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DESCRIPTION OF STUDY AREA Dillon Reservoir is located 112 km west of Denver, Colorado, in Summit County. The Denver Board of Water Commissioners owns and operates the reser- voir. It has a surface area of 1335 ha with 41.6 km of shoreline. The reser- voir is subject to an annual average drawdown of 3-5 m, but severe drawdowns up to 20 m have occurred. Dillon Reservoir is oligotrophic and supports four species of coldwater sport fish: rainbow trout (Salmo gairdneri), brook trout (Salvelinus fontinalis), brown trout, and kokanee salmon (Oncorhynchus nerka kennerlyi). The rainbow trout fishery is maintained through stocking catchable-size (200- 250 rom TL) fish annually. All other fish populations are maintained by natural reproduction. Fishing pressure is very high due to the close proximity of the reservoir to Denver. The study area included the reservoir itself and the Blue River. Some brown trout were collected from the reservoir, but the majority were collected in the river. The Blue River study area began at the river's confluence with the reservoir and ended 3.9 km upstream, where the river emerged from beneath large mounds of gravel tailings. The gradient within the study area was approximately 14 m/km (56 m/3.9 km). The average width of the stream was 13.2 m. The Blue River study area had varied trout habitat including riffles, runs, pools, undercut banks, boulders, and fallen trees. Riparian vegetation (i.e., willows, conifers, etc.) was abundant. METHODS Age, Growth, and Composition of Brown Trout in the Spawning Migration, 1979-1980 Brown trout were collected from Dillon Reservoir during June-September, 1979-1980. Collection sites were concentrated in the inlet bays. Fish were taken by gillnets and electrofishing. Spawning brown trout migrating up the Blue River were captured in a weir, which was installed 1 October 1979 and 3 October 1980 and removed 14 November 1979 and 1980. Scale samples were taken, and length, weight, and sex were recorded for most captured brown trout in both years. Scale samples were not taken during the 1980 spawning migration. Spawning fish were marked in 1979 to determine the percentage of brown trout returning to the Blue River to spawn in 1980. Brown trout less than 400 mm TL were marked with an adipose fin clip; brown trout over 400 rom TL were tagged with a yellow spaghetti tag. All brown trout collected during the 1980 spawning migration were given a left-pelvic fin clip so that they could be identified in future years. Unmarked brown trout migrating downstream were included in the estimated total number of fish spawning in the Blue River in 1980. No downstream migra- tion was observed in 1979. Ageing of Brown Trout Scale samples were used to age captured brown trout and to backcalculate growth. Length-frequency distribution and analysis of otoliths were used to 37