in tabic 24) are large at some of the 

 thermograph stations during the periods of 

 warm, sunny weather and cool evenings from 

 May through September. Maximum observed 

 diurnal temperature variations during the 

 survey period were: 3° F. in the Columbia 

 River at McNary Dam; 7° F. in the Columbia 

 River at Pasco; 2° F. in the Columbia River 

 at Rock Island; 3° F. in the Columbia River 

 below Grand Coulee Dam; 5° F. in the Snake 

 River; 9° F. in the Yakima River near 

 Richland; 15.5° F. in Lake Wenatchee outlet; 

 10° F. in the Wenatchee River at Monitor; 

 11° F. in Nason Creek; 10° F. in the Chiwawa 

 River; 10° F. in the Wenatchee River below 

 Dryden; 6.5° F. in the Wenatchee River above 

 Leavenworth; and 8° F. in the Wenatchee 

 River below Plain. 



Figures 35 and 36 illustrate the wide 

 range between the minimum and maximum daily 

 temperatures observed in each month of the 

 year during the survey period. For example, 

 the minimum temperature of 36° F. shown on 

 figure 35 for the Chiwawa River during May 

 is the coldest temperature observed in the 

 Chiwawa River during May of 1956. Daily 

 water temperatures in excess of 65° F. were 

 observed at all stations with the exception 

 of the Columbia River below Grand Coulee 

 Dam, Icicle Creek and the Chiwawa River. 

 Average-daily water temperatures in excess 

 of 65° F. persisted for a month or more in 

 the Yakima River near Richland, in the mouth 

 of the Snake River and in the Columbia River 

 at McNary Dam and at Pasco. They approached 

 this value in the Wenatchee River during 

 August 1956. 



Figure 37 is a plot of 1956 water 

 temperatures in the Wenatchee River from 

 Lake Wenatchee to Monitor, 5 miles from the 

 river mouth. It shows the moderating effect 

 of Lake Wenatchee on Wenatchee River water 

 temperatures. From September until March, 

 Lake Wenatchee discharges water warmer than 

 that found downstream while from March 

 through August, the lake discharges cooler 

 water than is found downstream. In August, 

 there is a marked temperature rise from the 

 lake outlet to Plain, 9 miles downstream, 

 followed by little temperature change through 

 Tumwater Canyon to below Dryden, a distance 

 of 31 miles. A marked temperature rise 

 again occurs between Dryden and Monitor (a 

 distance of 10 miles) as the river meanders 

 through a flood-plain offering little shad- 

 ing from solar radiation. In May and June, 

 the colder waters of Icicle Creek reduce 

 the river temperature below Leavenworth. 



Table 25 (page 90) is a tabulation of 

 water temperatures with depth observed in 

 Lake Wenatchee from July 1955 to December 

 1956. Figure 38 is a plot of these tempera- 

 tures at selected depths during 1956. Maxi- 

 mum temperatures occur in August or Septem- 

 ber. Vertical currents, set in motion by 

 wind action, warm the entire lake during the 

 spring and summer. Temperature measurements 

 were not made in the winter when the lake 

 was frozen because of the difficulty and 

 danger in getting over the snow-covered ice. 

 In the winter, it is likely that the upper 

 10-2 5 feet are at 32° F. and that the deeper 

 water is at the temperature of maximum den- 

 sity, 4° C. or 39.2° F. Thermoclines were 



Table 29. --Average monthly water temperatures in the Wenatchee River at Monitor, 

 U. S. Fish and Wildlife Service thermograph station, 5 miles from river mouth. 



Average 1954 



Average 1955 



Average 1956 



Minimum recorded 



Maximum recorded 



Maximum diurnal variation 



Minimum diurnal variation 



* Partial month 



98 



