FIG. 33 



year, relationship between upstream and 

 downstream air temperatures, shading 

 afforded by vegetation or land masses (or 

 other factors that will affect absorption 

 of solar radiation), tributary streams, and 

 the entrance of ground water. It is neces- 

 sary to know these normal temperature 

 changes if estimations are to be made on 

 the effect reservoirs have had on the tem- 

 perature of a particular stream. Few tem- 

 perature data are available on Pacific 

 Northwest streams for stream sections where 

 man has not already produced some structure 

 to change the original stream environment. 



Table 10 shows the noimal temperature 

 rise or fall in streams where the stream 

 section did not contain an impoundment or 

 a tributary of any significant magnitude. 

 In the streams listed, large impoundments 

 exist on the upstream waters. These pro- 

 duce a moderating effect on the water tem- 

 perature which is particularly significant 

 at the upstream station shown herein on 

 table 10. 



Only general stream characteristics 



are given as it is Impracticable to attempt 

 the computation of water temperature changes 

 in relationship with each of the influencing 

 variables. Data shown are for the period of 

 observation only and should not necessarily 

 be construed as being representative of 

 usual conditions. Temperature change values 

 are all reasonable and comparable excepting 

 for the lower Snake River in the early sum- 

 mer where a temperature fall of frcm 1.55 

 to 2.13° F- per 100 river miles was observed. 

 A temperature increase would have been ex- 

 pected because of the difference between 

 air and water temperatures and because of 

 absorption of solar radiation. In early 

 June of 19^5 and 1950 the Snake River was 

 experiencing its maximum yearly runoff, 

 which, with a late snow melt, might account 

 for the temperature decline. This is not 

 true in July of 19^5 where the temperature 

 decrease was still greater and the flow 

 much lower. It is possible that cold ground 

 water in appreciable quantities enters the 

 river in this section or that the points of 

 hand thermometer temperature readings were 

 not representative of the average river tem- 

 perature . 



$3 



