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The Firehole River, Yellowstone National Park, Wyoming, receives substantial amounts of geothermally heated, mineralized water from hot springs and geysers and is a sport fishery for rainbow trout, Oncorhynchus mykiss and brown trout, Salmo trutta. Geothermal effluents greatly increase the primary productivity and benthic insect biomass of the river (Armitage, 1958; Boylen and Brock, 1973), as well as somatic growth of both fish species during winter (Kaeding and Kaya, 1978). Summer water temperatures, however, can be high enough to adversely affect seasonal gonadal development in brown trout (Kaya, 1977), kill rainbow trout (Kaya, 1978) and inhibit somatic growth of both species (Kaeding and Kaya, 1978).
During summer, rainbow and brown trout use coolwater tributaries and mainstem areas upstream from the major sources of geothermal effluents as refuges from elevated Firehole River temperatures (Kaya et al., 1977; Kaeding, 1980; Koch, 1990). The objective of this study was to determine how use of two coolwater tributaries by rainbow and brown trout is affected by Firehole River temperatures during summer.
The Firehole River originates near Madison Lake (elev., 2550 m), in west-central Yellowstone National Park, and flows north for about 44 km before joining the Gibbon River to form the Madison River at Madison Junction (elev., 2070 m; [ILLUSTRATION FOR FIGURE 1 OMITTED]). Average stream gradient is 1.1%. Mean monthly discharge of the river is about 8 [m.sup.3]/sec most of the year but may reach 21 [m.sup.3]/sec during runoff in May and June (Druse et al., 1993). The thin siliceous soils of the 720-[km.sup.2] drainage cover bedrock dominated by rhyolite.
The Firehole River is a coolwater stream until it enters the Upper Geyser Basin about 17 km downstream from Madison Lake. In the basin and two similar geologic areas downstream, river temperatures are elevated by additions of geothermally heated water from hot springs and geysers (Allen and Day, 1935; Argyle, 1966; Burkhalter, 1979). Geothermal effluents constitute 20-40% of the Firehole River discharge during seasonal low-flows (Burkhalter, 1979). The river is classified as a sodium (187 ppm)-bicarbonate (140 ppm)-chloride (77 ppm) water due to the ions from these effluents (Wright and Mills, 1967). In addition to rainbow and brown trout, the only other fish species that occurs in the study area is brook trout, Salvelinus fontinalis, which predominately inhabit cooler headwater areas.
The study was conducted between July 1991 and July 1994. Temperatures ([+ or -]0.1 C) of the two coolwater tributaries (Sentinel Creek and the Little Firehole River), the Firehole River at three locations (upper, middle and lower), and Iron Spring Creek [ILLUSTRATION FOR FIGURE 1 OMITTED] were recorded hourly by electronic thermometers during most of this time except 1994. Temperatures at these locations were described primarily using mean monthly temperatures, calculated as the mean of the mean daily temperatures for that month.
The confluences of Sentinel Creek (SC) and the Little Firehole River (LFR) with …