PHYSICAL AND CHEMICAL LIMNOLOGY OF BROWNLEE RESERVOIR, 



1962-64 



By Wesley J. Ebel and Charles H. KosKr, Fishery Biologists 



Bureau of Commercial Fisheries Biological Laboratory 



Seattle, Washington 98102 



ABSTRACT 



The physical and chemical features of Brownlee 

 Reservoir are discussed in relation to seasonal change, 

 inflow and outflow, operation of the dam, modified 

 environment of rtver below the dam, and possible effects 

 of these factors on behavior, passage, and survival of 

 salmon {Oncorhynchus spp.)- 



Temperature, oxygen concentration, and current 

 were the most critical factors affecting the distribution 

 and survival of salmon. Water temperature above 

 26.7° C. and oxygen concentration below 3 p. p.m. 

 limited the area of suitable habitat during midsummer; 

 environmental conditions in the spring, fall, and 

 winter were suitable, if not optimal. The reservoir had 



a significant effect on the temperature and oxygen 

 concentrations of the Snake River below the dam. In 

 October, discharge from the reservoir was 5° C. warmer 

 than the water entering the reservoir; oxygen con- 

 centration was 5 p. p.m. lower than in the inflow. This 

 condition reversed in June and July. 



Current velocities and directions changed signifi- 

 cantly with changes in river inflow, discharge rate, and 

 surface level of the reservoir. Passage by juvenile 

 salmonids through the reservoir was affected by changes 

 in reservoir temperature, length, oxygen concentra- 

 tions, and velocity and direction of the current. 



Tlie U.S. Department of the Interior began in- 

 tensive research in 1961 to solve problems of fish 

 passage at high clams. Primary consideration was 

 given to the study of factors that might aifect the 

 survival and passage of adult and juvenile salmo- 

 nids {Oncofhynchiis spp.) under the conditions 

 anticipated for the Middle Snake River Basin. 

 Brownlee Reservoir, completed in 1958, provided 

 an area in which to examine the effects of a large 

 impoundment on migrating salmon. The Bureau 

 of Commercial Fisheries liegan linmological stud- 

 ies (physical and chemical) in July 19G2 and con- 

 tinued them through October 196-i; the study on 

 tlie effects of various environmental factors on the 

 behavior and passage of sahnonids was directed 

 principally toward water quality and current. 



Brownlee Reservoir lies at the head of the Snake 

 River canyon in hilly and seniiarid, open-range 

 country between the borders of northeastern Ore- 

 gon and midwestern Idaho (fig. 1). At full pool, 

 the reservoir begins about 16 km. downstream 



from Weiser, Idaho, and continues northerly in 

 a nearly straight coui-se for 92 km. to Brownlee 

 Dam. Average width is less than 0.8 km. ; maximum 

 depth at full pool is about 92 m. At full capacity, 

 the reservoir has an area of 6,100 hectares and 

 contains 217,000 hectare-meters, or 1.8 x 10^ m.^ 

 of water. 



Three main tributaries — the Snake, Powder, and 

 Burnt Rivers — and many small, intennittent 

 streams enter the reservoir. The upper 23 km. 

 above the moutli of Burnt River is relatively shal- 

 low and essentially a river. Depth increases sharply 

 below Burnt River; currents in the remaining 69 

 km. are weak when the reser\oir is at full pool. 

 The slope of tlie reservoii-'s basin, its depth, geo- 

 graphic location, and especially its thermal 

 characteristics, are such that it fits some of the 

 criteria used by Welch (1952) and Hutcliinson 

 (1957) to describe "temperate lakes of the second 

 oi-der." 



^\ji obvious difference between a reservoir such 



Published December 1968. 



FISHERY BULLETIN: VOL. 67, NO. 2 



295 



