private agencies whose contribution are 

 acknowledged at the end of this report. 



Causes of Water-Quality Change 



The natural water quality in a river 

 Is subject to change frcm four man-made 

 causes. They are: 



1. Impoundnent of water In reser- 

 voirs behind dams. 



2. Return flows froa Irrigation. 



3* Introduction of domestic sewage 

 and Industrial wastes. 



k. Soil erosion from fanning, log- 

 ging, or construction activities. 



3' Spray chemicals us«d in forestry 

 and agrlcultvire . 



Bipoundment of water 



The effect of water Impoundment on 

 water quality depends upon the tliae of 

 Impoundment, water depth, air temperatures, 

 character of reservoir bottom, whether 

 highly organic or inorganic, the physical 

 emd chemical quality of water entering the 

 reservoir, wind action to provide circula- 

 tory currents, and the point and depth of 

 water withdrawal from the resejrvolr. Ad- 

 verse water-quality factors in regard to 

 fish life that may arise from water impound- 

 ment are: high water temperature, low dis- 

 solved oxygen, high or low hydrogen-ion 

 (pH) concentration, excessive carbon diox- 

 ide, ammonia and hydrogen sulfide from 

 organic decomposition, siltation, and 

 accumulation of trace elements that may be 

 toxic to fish or their food supply, such 

 as copper, leaid, selenium, €uid zinc. Favor- 

 able water-quality effects that may arise 

 frcm Impoundment are: a lowering of the 

 downstream water temperature in the waim 

 season and a rsd.sing in the winter; 

 Increase In downstream flow, during the 

 noimal low period, that will more effec- 

 tively dilute pollutants. Release of 

 Impoiinded water will affect the stream 

 quality for some distance below the dam, 

 depending upon the water turbulence, air 

 temperatures, and the depth of water with- 

 drawal from behind the dam. 



Return flows fron irrigation 



In the irrigation of land, it Is 

 necessary that the soil be well -drained so 

 that the plant roots do not become water 

 sick and so that salts do not accumulate at 

 the soil surface. A favorable salt balance 

 is attained when the drainage water has a 

 higher salt content than the input water 

 (1). Most irrigation projects are provided 

 with drains or waste-ways which control the 

 direction of ground water movement in the 

 root zone by returning excess ground and 

 irrigation waters to a receiving stream. 



The amount of water required for irri- 

 gation varies from less than two to more 

 than ten acre -feet of water applied per 

 acre per yesir (2). Of this applied water, 

 from 20 to 60 percent may find its way back 

 to the stream as return flow. 



These return flow waters are more 

 mineralized and have different physical 

 properties frcm the input waters. Their 

 return to a stream will produce marked 

 water quality changes if the quantity of 

 return flow in relation to stream flow is 

 significant. 



Dcmestic sewage and industrial wastes 



The quantity of wastes discharged to 

 Inland waters is continually incresising. 

 Their content of polluting material is under 

 surveillance by, and is in the process of 

 being controlled by, water pollution control 

 agencies. Uncontrolled discharge of these 

 waste waters has, in many instances, caused 

 seriovis imptd-rmant in water quality to the 

 extent that fish life could not exist. It 

 is to be expected that these waste waters 

 will continue to cause less and less delete- 

 rious effects as waste treatment and other 

 control processes become more common. 



Soil erosion 



Poor land management, in the foim of 

 overgrazing or improper ciiltivation, together 

 with logging, mining, or construction activ- 

 ities that do not control soil erosion, fre- 

 quently imparts so much silt to a stream 

 that all other fonns of water-quality impair- 

 ment become minor in comparison. 



