Mancos Watershed Functional Assessment

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diversions on both the main Mancos River and on its tributaries. There are also several large storage reservoirs that are located above Mancos Valley itself, including Jackson Gulch and Weber reservoirs. Annual diversions from the Mancos River and its tributaries in the upper watershed were estimated by the Colorado Department of Natural Resources (Colorado Decision Support System 2001) to range between 14,600 acre feet and 67,000 acre feet per year for the period between 1974 and 2000, with an average of 42,100 acre feet per year. Additional human impacts in the upper parts of the watershed include livestock grazing, limited aspen timber harvests on the higher elevation plateaus, recreation, a few small roads along portions of several tributaries, and historic mining at higher elevations on the East Mancos (no mines are known to be active at present). The town of Mancos is situated directly on the Mancos River below where the major tributaries join together at the head of Mancos Valley. In recent years there also has been considerable new home construction in previously agricultural areas, as the population in the upper watershed has increased. A number of these homes are directly adjacent to the river or its tributaries. The lower part of the Mancos River watershed is largely undeveloped. Primary human use and impacts at the present time appears to be livestock grazing, as well as a road that runs along portions of the river itself in the lower part of Mancos Canyon. Hydrograph. A major impact that human activities like water diversions for agriculture and domestic water use can have on stream-riparian ecosystems is to change both the amount and timing of the water that moves through the river downstream from the diversion point(s). The pattern of stream flow past a specific spot is called a hydrograph. We are not aware of any detailed quantitative records of the amount of water that flowed in the Mancos River prior to the first water diversions. It is therefore impossible to know exactly what the original hydrograph would have looked like at various reaches within this watershed. However, it is likely that the Mancos River exhibited the same general flow pattern as does many other streams and rivers in the American southwest. Most precipitation in this region falls either during the winter, primarily as snow in mountains, or during the summer monsoon or rainy season, which usually takes place between July and October. This precipitation pattern leads to two peaks in the annual amount of water flow: a large increase in the spring, with the snow melt and runoff from the mountains, and a secondary peak that results from the summer rains. At other periods, and depending on the overall size of the watershed above a particular reach, flows tend to be much smaller. In some streams, and during some dry years, the amount of water in a particular reach may even fall to zero in the smaller area watersheds. Figure 3 is a hydrograph from the West Mancos River, above the town of Mancos, for the year 1939. These data were collected prior to the completion of the Jackson Gulch Reservoir project, which diverts water from the West Mancos and then stores it in the reservoir for subsequent release during the irrigation season. The hydrograph in Figure 3 shows the two peaks in stream flow (spring and late summer) discussed above, as well as the low flow periods over the winter and mid-summer. Most of the plants and animals that live in stream-riparian ecosystems in the American southwest are adapted to a lesser or greater extent to this type of water-flow pattern, although of course the aquatic system itself is very sensitive to periods when there is no stream flow whatever. Without further detailed analysis, it is impossible to know what the overall impact of the irrigation removals and water storage reservoirs has been on the stream-riparian ecosystems within the study area. The primary effect of the diversions is to decrease the total amount of water in reaches downstream whenever the diversion is active. A secondary effect, from the storage reservoir part of the irrigation system, is to decrease the annual variance in water flow. Part of the spring run off is captured by the reservoir, which then decreases the natural peak of water flow during that time. This stored water is then released throughout the summer, increasing the amount of water in at least parts of the river over what would normally be present (above any diversion of that water into irrigation ditches). It is impossible to tell how much of the hydrograph pattern in Figure 3 has been altered due to the combined effects of water 10