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<br />.,.',.. <br />i:{}1 <br /> <br />l\) <br />-..l <br />..... <br />C..'l <br /> <br />?:.;~tj <br />.-,:. <br /> <br />3 <br /> <br />PRESENT WATER USES AND HYDROLOGIC CONDITIONS <br /> <br />In a semi-arid area such as the Upper Colorado River Region, rivers <br />and streams are an important source of water supply for activities which <br />could not be sustained if they were to rely solely upon precipitation for <br />the necessary water. Furthermore, the small yield of rivers in this Re- <br />gion, as compared to that of rivers in more humid areas of the United <br />States, means that the consumptive uses which are made of these rivers <br />have a relatively large impact on their hydrologic regimes. Finally, of <br />course, the rivers and streams of the Upper Colorado River Basin are also <br />important ecological, recreational, and aesthetic resources. <br /> <br />Under these circumstances, it is. important to review the present <br />uses of water in the Basin and the resulting hydrologic conditions. The <br />sections which follow will address both surface and ground water. <br /> <br />SURFACE WATER <br /> <br />The present day flows of Upper Colorado River Basin streams are a <br />function of natural runoff conditions and the modifications caused by <br />man's activities. Likewise, present water quality conditions reflect the <br />comfosite effects of naturally occurring chemical, biological, and phys- <br />ica parameters plus those changes induced by man. The paragraphs which <br />follow briefly review each of these items. <br /> <br />Natural Flow Estimates <br /> <br />The natural, or virgin.,. flows of a river are defined as flows <br />which would have occurred if that river were in its natural state unaf- <br />fected by the activities of man. For the Upper Colorado River Basin, <br />new (1978) natural flow estimates for the period from 1906 through 1974 <br />have been made by the U.S. Bureau of Reclamation for this assessment. <br /> <br />Runoff conditions in the Upper Basin are characterized by two salient <br />features. The first is the extreme variability in the amount of runoff <br />which occurs from year to year. As can be seen in table 3.1, the estimated <br />maximum natural flow at the outflow points of the different WAUs during <br />the period 1906-1974 has exceeded the minimum flow by 300 percent to as <br />much as 600 percent. This variation in annual runoff conditions <br /> <br />3-1 <br />