486 Sir H. T. De La Beche's Memorandum 



the amount of water falling into them varies also, from the 

 comparatively small streams in dry weather to the heavy vo- 

 lumes discharged during freshets or floods, the mechanical 

 action on the bottom of the estuary is necessarily variable, 

 always allowing for a constantly prevailing action outwards ; 

 and unless there be sufficient compensating forces, general 

 changes will be effected according to those conditions which 

 are dominant. 



As there is no sufficient force to counteract the discharge 

 of detritus outwards, this will accumulate in the estuary, if the 

 ebb-tide be not able to force it beyond into the sea. 



The amount of detritus thrown into the estuary will depend 

 on the course of the river above it. If that course be rapid 

 to the tideway, and the water capable of forcing forwards gra- 

 vel, all minor detritus will be borne down, and the whole will 

 be discharged into the estuary, to be dealt with according to 

 its powers. And this is the general action in all kinds of river 

 courses, down to that which permits the river waters simply to 

 bear the matter of mud to the tideway, in which the velocities 

 may even be sometimes greater than in the river above. 



The banks across rivers formed where detritus, either me- 

 chanically suspended or pushed onwards, is brought to rest, 

 from a discontinuance of the conditions necessary to suspend 

 or move it, and commonly known as bars, occur, as might be 

 expected, very variously in rivers. When arms of the sea, 

 gradually diminishing inland, terminate in estuaries, these bars 

 are often far up the tideway, where the rush of river waters in 

 a freshet is met by the quieting action of the tide ; in other 

 tidal rivers the bars occur at their embouchures, where the 

 action of the breakers not only tends to force the detritus back 

 upon the coast, but also stops the velocity of the estuary waters 

 discharged on the ebb. 



In some situations, as in the sketch beneath, prevalent winds, 

 having action on the coast, force breakers along shore also 

 in a prevalent direction, A A, causing a beach, B B B, to 

 follow a line along shore, and across the mouth of the estuary ; 

 so that a formidable barrier to the discharge of the river- 

 borne detritus, seaward, by this and the other causes above 

 noticed, is presented, behind which shoals, C C C, accumu- 

 late, tending to fill up the lower part of the estuary, alluvial 

 flats forming above, D D. 



From the action of breakers on a coast (a most powerful 

 action, throwing up shoals and bars at their embouchures), 

 combined with the loss of velocity of the estuary waters sea- 

 ward, depositing the detritus they can no longer transport or 

 push onwards, estuaries as a whole are filling up, some slower, 



