102 MEMOIR ON BOSTON HARBOR. 



resulting from the confluence of streams approaching each other from opposite di ections. 

 The growth of the banks on both sides of the channel is probably now less rapid than it 

 has been on the external borders ; it will continue to diminish. And the reason of this 

 diminution is pregnant with instruction ; it is the gradual narrowing oi \\\e channel that 

 lessens the accumulation, the water being made so much more rapid in its course by this 

 contraction, that it carries its burden beyond this point to drop it in a more favorable place. 



It is perhaps in this narrowing of the channel, and the greater tendency to deposit on 

 the sides, that the explanation is to be found of the positive declaration of the late Mr. 

 Winslow Lewis, that there is now, somewhere in this part of tiie channel, seventeen feet 

 of water, where in 1814 there was twenty-one feet. It may be an instance similar to 

 those made known by the comparisons with Wadsworth's chart. Still, this deterioration 

 is going on ; the maximum velocity on a bank of gradual slope must, in the highest parts, 

 be insufficient to disturb the bottom. 



Having pointed out the mode of operation according to which the tidal streams cre- 

 ate deposits in Boston Harbor, it is worth while to turn for a moment to a consideration 

 of the peculiar character of the harbor, and of the artificial changes which have assisted 

 the working of natural causes. Of good tidal harbors on alluvial or other shores, pos- 

 sessing the means of self-preservation, there are several distinct kinds. There are those 

 which are merely river-courses, as that of Philadelphia and that of Savannah; those 

 which are bays forming the receptacles of rivers, as that of Hampton Roads; such again 

 as are kept open by a double communication with the sea, as Edgartown ; such as, hav- 

 ing the bay form, not only receive a river, but have a double communication with the sea, 

 as New York ; and lastly, those which, like Boston, have large reservoirs or basins behind 

 the port, receiving great quantities of tidal water, and keeping the channels of the port 

 open by the scouring power of the ebb tide. Though the Charles and Mystic are called 

 rivers, they are chiefly to be regarded as valuable reservoirs, the latter being a short 

 drain for Mystic Pond, and the former being affected by the tide only as far up as Water- 

 town. All harbors that have neither land-water nor back-water have a tendency to fill 

 rapidly, and when not very deep, like Wellfleet and Plymouth, soon lose their usefulness; 

 and, in the same manner, all tidal harbors like Boston have a constant tendency to deteri- 

 orate. The gradual diminution in capacity of the reservoir, or, in other words, the grad- 

 ual growth by accretion of the flats and marshes above Boston, as the South Bay, the 

 Back Bay, and the Mystic, is well known to every one who has occasion to observe them 

 from time to time. The deposits by which this gradual increase of the land is finally 

 effected, in the places here spoken of, occur principally on the flood tide. At " slack- 

 water," as it is called, the sedimentary matter will subside in every part of the harbor; but 



