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<br />3 <br /> <br />PRESENT WATER USES AND HYDROLOGIC CONDITIONS <br /> <br />INTRODUCTION <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 <br />for the necessary water. Furthermore, the small yield of rivers in this <br />region, 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, <br />of course, the rivers and streams of the Upper Colorado River Basin are <br />also 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, <br />The sections which follow will address both surface and ground water. <br /> <br />SURFACE \,ATER <br /> <br />. <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 <br />the composite effects of naturally occurring chemical, biological, and <br />physical parameters plus those changes induced by man. The paragraphs <br />which follow briefly review each of these items, <br /> <br />Natural Flow Estimates <br /> <br />The natural, or virgin flows <br />which would have occurred if that <br />fected by the activities of man. <br />new (1978) natural flow estimates <br />have been made by the U,S, Bureau <br /> <br />of a river are defined as those flows <br />river were in its natural state unaf- <br />For the Upper Colorado River Basin, <br />for the period from 1906 through 1974 <br />of Reclamation. <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 Water Accounting <br />Units (WAUs) during the period 1906-1974 has exceeded the minimum flow <br />by 300 to as much as 600 percent, This variation in annual runoff <br /> <br />. <br /> <br />3-1 <br />