waste Irrigation water. This is the warm- 

 est stream observed in the Basin. Afternoon 

 temperatures have reached 84.7° F. near the 

 creek mouth during unusually warm weather. 



5- The SnaJce River reaches a tempera- 

 ture in excess of 72° F. in August and less 

 than 35° F. in the winter. In August, it 

 is 8.5' F. wanner than the Columbia River 

 at its confluence and in December it is 8° 

 F. colder. In late July of 195^ (a waim 

 summer) , afternoon water temperatures 

 exceeded 77° F. 



6. The Yakima River has the largest 

 temperature rise during the summer, from 

 its headwaters to confluence with the Colum- 

 bia, of any stream in the Columbia River 

 Basin. In August, between Thorp and the 

 outlet at Richland (160 miles), the tempera- 

 ture increases from ^k' F. to 7I.3 ° F. 

 This high temperature rise of 17° F. is due 

 largely to irrigation return flows. In 

 late July of I956, temperatures of 67° F. 



at Thorp and 83° F. at Richland (corrected 

 for diurnal fluctuation) were observed. 



7. In the late summer, the Umatilla 

 River flow becomes very small due to a 

 seasonal reduction in flow and diversions 

 for irrigation. Water temperatures in 

 August reached 78° F. in the late afternoon. 



8. Deschutes River temperatures are 

 close to those of the Columbia at its con- 

 fluence. This large river (M.A.F. of about 

 6200 c.f.s.) has little influence on Colum- 

 bia River water temperatures. 



9. The lower Willamette River is 

 waim during the late summer. Water tempera- 

 tures in excess of 71° F. were observed in 

 late August of I955 when air temperatures 

 were below normal. 



10. The Lewis River is one of the 

 coldest tributaries of the Columbia River 

 during the summer. At Merwin Dam, during 

 the periods of observation, the water tem- 

 perature extended from a low of 39° F. in 

 March to a dally high of 56° F. in Septem- 

 ber. 



11. The Cowlitz, like the Lewis River, 

 has a cooling effect on the Columbia River 

 during the summer. The maximum daily tem- 

 perature observed in the Cowlitz River 

 during the summer of 195i^ was 6l° F. 



Colijmbia River temperatures 



Yearly temperatures are recorded on 

 the Columbia River at four locations, viz.: 

 Vancouver, Bonneville, Umatilla and Rock 

 Island. Tables 6-9 list the average mon- 

 thly temperatures and figures 26-29 depict 

 these temperatures for the period of record. 



Figures 26-27 for Vancouver and Bonne- 

 ville are very similar, as one would expect, 

 with water temperatures at Vancouver being 

 slightly higher during the summer and slight- 

 ly lower than at Bonneville during the re- 

 mainder of the year. The maximum temperature 

 always occurred in August, excepting for the 

 year 19*^1, when it occurred in July. In 

 July of 19^+1, the river flow was the lowest 

 on record because of the need to fill the 

 Grand Coulee Reservoir. Maximum August tem- 

 peratures range from 63° F. tosbout 69° F. 

 with an average of 67° F. while minimum tem- 

 peratures in January range from 3^° F. to 

 U2° F. with an average of 38° F. This gives 

 an average yearly temperature variation at 

 Vancouver and Bonneville of 29° F. 



At Unatilla (fig. 28) the Columbia 

 River is 1-2 degrees colder than at Bonne- 

 ville during the spring, summer and autumn 

 and is about one degree wanner in the winter 

 Maximum water temperatures in August range 

 from 63.5° F. - 67.5° F. with an average of 

 66.5° F. Minimum water temperatures in 

 January range from 35 -'+0° F. with an average 

 of 39° F. This gives an average yearly tem- 

 perature variation of 27.5° F. Umatilla 

 temperatures could be average only for the 

 limited period of 1950-1955- It is quite 

 probable that the temperature ranges woiild 

 be wider if data for earlier years were 

 available . 



In October, November, December and 

 January, the Columbia River at Rock Island 

 is about one degree warmer than at Iteatilla. 

 For the remainder of the year, it is 1-3 

 degrees cooler. Figure 29 shows a maximum 

 temperature variation in August of 61.5' F. 

 - 68.5° F. with an average of 65° F. The 

 minimum temperature in February ranges from 

 32° F. - 42.5° F. with an average of 37- 

 It is significant to note that the maximum 

 £ind minimum water temperatures occurred 

 prior to water impoundment at Grand Coulee. 



On figure 29, the average monthly air 

 temperature at Wenatchee (for period of 



-Vi 



