1988-04-11_PERMIT FILE - C1981008A

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In this report, these river drainages are placed in the Grand Division of the Upper Colorado River Basin and in the subdivision between the Gunnison and Green Rivers (Exhibit 7-7). Because of the gradual increase in the development of water resources within the Grand Division in this century, streamflow records used for this report (1914-1957) were adjusted to reflect the level of water-use developments (diversions, dams, irrigation, etc.) that existed in 1957. Although the 1957 levels of development varied from basin to basin, the adjustments made in the reported streamflow, records were useful for comparing stream characteristics among the different basins while accounting for the changes in streamflow and water quality caused by the increased developments. Since little additional activity (diversions, dams, etc.) has occurred within the San Miguel River basin since 1957, the information presented in lornsIet al . (1965) report is considered to be representative of present conditions. Regional Surface Water Flow. The precipitation that falls on the basins within the Upper Grand Division of the Colorado is generated from large air masses originating from the Pacific Ocean and the Gulf of Mexico. These air masses originating from these sources are pushed high into the atmosphere, losing much of their moisture prior to entering these basins. The Pacific air masses dominate from October through April, and the Gulf of Mexico air masses dominate during late spring and summer. The higher streamflows within the Grand Division largely result from snowmelt during late spring and early summer. By late July, most stream discharges recede to base flow. Little contributions are made to stream discharges resulting from the infrequent summer thunderstorms. In the lower altitudes of the subdivision between the Gunnison and Green Rivers (Exhibit 7-7), little snow accumulates during the winter months; therefore, most spring runoff events occur principally in the lead waters of the Dolores, San Miguel Rivers, and in the LaSal Mountains. On a more local basis, short duration, high intensity thunderstorms that fall on undeveloped (natural) watersheds provide some of the summer runoff. In addition, return water from irrigation, and water from localized ground-water reservoirs augment streamflow during the summer months. Figure 7-21 is a hydrograph showing the seasonal pattern of runoff in the Colorado River Basin between the Gunnison and Green Rivers, 1954 water year. This hydrograph illustrates that spring runoff is the principal contributor to flow during April , May and June with individual events during the summer months adding to the overall flow. Ground water also contributes to the stream baseflow during the summer months. 7-101 Revised 04/11/88