WATER AS A MECHANICAL AGENT. 



22T 



i / 



r-07i 



11 *15^ // 



1.1./ 



19 21 \..- 



But the wave-and-current drift, moving westward, finds its mouth open ; and the sand-bars 

 there made are so high and large that the greatest depth in the channel at low tide 

 is only 7 feet. 



The mouth of the Housatonic River, west of New Haven (a stream 100 miles long) is 

 in a worse plight ; for it gapes open directly eastward and faces the drift movement. The 

 greatest depth over the bar at low water 



is consequently only 3 feet. The tide is 200. 



7 feet ; but the tidal grounds are small. 



The harbor of New Haven (Fig. 199) 

 receives 3 rivers, the longest only 35 miles. 

 But, in contrast with the other cases, there 

 is a prolonged eastern cape of gneissoid 

 granite, and this forces the wave-and-current 

 drift to take a course more to the south- 

 ward over deeper water. The drift chokes 

 up the mouth of " West Eiver" (the west- 

 ernmost of the three), but leaves the rest 

 of the harbor mostly unharmed. Although 

 the tidal grounds are not large, the entrance 

 to the harbor has a depth over the bar of 20 

 feet ; and this it owes chiefly to its eastern 

 cape. 



Harbor-improvement. — The principle at 

 the basis of improvements of harbors at the 

 mouths of tidal rivers is made plain by the 

 preceding illustrations. It requires that there 

 should exist for each the largest possible 

 tidal grounds, in order that there may be the 



largest possible outflow of waters for channel scouring ; and where not existing, that they 

 should be obtained by the construction, from the capes either side of the entrance, of a 

 breakwater or levee as far out as the depths will allow ; that the breakwater should rise 

 so little above low-tide level that the tide may freely enter over it and fill the bay ; that 

 the windward side of the breakwater should have such a position and extent as will carry 

 the wave-and-current drift far enough out to clear the leeward cape, if possible. A harbor 

 with a large breakwater receives great aid for channel scouring from the waters that are 

 piled in by the storm-winds. These winds sometimes keep driving in water, and making 

 an undercurrent out of the channel, through all states of the tide. As to one or more 

 additional channels to the harbor, the engineer has to decide after examination. In the 

 case of a tideless river, like the Mississippi, the channel may be improved by embank- 

 ments alongside of it ; but not so that of a tidal river. 



The harbors made by coral-reef barriers about Pacific islands are in accordance with 

 the best models ; and an atoll with a ship entrance to the lagoon is such a harbor isolated 

 in midocean. The outflowing tidal waters keep the channel in good condition. 



2 1 _ _2_ _3 -."--IQ, 9 22 

 r/sVljfu "fl3'l.'22 



J 7 ll.'u / 19 



17 1_6 /22 " « 



B^' """"24 34 40 



19 3' 



40 



40 



42 45 



Mouth of the Housatouic. 



The formation of sand-bars in Long Island Sound, and the variations in 

 depth, are due mainly to variations in the velocity of the outflowing tide, as 

 partly explained on page 216, the rivers being the chief source of new material. 

 But the position of the deeper channel has come down to a large extent from 

 preceding geological time, and especially from the Glacial and Champlain 

 periods, when depositions were on an enormous scale. In other cases over 

 the coast region the shoals indicate the forms, and partly the positions, 



