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002347 Spring, and Dry Creeks (figs. 1 and 2), can affect water quality of the Uncompahgre River during periods of peak snowmelt, which occur from March to May depending on weather conditions. After the snowmelt season, stream dis- charge in these tributaries upstream from irrigated areas is small, and during the irrigation season most of the water discharged from the tributaries into the Uncompahgre River is irrigation return flow. During the irrigation season, the Uncompahgre River at the South Canal terminus (figs. I and 2) will be a mixture of water diverted from the Gunnison River and water in the Uncompahgre River upstream from the South Canal. Downstream from the South Canal, the quality of water in the river is altered by canal diversions, irrigation return flows, and subsurface drainage. Water samples for inorganic analysis were collected at four sites at Sweitzer Lake (fig. 8); site 5 (Garnet Canal diversion at Sweitzer Lake) is a diversion ditch from the Garnet Canal into the lake; site 6 is a drainage ditch from fields south of the lake; site 7 is Sweitzer Lake; and site 8 is the outflow from the lake. Sites 5 and 6 were the only two ditches that had water flowing into the lake during the initial reconnaissance of sites in July 1987. Water samples were collected from Sweitzer Lake from near the surface and from near the lake bottom near the middle of the lake when possible; In January, the sample was collected near the dam because of safety considerations regarding the ice on the lake and was a composite of water sampled at three depths. An additional water sample was collected near the dam in March from the deepest area in the lake to obtain a sample from a water layer that had almost zero dissolved-oxygen concentration and a much larger specific conductance than the water layer immediately above it. The anaerobic water layer was not thick enough at the mid-lake site to collect a water sample that was representative of the anaerobic layer. Water samples for pesticide analysis were collected in July (table 10) at stream sites that were downstream from irrigation drainage on the Gunnison River (site 11) and on the Uncompahgre River (site 9), and at Sweitzer Lake (sites 5 and 7). These samples were collected during the period when pesti- cides normally are applied in the study area. Bottom-sediment samples for inorganic analysis were collected at sites 3 and lIon the Gunnison River, at site 9 on the Uncompahgre River, and at Sweitzer Lake (table 9). Site 4 on the Uncompahgre River at Colona also was sampled because concentrations of heavy metals in bottom sediments in the Uncompahgre River may be greater than baseline concentrations because of more than 100 years of metal mining and associated mine drainage in the headwaters of the river. The Uncompahgre River upstream from Ridgway (fig. 1) has been severely affected by heavy metal pollution (Wentz, 1974). The extent of the metal pollution downstream from Ridgway into the study area was uncertain. The Gunnison River upstream from Escalante Creek (site 11) and Sweitzer Lake (site 7) were the only sites sampled for pesticides (table 9) in bottom sediment. Bott~m-sediment samples were scheduled to be collected only in November, when maximum accumulation of potential contaminants from irri- gation drainage was expected to occur; however, a malfunctioning bottom- sediment sampler prevented collection of a bottom-sediment sample from a mid-lake location at Sweitzer Lake. Therefore, the sample collected in November was collected from bottom sediment at the south (inflow) end of the lake in shallow water. In January, a second bottom-sediment sample in the lake was collected about 25 ft from the lake bank near the dam (table 10). 33