on Estuaries and their Tides, 489 



whole, be a constant, any deepening of the bed of any part of 

 the estuary, as for instance at H I, will, it is* obvious, not 

 alter the amount of the general slope from A to B ; for the in- 

 fluence of any extra water lodged in the cavity H I, so long 

 as H retained its height, supported by the bottom HBC, 

 would be nothing as to scour of the bed, since it would merely 

 constitute a pond or pool in the general channel, that would 

 be again filled up by detritus brought down from above, in the 

 usual way, along the bottom D A I. 



Assuming that the action of the tidal water introduced at 

 the flood-tide is as great upon the bed of an estuary as during 

 the ebb, the same volume of water, viewing only the tidal 

 water, passing in and out, — an assumption not strictly correct, 

 inasmuch as the flood-tide flows up an inclined plane, and the 

 ebb runs down it, — but, for convenience, assuming this action 

 to be equal, we should anticipate that when the back-ponded 

 river water can be let loose upon the ebb, the greatest scour- 

 ing effects outwards would be when the volume and velocity of 

 the united waters, viewed in combination, should be the great- 

 est, and that that would be not immediately after high water, 

 when the pressure of the supporting tide is slowly removed, 

 but towards the middle of the tide, when (though the volume 

 was greatest nearer high water) the velocity was so much in- 

 creased, as, with the volume of water still remaining, to pro- 

 duce the greatest effects. This anticipation is borne out by 

 observation, which appears to show that from about two to 

 four hours' ebb the greatest scour of the estuary bed is ef- 

 fected. 



All observation tends to show, that the greater the volume 

 of tidal water coming up an estuary with the flood, the greater 

 is its mechanical or scouring action when its volume and ve- 

 locity are increased by the addition of the ponded back river 

 water, and the ordinary river discharge. And it should be 

 borne in mind, that it is only during the ebb that the river 

 water passes out as a whole, though in some estuaries, during 

 heavy freshets, the river water, from its less specific gravity, 

 has been known to run downwards on the top of the flood, 

 coming upwards : proved by large vessels, such as men-of- 

 war, drawing several feet of water, riding at anchor with their 

 heads to the first of the flood-tide, while boats alongside, draw- 

 ing little water, laid with their heads up stream against the 

 freshets. It may be here observed, that when conditions are 

 favourable, the flood-tide running in lifts the outgoing ebb, 

 particularly when composed of little else than river water, in a 

 wedge-like manner : from this, to the conditions where the 

 flood suddenly overcomes the ebb-tide by a great wave, or 

 bore, as it is termed, there are various modifications. 



