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N W W o in anyone year (day-to-day fluctuations are also common). As a function of the precipitation patterns noted above, runoff from snowmelt during the spring and summer months with 65 to 75 percent of the annual total arising, on the average, during the period April through July (see fig. 2.5) . Fish and wildlife resources are important components of the Upper Colorado River Region's natural environment. The Region supports out- standing, although inherently fragile, fish and wildlife habitats. Sport fishing is especially important, attracting thousands of people each year. Game animals also attract thousands of visitors and residents alike, making hunting an impor<snt economic contributor to the Region. Finally, the humpback chub, bony tail chub, and Colorado River squawfish are on the Federal endangered species list in this Region. Oil Shale and Coal Resources The Green River Formation is the geologic source of the most abundant oil shales in the United States (see fig. 2.6). Deposition of these oil shales occurred in a lacustrine (lake) environment. The three ancient lakes in which the Green River Formation was deposited were stable, two- layer lakes which did not overturn annually as most lakes do, probably because of their great depth. The resulting thermal and chemical char- acteristics of the lake allowed the deposition of thin-bedded oil shales and of a number of relatively rare carbonate minerals which occur through- out the formation. The chemistry of deposition is extremely complex. The organic matter that was deposited has become kerogen. The additional minerals include nahcolite, a sodium bicarbonate material which could be used as a source of soda ash or which could find a market as is for use in air pollution control devices. A second mineral is dawsonite, which could become a source of alumina rivaling bauxite deposits in the United States. In southwestern Wyoming, a third metal, trona, is currently being mined commercially to produce soda ash. Finally, halite, a common salt, is also abundant in the deposits. It is difficult to accurately estimate the total amount of the Upper Basin's oil shale resource, but geologists generally agree that it is vast. The Green River Formation is estimated to contain more than 3 trillion barrels of oil-equivalent in-place. In Colorado, continuous oil shale sections 15 to 2,000 feet thick, averaging at least 15 gallons per ton (gpt), exist under 1,380 square miles. These sections contain richer portions averaging 25 gpt that represent 400 billion barrels 2-7