Stbeam Pollution 19 



Pollution in Small Streams 



The effects of pollution in such a small stream are not, however, 

 summed up completely by any means in the previous paragraph. 

 The same contamination which renders the area unfit for reproduc- 

 tive purposes destroys the smaller organisms of the water. The 

 insect larvae are killed, and the smaller crustaceans and the micro- 

 scopic animals and plants suffer a similar fate. Now, these organ- 

 isms multiply most rapidly in the shallower areas. They are pro- 

 duced constantly in enormous numbers in such places, and are 

 carried on in part directly and in part through the medium of 

 smaller fish to replenish the food supply of the other parts of the 

 river system, and thus to furnish substances ultimately for the adult 

 fish which immediately interest man. The effect of contaminating 

 the smaller stream is seen in a diminution of the general food supply 

 in the water system of which it is a part. 



Effects on Larger Streams < 



These influences, however, are not confined to the smaller streams 

 with which the discussion started. They affect equally the rivers 

 and lakes with which these smaller streams connect. So long as the 

 amount of pollution is small, the larger stream does not show the 

 effect so quickly. It may be, indeed, that for a long time the amount 

 of pollution affects the large stream merely within a limited area or 

 only so much that the life in it does not find the opportunity for 

 vigorous development, and the investigator examining the water has 

 to record that various elements of the fauna and flora are scanty, 

 that aquatic life is poorly developed or absent from some regions 

 and confined to the more productive areas, or to portions in which 

 the contribution from uncontaminated water in certain tributaries 

 makes an area more favorable for the development of life. But, as 

 the subsidiary streams become more and more affected, the main 

 stream assumes an increasingly unfavorable condition, until the 

 organisms of pure water are gradually but more or less completely 

 replaced by those that characterize polluted waters. This replace- 

 ment occurs first at the points of greatest pollution, such as near the 

 discharge of an industrial waste or the inflow of a highly polluted 

 side-stream. It extends down stream and over the general area of 

 the larger water body, gradually eliminating all of the characteristics 

 of the original river and substituting those of the open sewer. 



Influence of Seasonal Changes 



These conditions evidently vary with the seasons. When the 

 natural flow in the stream is lowest, and the dilution of the water 



