4-88 Sir H. T. De La Beche's Memorandum 



It will also be readily seen, that when the strength of a tide 

 impinges on a bank (sufficiently hard to prevent its being worn 

 away), that any artificial alteration in the form of that bank 

 would produce a change in the direction of the tide, the shoal- 

 forming or channel-scooping influences of which will be felt 

 in proportion to the amount of change so made, due regard 

 being had as well to the new effects produced upon the flood 

 as upon the ebb-tide. 



As, at any given time, the forms of natural banks in estuaries 

 are but adjustments to the general conditions existing at that 

 time, it follows that, if these forms be artificially altered, this 

 adjustment is destroyed, and a new state of things arises, 

 which may be considered local or otherwise, according to the 

 amount of change. In strictness, viewing an estuary as a 

 whole, and the effects produced by changes of this kind upon 

 the two tides, that which may appear of very little importance 

 should be considered as, to a certain extent, producing gene- 

 ral results. 



As the velocity of equal volumes of water in rivers depends 

 on the amount of fall of their channels or courses, and as the 

 same necessarily holds good in estuaries, to that level which 

 may be considered as low water (when such is not outside, the 

 whole estuary being above real low water on the coast), the 

 fall of the channel of the estuary from its head to this level, 

 all other things being equal, gives the general velocity of the 

 waters of the ebb-tide ; whence it follows that whatever scour 

 or mechanical action of the water on the bottom may be due 

 to the influence of this fall, it cannot be altered, as a whole, 

 so long as these two points remain at the same distance from 

 each other, and the difference of their levels is constant, equal 

 volumes of water being understood. 



Let, in the annexed sketch, the line D A B C represent, 



at an exaggerated angle, the bottom of part of a river, above 

 an estuary, the bottom of the estuary itself, and a portion of 

 the sea bed ; the line G A being that of high water in the 

 estuary, and E B that of low water ; and let A B be the bot- 

 tom of the estuary, and B C a part of the sea bed beyond it. 

 So long as A, the head of the estuary, and B, the level of low 

 water, remain at the same distance, and at the same difference 

 of level, and the volume of water passing down, or rather the 

 excess of influence of the ebb-tide over the flood, viewed as a 



