the dam were affected mainly by the presence or 

 absence of spillway discharges even when the 

 reservoir level was down 27.1 to 15.5 m. (fig. 5). 

 Surface currents in the middle and upper sections 

 of the reservoir, however, appear to be little af- 

 fected by spillway discharges. Reservoir level and 

 volumes of flow liad the greatest effect on the 

 orientation and stability of the surface currents in 

 these areas (fig. 5). 



TEMPERATURE CYCLE 



The volume of flow and water level of the reser- 

 voir also influenced the characteristics of tempera- 

 ture and oxygen in the impoundment. Figure 6 

 shows a generalized annual tliermal cycle in the 

 reservoir. From January to mid-March, isotlierms 

 of 1° t« 6° C. were vertically aligned. Horizontal 

 alignment of isotherms began in late March, and 

 thermal stratification developed by early June. A 

 sharp convergence line (not shown in fig. 6) was 

 formed in the upper reservoir in late spring of 

 1963 when cold, dense river water sank below the 

 warmer surface water (Ebel and Koski, 1968). In 

 late June, the temperatures of the river water and 

 surface water were similar and the line disap- 

 peared. By mid-July the entire reservoir was 

 usually stratified with well-defined epilimnion, 

 thermocline, and hypolimnion. In late summer, 

 water temperatures ranged from 5° C. in the liy- 

 polimnion to as high as 24° C. at the surface. In 

 mid-October a convergence line was again formed 

 by rapidly cooling water from the Snake River 

 and the thermocline was gradually eroded. Ver- 

 tical alignment of isotherms began again in early 

 December. 



DISSOLVED OXYGEN CYCLE 



The dissolved oxygen content of the reservoir 

 followed a similar cycle each year (fig. 7). The 

 reservoir was near saturation and relatively stable 

 from January to mid-March, at which time oxy- 

 gen concentrations began to decline in the deeper 

 parts. Depletion of oxygen continued through the 

 summer until August when all water below 30 m. 

 had less than 3 p.p.m. In September, the cooler, 

 oxygenated water from Snake River began to sink 

 below the surface of the upper reservoir. By No- 

 vember, most of the water had 7 to 9 p.p.m. of 

 oxygen. 



These seasonal changes occurred each year, "but 

 with certain differences. In 1965, a drawdown of 

 28.3 m. and a late filling period caused: (1) late 

 formation of a thermocline, (2) higher average 

 temperatures from top to bottom, (3) lower oxy- 

 gen concentrations during August and September, 

 and (4) currents that were consistently oriented 

 downreservoir through May. Drawdown was sig- 

 nificant in 1964 (26.7 m.), but the filling began 

 earlier and volumes of inflow and outflow were 

 smaller. Temperatures were lower and concentra- 

 tions of dissolved oxygen were higher in 1964 than 

 in 1965, but conditions were less favorable for fish 

 than in 1962 or 1963. A reservoir drawdown suf- 

 ficient to allow sustained downreservoir current 

 velocities can prevent a sharply defined conver- 

 gence line from forming. For this reason, no con- 

 vergence line formed in the upper reservoir in the 

 spring of 1962, 1964, or 1965. 



Generally, the temperatures and dissolved oxy- 

 gen in the reservoir were within acceptable limits 

 for survival of salmon during their spring migra- 

 tion (March- June). These conditions, however, 

 began to deteriorate by late June and were mar- 

 ginal to restrictive until late September. A more 

 detailed report of the environment was presented 

 by Ebel and Koski (1968) . 



DISTRIBUTION AND MOVEMENT OF 

 NATIVE STOCKS OF SALMON 



Juvenile salmon from four populations were 

 indigenous to Brownlee Reservoir: Two were 

 progeny of spring chinook salmon, one of fall 

 Chinook salmon, and one of kokanee. The account 

 of movement and behavior of juvenile salmon 

 while passing through the reservoir environment 

 is presented by species and population. 



SPRING CHINOOK SALMON 



Offspring of spring chinook salmon enter the 

 reservoir from two areas : the Weiser River, a trib- 

 utary of the Snake River, and Eagle Creek, a 

 tributary of the Powder River (fig. 1). Migrants 

 from Weiser River must traverse the entire reser- 

 voir on their seaward migration, whereas Eagle 

 Creek migrants have less than one-half of the 

 reservoir to negotiate. The two populations also 

 differ in average age, size, and season of entry. 

 Table 1 gives yearly estimates of the numbers of 

 young fish that entered the reservoir in 1962-65. 



JUVENILE SALMON DISTRIBUTION AND MOVEMENT IN BROWNLEE RESERVOIR 



225 



