THE BACTERIA IN NATURAL WATERS 7 



meter; and furthermore the amounts of organic and 

 mineral matters which serve as food materials, and thus 

 become a factor in later multiplication of organisms, 

 are greatly increased. 



In the larger streams several conditions combine to 

 make these enormous bacterial numbers somewhat 

 lower. Ground-water containing little microbic life 

 enters as a diluting factor from below. The larger 

 particles of organic matter are removed from the flow- 

 ing water by sedimentation; many earth bacteria, 

 for which water is an unfavorable medium, gradually 

 perish; and in general a new condition of equilibrium 

 tends to be established. It is difficult, however, to 

 find a river in inhabited regions which does not con- 

 tain several hundreds or thousands of bacteria to the 

 cubic centimeter. Furthermore, heavy rains which 

 introduce wash from the surrounding watershed may 

 at any time upset whatever equilibrium exists, and 

 surface-waters are apt to show sudden fluctuations 

 in their bacterial content. 



Seasonal Variation of Bacteria in Surface Waters. 

 Sharp variations in bacterial content are particularly 

 apt to occur in the spring and fall as a result of the 

 rain and melting snow at those seasons. The high 

 numbers shown for various rivers in the table on page 

 8 illustrate this point. 



The rainfall is the main factor which causes these sea- 

 sonal variations; but its specific effect differs with dif- 

 ferent streams. The immediate result of a smart shower 

 is always to increase contamination by introducing fresh 

 wash from the surface of the ground. More prolonged 



