Thermally Stratified Pools and Their Use by Steelhead in Northern California Streams

-Thermal stratification occurred in pools of three rivers in northern California when inflow of cold water was sufficiently great or currents were sufficiently weak to prevent thorough mixing of water of contrasting temperatures. Surface water temperatures in such pools were commonly 3-9°C higher than those at the bottom. Cold water entered pools from tributaries, intergravel flow through river bars, and streamside subsurface sources. In Redwood and Rancheria Creeks, cold water was protected where gravel bars encroached into pools that were scoured along bedrock banks, creating isolated backwaters. Sixty-five percent of the juvenile steelhead Oncorhynchus mykiss found in the Rancheria Creek study reaches moved into adjacent stratified pools during periods of high ambient stream temperatures (23-28°C). Fish showed a decline in forage behavior and increased agonistic activity just before movement into stratified pools. In the Middle Fork Eel River, pools deeper than 3 m stratified when surface flow decreased to less than 1 m3/s. Summer-run steelhead adults were found in deep stratified pools on the Middle Fork Eel River throughout summer when midday ambient stream temperatures ranged from 26 to 29°C and coldwater pockets averaged 3.5°C cooler. Thermally stratified pools provided refuge habitat for significant numbers of young-of-the-year, yearling, and adult steelhead in marginal river habitats where stream temperatures reach upper incipient lethal levels. In northern California many streams are characterized by high sediment loads, low summer flows, and elevated stream temperatures. During periods when ambient stream temperatures reach upper incipient lethal levels, available fish habitat may be considered marginal. Pools stratified vertically by temperature may provide salmonids important refuges from warm water during summer low flows. Stream temperature plays a critical role in salmonid energy conversion by pacing the metabolic requirements for food and governing the rate of food processing (Brett 1952; Brett and Groves 1979; Pandian and Vivekanandan 1985). Rainbow trout Oncorhynchus mykiss, of which steelhead is the anadromous race, reach their physiological maximum at 17.2°C (Hokanson et al. 1977). Fish subjected to high water temperatures (20– 24°C for rainbow trout; Hokanson et al. 1977) exhibit high metabolic demands, which can lead to growth suppression and early mortality (Brett 1979). Salmonids have been observed to actively –––––––––––– 1 Present address: Hopkins Marine Station, Stanford University, Department of Biological Sciences, Pacific Grove, California 93950-3094, USA. seek cold water when ambient stream temperatures were high (Mantleman 1960; Gibson 1966; Kaya et al. 1977; Berman and Quinn 1991; Keller et al., in press). Poikilothermic fish do not regulate their temperature physiologically, but do compensate for thermal conditions behaviorally by adjusting activity rates and metabolic demand in adverse thermal conditions (Coutant 1985; Priede 1985). Thermal refuges are known to be important for other species in unstable aquatic environments (Magnuson et al. 1979; Coutant 1985). Temperature stratification in stream pools has been described previously (Neel 1951; Bilby 1984; Moses 1984; Keller et al., in press), but the relative importance of factors leading to stratification in different stream channels has not been evaluated. Thermal mixing can be inhibited by the buoyancy of warm water overlying cold water. This force, however, is extremely small. The difference in density between water that is 10°C and 20°C is only 0.15% and the corresponding buoyancy force equals 0.15 N/m3. In comparison, a characteristic difference in density between fresh and sea water is 2.5%. This suggests that isolated bodies of cold water in stream pools can exist only where there are large influxes of cold water or where mixing is