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LXVIII. Memorandum on Estuaries and their Tides. 

 By Sir H. T. De La Beche, F.R.S., F.G.S., $c* 

 T^HE existing state of any estuary, or tidal river, may be 

 considered as an adjustment, for the time, of certain con- 

 ditions, changes in any of which conditions effect alterations 

 in that state, productive of injury or benefit to the purposes 

 for which we employ, or may be desirous of employing, such 

 estuary, according to circumstances. 



The action of the flood-tide in an estuary, or tidal river, is 

 to pond back, during its continuance, the river waters, which 

 would otherwise have flowed outwards for that time; so that 

 when the ebb-tide makes downwards, the tidal waters which 

 came up with the flood-tide are increased by the volume of 

 water so ponded back, independently of the ordinary dis- 

 charge of the river waters during the ebb. 



The ebb-tide, therefore, is composed of the tidal waters 

 which came up with the flood, plus the water ponded back, 

 and the river water continuing to flow out during the ebb. 



It follows that the mechanical effects produced on the bot- 

 tom of an estuary by the ebb-tide must be far greater than on 

 the flood, and that there is a constant tendency to force mud, 

 silt, sand and gravel outwards to the sea. 



The mode in which these substances, commonly termed de- 

 tritus, are carried along is twofold: — 1st. When the move- 

 ment among the particles of water is sufficiently considerable, 

 the water will take up the detritus in mechanical suspension, as 

 it is termed, according to the amount of this movement ; that 

 is, the detritus is really lifted up and borne onwards actually 

 suspended in the water. 2ndly. The mud, silt, sand and gra- 

 vel, as the case may be, will be pushed or forced along upon 

 the bottom, according to the pressure and velocity of water 

 above them, producing the friction required. 



It follows, where there are unequal velocities of waters in 

 tideways, that the same substances which may be mechanically 

 suspended in the water in one place, will be thrown down on 

 the bottom in another ; and that in situations where the water 

 remains for any sufficient time at rest, such places will be 

 gradually filled up by deposit of the substances mechanically 

 suspended in the water flowing into them. 



In like manner also, substances which are pushed or shoved 

 onwards along the bottom by the weight and velocity of the 

 water above them in some situations, come to rest in others, 

 when such weight and velocity are sufficiently diminished. 



As the volumes and velocities of tides in estuaries vary, and 



* From the First Report of the Commissioners on Metropolis Improve- 

 ments, 1844 : Appendix, p. 205. 



