HYDROLOGY OF NEW Y'ORK 



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what further into the detail than this statement implies. The 

 clearing up of lands and drainage of towns, together with farm 

 drainage, are all efficient causes for floods. In a general way, the 

 water falling in the form of rain or snow runs off by these various 

 means quicker than formerly. In the meantime the channels of 

 rivers have not been correspondingly enlarged to meet the in- 

 creased demands upon them, with the result that overflows fre- 

 quently ensue. An examination of rainfall statistics shows that 

 as a whole the rainfall is no greater now than formerly, and the 

 increased frequency of floods must therefore be ascribed to not 

 only irregularity in the rainfall, but to the greater rapidity with 

 which water runs off. Broadly, we may say that the higher the 

 degree of civilization, the more quickly will surface water be 

 discharged, and hence, without there is a corresponding increase 

 in the discharging capacity of streams, floods will become more 

 frequent, with their attendant evils. 



Frequency of floods. This matter has been referred to in a 

 preceding chapter, discussing floods on the Tiber and Seine. 

 Very serious floods have occurred on these streams as often as 

 about once in thirty to fifty years. In the United States there 

 are no records long enough to show certainly how often floods may 

 be expected, but probably their periods will not be materially 

 different from these streams — in some cases, heavy floods have 

 occurred on the Tiber only a few years apart and it is intended to 

 indicate here only very general averages. 



Are storage reservoirs effectual in preventing floods f There 

 is considerable doubt about storage reservoirs being a universal 

 remedy for floods. On some streams excessive cost would pre- 

 clude their use— that is to say, the benefits received would not 

 be commensurate with the expense. On other streams their use 

 may be of the greatest value. On the Genesee river, 41 per cent of 

 the total catchment area can be controlled by a reservoir; on 

 Salmon river, 55 per cent; on Black river, 90 per cent, and on the 

 upper Hudson river, 50 per cent. On all these streams the percent- 

 age of control is large enough to prevent destructive overflows. 



Views of French engineers. The utility of storage reservoirs 

 in diminishing damage from floods was taken under considera- 

 tion in France in 1856. Investigations made in the valleys of 



