8 



ELEMENTS OF WATER BACTERIOLOGY 



SEASONAL VARIATIONS IN BACTERIAL CONTENT OF 

 RIVER WATERS. BACTERIA PER C.C. MONTHLY 

 AVERAGE 



River. 



June. 



Thames ^ 



Lea 1 



New 1 



Mississippi ^ . . . 



Potomac ^ 



Merrimac * . . . . 

 Susquehanna *. 



382 



598 

 198 

 2,007 

 2,300 

 9,600 

 2,903 



River. 



Dec. 



Thames ^ . . . . 



Leal 



New 1 



Mississippi ^ . . , 

 Potomac '...., 

 Merrimac * . . . . 

 Susquehanna ^ 



740 

 2,050 



621 

 2,021 

 6,900 



37,525 



1 Houston, 19060, 19066. * Massachusetts, 1906. 



2 New Orleans, 1903. ^ Harrisburg, 1907. 

 ' Figures obtained through courtesy of F. F. Longley. 



moderate rain, however, exerts an opposite effect, and 

 after the main impurities which can be washed away 

 have been removed, may dilute the stream with water 

 purer than itself. What the net effect of rain may be 

 depends, therefore, on the character of the stream. 

 A river of fairly good quality shows its highest numbers 

 in rainy periods. With a highly polluted stream, on 

 the other hand, the constant influx of sewage over- 

 balances occasional contributions of surface contamina- 

 tion. Thus Gage (1906) shows in the following table 

 that the bacterial content of the Merrimac is highest 

 when the stream is lowest, that is, when its sewage 

 content is least subject to dilution. 



