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. ..,,"'.. W f\..:. l\) OTHER GEOGRAPHIC .FACTORS* For the purposes of this study, the Upper Colorado River Region includes: (a) the surface drainage area of the Golorado River above Lees Ferry, Arizona, and (b) the Great Divide Basin. The Region thus defined encompasses 113,496 square miles. For a more detailed discussion of the following material, please see Appendix A. The Region's character revolves mainly around one theme: the dry- ness of its climate. Only 15 percent of the total area receives 85 percent of the Region's moisture that falls each year--that is, elevations above 8,000 feet receive more than 30 inches annually, while lower elevations receive less. than 30 inches of precipitation each year. The low rainfall, in turn, contribute~ to low humidity. High heat is another characteris- tic of the Region, caused primarily.by the high insolation received. These factors--low humidity and high temperatures--plus high winds, cause large evaporative losses of moisture that does fall. In fact, only about 15 percent of the total precipitation contributes to stream flow (Weather- ford and Jacoby, 1975). . Variations in regional stream flows due to the geographic influence of mountains and the above climatic factors are another regional charac- teristic. The majority of the precipitation comes in the form of snow that is deposited on the higher elevations, especially along the Region's eastern mountains such as the Wind River, Front, and San Juan ranges. The role of. the mountains as precipitation collectors, in fact, is very important to the Region. If mountains did not exist, it is likely the Region would be much more arid than it is. Because Colorado, southwestern Wyoming, and northeastern Utah claim the bulk of the mountainous .area, most of the stream flow in the Region originates there. The geology of the Region and the sparse precipitation causing low stream flows (which in turn reduce the dilution possibilities of streams), produce an insidious problem--high salinity (total dissolved solids) con- centrations in much of the Regions's surface and ground waters. Most of the rocks are sedimentary types which contribute large quantities of soluble minerals such as calcium sulfate, sodium chloride, and potassium salts. Shales cover a large portion of the Region. and are primary salt contri- butors to the Region's waters. In addition, claystones, siltstones, mud- stones, and other highly erodable rocks in the Region add to the large sediment loads of streams. Climate, topography, and geology create three vegetation zones in the Region--(l) alpine land, (2) desert rangeland, and (3) miscellaneous pasture, cropland, urban land. and barren land. For a descriotion of the vegetation. within each zone, see Appendix A. . Between 1950 and 1975 the population of the Region grew from 281,154 '. *References Eor this section appear in the bibliography at the end of Appendix A: Geography. 2-6