HYDROLOGY OF NEW YORK 



127 



between the runoff and the rainfall, runoff increasing as rainfall 

 increases, and conversely. 



As regards the division of streams into classes in proportion to 

 size of catchment area, it appears, therefore, that aside from 

 floods one is not, on present information, justified in such classi- 

 fication, and even in cases of floods it is quite probable that there 

 are other considerations of such importance as to render a 

 classification of this character inexpedient. 



Since there is no very definite relation between size of catch- 

 ment and runoff there is no reason why comparison may not be 

 made of streams having such large difference in size of catchment. 

 For some streams, as for instance, Pequannock river, where the 

 slopes are very steep, the runoff is somewhat higher than it would 

 be with other conditions the same, but with flatter slopes. But 

 generally the degree of forestation and other elements exercise so 

 much inore important an influence that a comparison, without 

 regard to size of catchment area, may be legitimately made. 

 Nevertheless, this proposition is possibly debatable, and for the 

 present the conclusions drawn are tentative merely. 



The extreme low-water period . In the discussion of the streams 

 the writer has given the low water of the minimum year, but this 

 does not usually include the extreme low-water period, which is in 

 almost every case much more than one year. Space will not be 

 taken to show the extreme low-water periods of all the streams. 

 It is considered that illustrations from Muskingum and Genesee 

 rivers are sufficient. 



On the Muskingum river three low-water periods have occurred 

 during the time covered by the gagings. The first was from 

 December, 1887, to November, 1889, inclusive, a period of twenty- 

 four months, during which the total runoff was 18.55 inches, or 

 if we assume a reservoir on said stream of 20 square miles water 

 surface, the total net runoff becomes 18.15 inches. The computa- 

 tions of evaporation, etc. for such a reservoir, neglecting variation 

 in water surface, are as follows. Assume an annual evaporation 

 of 40 inches and with distribution for the several months as per 



