2l6 SEA AND LA AW 



depreciation. So far no successful means have been discovered 

 whereby the effect arising froni this constant importation of 

 sediments into a harbor may be arrested. The only recourse 

 seems to be dredging — a slow and costly process, by which the 

 deposits of mud are removed from the important channels and 

 conveyed by boat to some point where they will not return to 

 the basin. 



It is fortunate that, thouMi our bavs and other harborasfe 

 places are generally entered by rivers, only a few of these 

 shelters suffer any rapid depreciation by the action of the 

 streams. Moreover, in the case of very many of our rivers 

 of this continent, as well as those of northern Europe, there 

 are certain conditions which fortunately limit the amount of 

 detritus which the flowinsf water brinsfs down to the sea level. 

 Because they lie in glaciated districts the streams of the 

 northern lands generally drain from regions which are deeply 

 covered with sandy and pebbly waste. In such cases the 

 greater part of the rain-water passes into the porous drift 

 material, and is only gradually yielded to the streams. Thus 

 the river system of a glaciated district has in proportion 

 to the outflow very few torrents, such as characterize the sur- 

 face of regions not underlaid by glacial waste. It is the swift- 

 moving currents of these torrents which do almost all the 

 work of breaking up rocky matter into the shape in which 

 it may be transported as mud by the largest streams. 



Almost all the considerable rivers of glaciated districts 

 pass through lakes, where their waters are for a time brought 

 to a state of almost complete rest and are thus led to lay down 

 their burden of sediment before they flow on to the sea. An 

 excellent instance of this is found in the case of the St. Law- 

 rence, which has a chain of great lakes in its main path, in 



