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2 l ~ ~ I I , I ! measured at Lee's Ferry, Arizona. Sampling in the Gunnison River was conducted from Escalante Canyon, Colorado to the confluence of the Colorado River at Grand Junction (Figure 1). This 42-mile reach was divided into three strata varying from 12 to 15 miles in length. Several water diversion and power-generating structures have been constructed on the Gunnison River. Redlands Diversion, built in 1907 for irrigation purposes, is the only structure found within the study section. Unused irrigation water from Redlands Diversion flows through Redlands Canal and empties into the Colorado River, leaving the Gunnison River below the dam stagnant during the growing season. A diversion tunnel (BR Uncompahgre Project) was constructed in 1910 on the upper Gunnison above Delta, Colorado to irrigate the Uncompahgre Valley. Unused irrigation water diverted through this tunnel returns to the Gunnison River via the Uncompahgre River. The Gunnison River is also the site of the BR Curecanti Project (completed in 1977), which consists of three water storage and hydroelectric dams above Montrose, Colorado. Dolores River b, The Dolores River is a major tributary of the upper Colorado River. Sampling of the Dolores River was conducted for 67.6 miles from Paradox Valley, Colorado to the confluence with the Colorado River near Cisco, Utah (Figure 1). The study reach was divided into four strata ranging from 10 to 22.3 miles in length. The Dolores River has several modifications associated with agriculture and mining. A dam was constructed by the Montezuma Valley Irrigation Company north of Dolores, Colorado to irrigate arid farmland. This dam diverts nearly all of the river during times of low flow in the summer and fall (July- November). No water returns to the Dolores River after this irrigation diversion, and the river is stagnant below the dam to the San Miguel River confluence. At this point, the San Miguel River contributes nearly all the water flowing in the lower Dolores River. Uranium, gold and salt mining operations also occur along the Dolores and San Miguel rivers. Spills from these mining operations have resulted 1n several fish kills (Sigler et al. 1966, Nolting 1957). Sample Design Sampling was conducted on the Gunnison River from 30 July 1979 through 16 October 1981. The Dolores River was sampled from 28 April 1981 through 23 October 1981. Every stratum was sampled before, during and after spring runoff; for two complete years on the Gunnison River and in 1981 only on the Dolores River. Sampling was not conducted from November to March because of adverse weather conditions. Field crews consisting of 2-4 investigators spent 3 days sampling each stratum, and made 8-9 field trips per year on each river.