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<br /> <br />the Dolores Riverand finally to the Colorado River. These river systems are explained thoroughly <br />in lorns' et al. (1965) USGS Professional Paper, Water Resources of the Uoper Colorado River <br />Basin -Technical Reoort. In this report, these river drainages are placed in the Grand Division of <br />the Upper Colorado River Basin and in the subdivision between the Gunnison and Green Rivers. <br />Because of the gradual increase in the development ofwater resources within the Grand Division <br />in this century, streamflow records used forthis report(1914-1957)were adjusted to reflectthe level <br />ofwater-use developments (diversions, dams, irrigation, etc.) that existed in 1957. Although the <br />1957 levels of development varied from basin to basin, the adjustments made in the reported <br />streamflow records were useful for comparing stream characteristics among the different basins <br />while accounting for the changes in streamflow and water quality caused by the increased <br />developments. Since little additional activity (diversions, dams, etc.) has occurred within the San <br />Miguel River basin since 1957, the information presented in lorns' eta1. (1965) report is considered <br />to be representative of present conditions. <br />Regional Surface Water Flow. The precipitation that falls on the basins within the Upper Grand <br />Division of the Colorado is generated from large air masses originating from the Pacific Ocean and <br />the Gulf of Mexico. These air masses originating from these sources are pushed high into the <br />atmosphere, losing much of their moisture prior to entering these basins. The Pacific air masses <br />dominate fromOctoberthroughApril, andtheGulfofMexicoairmassesdominateduring latespring <br />and summer. The higherstreamflowswithintheGrandDivisionlargelyresultfromsnowmeltduring <br />late spring and early summer. By late July, most stream discharges recede to base flow. Little <br />contributions are made to stream discharges resulting from the infrequent summerthunderstorms. <br />In the lower altitudes of the subdivision between the Gunnison and Green Rivers, little snow <br />accumulates during the winter months; therefore, most spring runoff events occur principally in the <br />head waters ofthe Dolores, San Miguel Rivers, and in the LaSal Mountains. On a more local basis, <br />short duration, high intensity thunderstorms thatfall on undeveloped (natural) watersheds provide <br />some of the summer runoff. In addition, return water from irrigation, and water from localized <br />ground-water reservoirs augment streamflow during the summer months. <br />• (REVISED 6!28/01) 2.4.7-2~ <br />