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<br />~ <br />~ <br />~ <br />w <br /> <br />interior basins. The sedimentary rocks, rich in carbons and hydro- <br /> <br />carbons, form the basis of much of the Region's mineral wealth. <br /> <br />The composition of these sedimentary rocks also affects water qual- <br /> <br />ity. Salts leach into surface and groundwaters or are dissolved when <br /> <br />surface water flows directly over salt domes. The siltstones, clay- <br /> <br />- I <br /> <br />stones, and mudstones add large volumes of sediment to the streamflows. <br /> <br />The climate is dry and has extreme temperature variations between <br /> <br />various locations and daily at any given location. Relative humidity is <br /> <br />low, generally from 10 to 60 percent. The combination of wind, tempera- <br /> <br />ture, and low humidity results in a high rate of evaporation. <br /> <br />Precipitation is heaviest in the winter and spring, except in the <br /> <br />extreme south, where most moisture comes from summer thunderstorms; <br /> <br />it varies from 6 inches in the valleys to 50 inches in the mountains. <br /> <br />Most of the Region receives total annual precipitation of 10 to 20 inches. <br /> <br />Annual snowfall ranges from 5 inches in the lower valleys to 200 - 300 <br /> <br />inches in the higher mountains. <br /> <br />Surface stream runoff has extreme variability. Estimated natural <br /> <br />flows of the Colorado River at Lees Ferry, Arizona], have varied by more <br /> <br />- <br /> <br />than a factor of two from one year to the next with record flows ranging <br /> <br />from 7 million acre feet (maf) to 24 maf annually. Significant monthly <br /> <br />] <br />In this report, Lees Ferry refers to the U.S. Geological Survey <br />gaging station, while Lee Ferry refers to the demarcation point between <br />the Upper and Lower Colorado River basins in the 1922 Colorado River <br />Compact. <br /> <br />2-4 <br />