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~u2339 The primary source of water to the Uncompahgre Project is the Gunnison River. Taylor Park Reservoir provides about 50,000 acre-ft of storage for the project in the upper Gunnison River basin. Water from the Gunnison River is diverted downstream from Blue Mesa Reservoir through the Gunnison Tunnel (capacity 1,135 ft3/s) into the South Canal (figs. 2 and 3). In water year 1988, 399,186 acre-ft of water was diverted through the Gunnison Tunnel (James Hokit, Uncompahgre Valley Water User's Association, oral commun., 1988). There are laterals and direct diversions from the South Canal to irrigate areas east of Montrose, but most of the water in the South Canal flows to the Uncompahgre River. The South Canal has a capacity of 800 ft3/s at its terminus at the Uncompahgre River. There, water is carried by flume across the river into the West Canal (capacity 175 ft3/s), the remainder discharges into the Uncompahgre River to augment flow for downstream diversions to the six canals (Montrose and Delta, Loutsenhizer, Selig, Ironstone, East, and Garnet) shown in figure 3. There is considerable reuse or rediversion of irrigation water by the downstream canals, such as East and Garnet Canals. The streamflow in the Uncompahgre River often is minimal during summer imme- diately downstream from the diversion for the East Canal at Olathe; therefore, the Garnet Canal, which supplies water to Sweitzer Lake, diverts water that primarily might be return flow from upstream irrigated lands. During this study, water diversion through the Gunnison Tunnel was stopped October 31, 1987, and started again on April 5, 1988. Water is maintained in some of the canals during winter for livestock watering. Water flows through about 70 percent of the system at a maximum flow of about 20-percent capacity during winter (U.S. Bureau of Reclamation, 1984). Because most of the facilities of the Uncompahgre Project were built before 1915, most are old and in need of repair. More than 80 percent of the canal and lateral structures were constructed of timber (U.S. Bureau of Reclamation, 1984). Currently (1989), more than 90 percent of the canals and laterals are not lined, which results in conveyance losses in the distribution system. Much of the irrigation drainage and return flow from lands east of the Uncompahgre River discharge into the Uncompahgre River. However, tailwater and part of the surface return flow and subsurface drainage from areas served by the Selig, East, and Garnet Canals discharge into the Gunnison River between Delta and Peach Valley Arroyo (fig. 2) through drainage canals, ditches, and natural drainages. Much of the irrigation drainage from lands on the west side of the Uncompahgre Valley also discharge into the Uncompahgre River. However, some tailwater and irrigation drainage from the Montrose and Delta and Ironstone canals discharge into the Gunnison River downstream from Delta through Roubideau Creek and a few small drainages. Sweitzer Lake Sweitzer Lake has a storage capacity of 1,330 acre-ft and a surface area of 135 acres. The maximum depth is 28 ft. According to park personnel at Sweitzer Lake State Park, the lake usually is full, and water flows over the spillway at the dam. Apparently, there is sufficient surface and subsurface inflow to keep the lake full or almost full throughout the year. Surface 17 /