in conformance with requirements of the 

 Pollution Control Commission (14) . Irri- 

 gation return flows are minor in the river 

 basin. Assuming a net consumptive use of 

 2.0 acre-feet per acre (2) and an annual 

 irrigation application of 4 acre-feet per 

 acre, the return flow prorated over 12 

 months would be about 71 c.f.s. This 

 amounts to 2.5 percent of the mean annual 

 flow. 



The Columbia River between Beebe 

 Orchard Bridge and Priest Rapids receives 

 very little polluting material in relation 

 to the river volume. The City of Chelan 

 discharges sewage to the Chelan River 

 through a primary treament plant serving 

 a population of 2,300 persons. Wenatchee 

 and East Wenatchee discharge untreated 

 sewage to the Columbia River from a con- 

 nected population of about 14,000 persons 

 (13). Sewage treatment is being planned 

 for these cities. Other cities in the 

 area do not discharge sewage to the river. 

 All industrial waste discharges in the 

 river section, including the aluminum and 

 electro-metals plants at Malaga and Rock 

 Island, meet the requirements of the Pol- 

 lution Control Commission with the excep- 

 tion of a few food processing plants in 

 Wenatchee. Irrigation return flows in this 

 section of the Columbia River are small. 

 Anticipated return flows of 233,000 acre- 

 feet from the Columbia Basin Project will 

 not be realized until the area is all under 

 irrigation and not until the ground water 

 table has been raised sufficiently to per- 

 mit this return flow. A return flow of 

 233,000 acre-feet, prorated over 12 months, 

 would be equivalent to a flow of 325 c.f.s. 

 or 0.3 percent of the mean annual flow of 

 the Columbia River at Trinidad. 



WATER QUALITY CHANGES IN A RIVER 



In a given river section, unaffected 

 by marfs activity, the quality of the river 

 water is subject to change by natural 

 causes as it flows through this section. 

 The magnitude of the change will vary with 

 the length of the section, depth of flow, 

 shading afforded, elevation of the ground 

 water table, turbulence and, it will vary 

 with the physical and chemical character- 

 istics of the ground over which the river 

 flows. Tributary streams will of course 

 have an effect on the water quality in the 

 stream under consideration. Water quality 



changes that usually take place in a river 

 section are as follows: 



1. Increase in dissolved mineral 

 matter and conductance. 



2. Increase in water temperature 

 during the summer months. 



3. Decrease in water temperature 

 during the winter if a large 

 impoundment exists above the 

 stream section. 



4. Increase in pH if the area con- 

 tains alkaline soil. 



5. Color may be increased or re- 

 duced, depending upon the solar 

 radiation received and on the 

 nature of the surrounding soil. 



6. Turbidity may be increased or 

 reduced, depending upon the water 

 velocity and the nature of the 

 surrounding soil. 



7. Dissolved gases, such as carbon 

 dioxide, will decrease in a river 

 section unless entrained organic 

 matter is undergoing rapid decom- 

 position. Dissolved oxygen will 

 increase towards saturation or 

 remain in a saturated state un- 

 less rapid decomposition removes 

 oxygen faster than it is replen- 

 ished by reaeration. 



The natural water quality in a river 

 is subject to change from 4 man-made causes. 

 They are: 



1. Impoundment of water in artifi- 

 cial reservoirs behind dams. 



2. Return flows from irrigation. 



3. Introduction of domestic sewage 

 and industrial wastes. 



4. Soil erosion or vegetative cover 

 changes from farming, logging, or 

 construction activities. 



Impoundment of Water 



The effect of water impoundment on 

 water quality depends upon the time of 

 impoundment, water depth, air temperatures, 



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