sTirface. Oxygen depletion at the lower depths also 

 may have induced them to move towai'd the sur- 

 face. 



TMurnal studies in the sprinji and fall did not 

 sliow significant variations in either temperature 

 or chemical charactei'istics of water that might 

 have noticeably atfccted the movement or be- 

 havior of fish. In general, the environment in 

 Brownlee Reser\"oir was suitable for juvenile sal- 

 monids in the spring, winter, and late fall, but 

 marginal during the late summer and early fall. 



Movements of salmonids were affected by cur- 

 rents during drawdown and periods of increased 

 inflow and outflow. The maxinunn eti'ect came dur- 

 ing maximum drawdown when current velocities 

 were highest and direction was strongly oriented 

 downstream. Laboratory experiments by Gregory 

 and Fields (196'2) indicated that juvenile coho 

 and Chinook salmon showed significant responses 

 to currents with a velocity as low as 0.0039 m.p.s. 



Studies at Brownlee Reservoir showed that the 

 direction of water currents significantly influenced 

 the passage of young salmonids. This finding was 

 substantiated by analyses of changes in water cur- 

 rent and subsequent changes in success of passage 

 by juvenile migrants.^ 



The most obvious downstream orientation of 

 currents was at maximum drawdown, minus 27.1 

 m., and at minus 13.4 m. at high inflow and out- 

 flow. Conversely, the downstream current was 

 weakest at full pool. Studies by Durkin and Park * 

 on rates of movement and by Sims ' on escapement 

 of juvenile salmonids indicated that passage 

 through the reservoir was more successful in the 

 spring of 1964 than in the two previous springs. 

 In 1964, two favorable conditions existed: (1) a 

 fairly high river inflow and (2) relatively low 

 reservoir level throughout most of the downstream 

 migration i)eriod. Conceivably, then, fingerlings 

 would have greater success in passing through res- 

 ervoirs of this type if it were possible to create 

 conditions similar to those described without in- 

 flicting heavy financial loss to power companies or 



^ Durkin. Jo.seph T.. and Donn L. Park. Behavior of juvenile 

 salmonids in Brownlee Reservoir. Bureau of Commercial Fish- 

 eries, Fish-Passage Research Program, Seattle. Wash. [Manu- 

 script in preparation.] 



' See footnote 3. 



' Sims, Carl W. Escapement of juvenile salmonids from Brown- 

 lee Reservoir. Bureau of Commercial Fisheries, Fish-Passage Re- 

 search Program, Seattle, Wash. [Manuscript in prepiiration.] 



greatly interfering with wat«r conservation and 

 irrigation practices. 



Fall chinook salmon are the most likely of adult 

 migrant salmonids to be affected by the reservoir 

 environment. Water temperatures below Oxbow- 

 Dam during peak migration were 2.7° to 4.9° C. 

 higher than corresponding temperatures in the 

 Snake River above the impoundments. At the 

 same time, dissolved oxygen concentrations below 

 Brownlee and Oxbow Dams were considerably 

 lower than concentrations in the inflowing water 

 abo^-e these reservoirs. The combination of these 

 two conditions could place additional stress on 

 adult migrants during fall. 



SUMMARY AND CONCLUSIONS 



Studies of the physical and chemical limnology 

 of Brownlee Reservoir in relation to migrations of 

 salmon began in July 1962 and continued through 

 October 1964. Sampling stations were established 

 throughout the reservoir, above and below the res- 

 ervoir, and in the main tributaries. Factors studied 

 were water temperature and chemistry, turbidity, 

 weather, river inflow and outflow, and currents. 



Temperatures and oxygen concentrations in the 

 spring, late fall, and winter were suitable for sal- 

 monids. On the other hand, excessively high epi- 

 limnial temperatures (up to 27.2° C.) and danger- 

 ously low oxygen concentrations (0^ p. p.m.) pre- 

 vailed in August and September. Salmon were 

 forced to occupy a relatively marginal area for sur- 

 vi\'al. Trends in oxygen concentration and tem- 

 perature in the epilimnial layer of the upper end 

 of the re.servoir wei'e significantly influenced either 

 by large volumes of water entering from the Snake 

 River and tributaries or by large discharges at the 

 dam. 



A thermocline or metalimnion developed each 

 year. The formation was accompanied by severe 

 oxygen depletion in all areas below the ejjilinuiion. 

 The thermocline was formed fully 2 months earlier 

 in 196.3 than in 1962. Time of formation appeared 

 to depend more on the extent of the drawdown and 

 timing of the filling period than on air tempera- 

 ture and volume of water entering the reservoir. A 

 sub,stantial drawdown (minus 12.2 m.) followed by 

 a prolonged filling period delayed formation of 

 the thermocline in 1962, whereas a minimal draw- 

 down (minus 6.1 m.) accompanied with rapid fill- 

 ing accelerated the formation in 1963. 



LIMNOLOGY OF BROWNLEE RESERVOIR 



333 



