CONTROLLING TJDAL STREAMS 227 



in them in i)roportion to tlie mass of tlie current than on 

 the shoaler bottom at either side and so the currents tlow 

 faster, thus preventing- the deposition of sediment during 

 the period of ebb and flow and removing such materials as 

 may have come to rest at high and low tide when the currents 

 cease to act. 



Fortunately for the interest of ports which are visited 

 by tides, the forces which they apply to our harbors are 

 often readily controllable by engineering constructions. By 

 means of jetties it is fre(|uently practicable to direct the run 

 of streams or by concentrating them to increase their energy 

 in such manner that the character of the exits may be im- 

 proved. It has been found to be an important precaution in 

 the management of harbors to restrict all natural or artificial 

 chansfes which tend to diminish the volume of the tidal 

 water that moves through the inlet. This is the case where 

 about the great cities which have developed on the margins 

 of our harbors it is found commercially desirable to fill in a 

 portion of the shallow sea bottom. The harbor autliorities 

 properly require that a compensation be made in the way of 

 dredging other portions of the basin so that the total amount 

 of water which moves with each tide ma\' not be diminished. 

 Those ports are fortunate which have a great extent of tidal 

 waters extending inland beyond the points where the ships 

 seek access to the shore, for such an extensive storage in- 

 sures a strong flow of tide, and consequentK- open pathways 

 to the sea. Thus in the case of the port of New York, the 

 Hudson, which is a marine inlet rather than a river, aftords 

 a vast storage basin for the water which passes through the 

 harbor mouths. 



The effect of tidal action is much like that brought about 



