EFFECTS OF TIDES OX HARBORS 225 



Other conditions which serve to accent in a certain measure 

 this outward setting action which takes place during the period 

 of ebb. Ahnost all harbors receive a certain share of river 

 water. Owing to the momentum of the tide in entering this 

 water is in a measure banked up ; if the discharge from the 

 streams be not large it may all be retained in the basin until 

 the tide turns. This retained water is then permitted to flow 

 out, increasing the energy with which the harbor water goes 

 seaward. The admixture of fresh water, provided it be not 

 heavily charged with mud. probably somewhat increases the 

 capacity of the tide to dissolve and bear along sedimentary 

 materials. 



Where, as is often the case, the tide rises higher in the 

 interior parts of the harbor than at the mouth, the currents 

 which are generally in proportion to the altitude of the rise 

 weaken as they pass toward the entrance of the inlet. The 

 effect of this action is unfortunate, for the stronger currents 

 near the head of the harbor are likely to stir up and transport 

 toward its mouth more or heavier sediment than the weaker 

 streams. In this way the tidal currents are apt to cooperate 

 with other agents in producing something like a bar oil the 

 mouth of the port. On the other hand if the harbor have 

 considerable area and depth of water and the tidal rise and 

 fall is great the outrush is certain to keep a channel open for 

 a distance sufficient to cross the zone of migrating sands which 

 borders the shore. The tidal channel does not usually arrest 

 the movement of the coastwise journe\ing detritus. The 

 effect of the migrating sands is generally to push the tidal 

 scourway against the side of the bay toward which the dtbris 

 is journeying until after a time it breaks a fresh opening 

 through the partial barrier. Where, as in the principal 



