which leach into surface and ground waters. Salt loading <br />also occurs when surface water flows directly over salt domes. Shales <br />are the primary rock types containing soluble salts. The siltstones, <br />claystones, and mudstones add large volumes of sediment to the stream <br />flows. Potential problems with siltation and salt loading can be created <br />when these sedimentary rocks are disturbed. <br /> <br />The Region's climate is characterized by dryness and extreme <br />temperature variations (when one compares winter temperatures at higher <br />elevations with summer temperatures at lower elevations or when one <br />takes into account diurnal fluctuations at any given location). Relative <br />humidity is low, with readings generally from 10 to 60 percent throughout <br />the year. The combination of wind, temperature, and humidity accounts <br />for a high rate of evapuration. <br /> <br />Precipitation is heaviest in the winter and spring, except in the <br />extreme southern portion of the Region, where most of the moisture is <br />in the form of summer thunderstorms. Snowfall varies from 5 inches <br />per year in the lower valleys to 200 or 300 inches per year in the <br />higher mountains. Total precipitation varies from 6 inches of water <br />in the valleys to 50 inches in the mountains, although most of the <br />Region receives from 10 to 20 inches of precipitation annually. <br /> <br />Surface stream runoff in the Upper Basin is characterized by two <br />salient features. The first is the extreme variability in the amount <br />of runoff that occurs from year to year. The estimated natural flows <br />of the Colorado River at Lees Ferry, Arizonal have varied by more than <br />a [actor of two from one year to the next. Figure 2.4 illustrates this <br />point. The second characteristic is the monthly variation that occurs <br /> <br />1. "Lees Ferry" is the name of the U.S. Geological Survey gauging station <br />located on the Colorado River upstream from the town of Lees Ferry and <br />above the confluence of the Paria River with the Colorado. "Lee Ferry" <br />is the point on the Colorado River one mile below the mouth of the Paria <br />which serves as the point of demarcation between the Upper and Lower <br />Colorado River basins for the purposes of the 1922 Colorado River Compact. <br />As used in this report, "Lees Ferry" will mean the gauging station, while <br />the phrase "compact point" will be used to refer to "Lee Ferry." <br /> <br />2-5 <br />