HYDROLOGY OF NEW YORK 



423 



the bed, together with the alluvial banks, permit of sudden and 

 extensive changes in channel location at times of floods. This 

 concavity of profile, with its resulting diminution of velocity, is one 

 of the potent factors in the causation of floods, since it permits 

 flood waters to be brought to points having sharp concavities of 

 profile or abrupt flattening of grades more rapidly than the chan- 

 nel will carry the same away, producing at such points temporary 

 accumulations of water with an attendant overflow. 



It is not to be understood that the profile of any stream is a 

 perfectly smooth concave curve, nor that the grades grow 

 progressively flatter without interruption as one passes down 

 stream. Local causes interfering with regularity of flow, and 

 geological formations interfering with the vertical erosion of 

 channels, cause interruptions in the regularity of the concave 

 profile. The Genesee river is a characteristic type of this inter- 

 ruption by geological causes. Whatever the regularity or irregu- 

 larity of the profile may be, however, it is safe to expect that if 

 floods occur at all on a stream, they are more certain to occur 

 where the stream slope grows suddenly or decidedly flatter or 

 where extensive local obstructions or restrictions occur. 1 



In order to correct the excessive flows produced by the fore- 

 going conditions the stream may be trained or regulated in a 

 number of ways; such training is commonly called river regula- 

 tion or river conservancy. 



Antiquity of river regulation or conservancy. River regulation, 

 or river conservancy, is very old, and there is scarcely a phase of 

 it that has not been considered at some time in the Old World. 

 The Chinese rivers, particularly the Hoang-ho, were regulated by 

 dykes and embankments over 4000 years ago. The same is true 

 of the Euphrates and many other rivers on which were situated 

 the cities of the ancient world. This statement is specially true 

 of the river Tiber, at Rome. In the year 53 B. C. — 1957 years 

 ago — a proposition was brought forward in the Roman senate for 

 moderating the frequent inundations of this stream, which re- 

 sulted in the appointment of five senators as a river conservancy 

 commission, to whom was assigned the task of so regulating the 

 volume of water in the river that there might be " no deficiency 



i Abstract from the Report of tlie Water Storage Commission. 



