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A"' ~,'1~1 ,,~ ")' <:~, -3 ,y t ~ '-' . B. Distribution of Short-Term Runoff by Reach. The gages at the confluence of the West, Middle, and East Mancos rivers were discontinued in 1952. Consequently, there was no upper Mancos gage data available during most of the 1951 through 1980 study period. The La Plata drainage basin above the Hesperus gage is located adjacent to the upper Mancos basin and the upper Mancos and La Plata rivers drain the west and east slopes, respectively, of the La Plata Mountains. Thus, a precipitation event that causes a sharp rise in the flow of the upper Mancos River at the West, Middle, and East Mancos gage locations would normally also cause a sharp rise in the upper La Plata River. Conversely, if a rise did not occur in the La Plata River, it was more probable that most of the observed runoff on the Mancos River was contributed from . areas downstream of the upper Mancos gage locations. Based on this relationship, Table 16 is a summary of the estimated short-term runoff measured at the gage near Highway 666 which originates downstream of the upper Mancos gage locations. The portion of short-term runoff which originates in each reach of the Mancos River basin was calculated by the following method: I. The volume of short-term runoff originating within each reach of the Mancos basin downstream of the West, Middle, and East Mancos gage sites was considered to be proportional to mean annual precipita- tion within each reach. 2. The mean annual precipitation values for the Mancos River basin were obtained from the map entitled "Colorado Average Annual Precipitation 1951-80," published by the U.S. Geological Survey (Scale 1:500,000). Precipitation in the Mancos basin varies from less than eight inches per year in the Highway 666 gage location to over 40 inches in the La Plata Mountains. Isohyetals of mean annual precipitation are shown on Figure 4. . -35- i