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Figure 3. Hydrograph from the West Mancos River in 1939, prior to the construction of Jackson Gulch <br />Reservoir, which was completed in 1949. The hydrograph shows an annual pattern in stream flow levels <br />typical of many streams and small rivers in the southwest, with the highest flows during the spring snow melt, <br />and a secondary peak during the summer rainy season. Low flows occur during the winter months, and <br />during mid to late summer before the summer rains. The watershed area of the West Mancos River that feeds <br />into the measured water flow location in this figure is 42 square miles, and has an average discharge of 27,490 <br />acre feet per year (USBR, 2007). <br />storage and diversion. However, overall, these factors are likely to have the following impacts: 1) reduce <br />the peak amount of water that flows in downstream reaches, particularly the scouring flows and overbank <br />flooding that often occur during the spring run-off, and 2) reduce the total amount of water that flows in <br />the furthest downstream reaches during the water diversion/irrigation season, although this might be <br />balanced to some extent in some reaches by water releases and irrigation ditch return flows during the <br />period of low precipitation in the early summer. The Mancos watershed, particularly the upper part, is <br />relatively small; whether or not any of the reaches in Mancos Valley and surrounding areas ever went <br />totally dry before the development of intensive irrigation remains problematic. <br />Salinity. Much of the middle and lower sections of the upper Mancos River watershed (upper plateau and <br />canyon, and Mancos Valley, sections) is underlain by salt bearing formations that include Mancos Shale <br />and Dakota Sandstone. There has been considerable leaching of the salts from exposed beds of these <br />formations, and the impact of the salts that are then transported downstream by the Mancos River on the <br />overall salinity of the San Juan and Colorado River basin system has been the subject of considerable <br />study (e.g., Yochum 2004). An examination of the possible effects of high salt loads in the Mancos River <br />on the overall functioning of the stream-riparian ecosystem was beyond the scope of this study. However, <br />we did not observe any obvious direct effects of this factor in any of the reaches examined. <br />11 <br />