WSP03310 - Laserfiche WebLink

Home Browse Search

,ei'1" -.;"1::,.' '~:~0 I\:) 00 ~ ...... t:J ~d) The geology of the Region consists in general of igneous and meta- morphic rock boundaries (found on the mountainous perimeter) with interior basins of sedimentary rocks. These sedimentary.rocks, which formed in the Mesozoic era (a period of marine deposition), are richly supplied with vegetative sources of carbons and hydrocarbons. Thus, they form the basis of much of the Region's mineral wealth. These sedimentary rocks are also the source of actual and potential degradation of water quality, many of the rocks contain soluhle minerals (i.e., . salts) which leach into surface and ground waters. Salt loading also occurs when surface water flows directly over salt domes. Shales are the primary rock types containing soluble salts. The siltstones, claystones, and mudstones add large volumes of sediment to the stream flows. Potential problems with siltation and salt loading can be created when these sedimentary rocks are disturbed, The Region's climate is characterized by dryness and extreme temper- ature variations (when one compares winter temperatures at higher eleva- tions with summer temperatures at lower elevations), Relative humidity is low, with readings generally from 10 to 60 percent throughout the year, The combination of wind, temperature, and humidity accounts for a high rate of evaporation. Precipitation is heaviest in the winter and spring, except in the extreme southern portion, where most of the moisture is in the for.m of sum- mer thunderstorms.. Snowfall varies from 5 inches per year in the lower valleys to 200 or 300 inches per year in the higher mountains, Total pre- cipitation varies from 6 inches in the valleys to 50 inches in the mountains, although most of the Region receives from 10 to 20 inches of precipitation annually. Surface stream runoff in the, Upper Basin is characterized by two sal- ient features. The first is the extreme variability in the amount of run- off which occurs from year to year. The estimated natural flows1 of the Colorado River at Lees Ferry, Arizona, which have varied by as much as a factor of two from one year to the next, illustrate this point (see fig. 3.1). The'second characteristic is the monthly variation which occurs in anyone year. As a function of the precipitation patterns note~ above, I. The natural, or virgin, flows of a river are defined as those flows which would have occurred had the river and its drainage area been in a natural state unaffected by the activities of man. . xvii .