Great Basin Naturalist


During July to September 1994–1996, I examined water temperatures at the lower end of the elevational distribution of redband trout (Oncorhynchus mykiss gairdneri) in 4 streams in the Owyhee Mountains in southwestern Idaho. Maximum water temperatures in Castle, Shoofly, Little Jacks, and Big Jacks creeks during low flows during a drought in 1994 ranged from 26.7°C to 29.0°C. Water temperatures fluctuated 9.5–11°C during the 24-h period maximum temperatures were observed. Stream flows at the lower end of Big Jacks and Little Jacks creeks in 1994 were <0.003 m3 s−1 and subsided underground 50–130 m downstream of pools inhabited by trout. Trout were distributed to lower elevations where drainage basin area was larger in 2 of 3 yr (P < 0.03). Lower elevational limits of redband trout distribution in Big Jacks, Little Jacks, Castle, and Shoofly creeks were 920, 934, 972, and 1090 m above sea level, respectively, in 1994. With higher stream flows in 1995–96, trout were found 3–6 km farther downstream in Castle, Big Jacks, and Shoofly creeks at elevations of 860, 891, and 998 m, respectively, and tolerated maximum temperatures of 25.5–29.0°C. Trout were not distributed farther down Little Jacks Creek because of poor channel conditions. Maximum daily water temperatures of 29.0°C may have limited trout distribution in Big Jacks Creek, as flows and suitable channel conditions (but higher temperatures) continued >5 km downstream of the lowest pool inhabited by trout in 1995–96.