on Estuaries and their Tides. 



487 



some faster, according to conditions; a fact which, though 

 not always apparent in some places, during what have been 



termed historical times, is abundantly shown in others, and, 

 when considered geologically, satisfactorily proved. 



New channels may be formed from an increase of deposit 

 in one place, giving a new direction to the waters in another ; 

 and it is highly instructive to observe how an apparently very 

 small cause may produce important modifications and altera- 

 tions, changing the channels and shifting the banks or shoals ; 

 but the general resulting action is a Jilling-up, viewed as a 

 whole. 



Such are the variable conditions existing in estuaries, and 

 so complicated are often the effects of the causes in action, that 

 it becomes of the utmost importance well to study and reflect 

 upon the value of each cause before we attempt changes in 

 connexion with an estuary intended for our advantage ; and 

 this more especially when a great commercial port is situated 

 on part of such estuary. 



It will be obvious, if bridges traverse an estuary, that not 

 only will the piers stop the tide flowing upwards (in propor- 

 tion as the piers occupy the breadth of the river), thereby 

 bringing the head of the estuary more seaward, but that these 

 obstructions would tend to produce shoals in the bed of the 

 main channel ; for though the ebb, with its back-ponded river 

 waters, would pass between the piers with great velocity (be- 

 cause passing through what may be considered as a dam 

 pierced in many places), scooping out corresponding channels, 

 deposits would be formed in the eddies behind the piers, the 

 influences of which are prolonged into the main channel. 



