612 PROP. W. J. SOLLAS ON THE ESTUARIES OP 



favour ; yet I shall hope to show that it is one, at all events, supported 

 by evidence of considerable weight. 



When, however, all these sources have been admitted as genuine, 

 there still remains one difficulty which has much exercised the minds 

 of many painstaking observers. The Severn and its tributaries are 

 not, except when flooded, very muddy rivers ; the wash of the cliffs 

 is not, as a rule, excessive ; the sea, if it furnishes anything, certainly 

 cannot furnish much ; and yet the vast body of estuarine water 

 which extends from "Weston to Portishead is never otherwise than a 

 sea of more or less diluted mud*. 



The explanation of this lies in the fact that the water in the tidal 

 portion of the Severn channel flows up and down twice daily 

 at the rate of from 6 to 12 miles an hour, a velocity much greater 

 than that required to move along large boulders of rocks. Water 

 moving at this rate is far more likely to denude than to deposit 

 material ; and indeed in certain parts of the Severn it is, by scouring 

 along great masses of boulders, deepening the channel ; and to its 

 past action in this way the deep water known as the " shoots " is 

 attributed by Mr. Eichardson. 



In this rapidly moving body of water, the direction of which is 

 reversed twice daily, the mud discharged by the rivers and washed 

 from the beach accumulates, and from it sediment is supplied to all 

 the tributary estuaries during flood tide ; a sufficient diminution in 

 the velocity of the current will of course be marked by the subsi- 

 dence of sediment, and when the velocity sinks to zero sedimentation 

 is copious. Such a cessation of movement appears to take place in 

 the Avon during ebb tide, as Mr. W. R. Browne has well shown by 

 a series of experiments made with an ingenious current-meter devised 

 by my colleague Prof. H. S. Shaw. I give Mr. Browne's results in 

 his own words f : — " In ordinary tidal channels, such as the Avon 

 below Bristol, the course of events during an ebb seems to be as follows. 

 At first the slope of the surface is exceedingly small (in the Avon it 

 was about 1J foot in 7| miles), and, while the velocity at the surface 

 is considerable, it diminishes rapidly from thence downwards, and 

 at some distance from the bottom becomes nil. This continues for 

 about two thirds of the ebb, the surface-velocity increasing up to a 

 certain point, and then becoming nearly constant. During all this 

 time not only is no scour going on at the bottom, but, if the waters 

 be muddy, an actual deposition of silt is taking place. At this 

 time, after about two thirds of the ebb, the water has fallen about 

 three quarters of its total height, the slope of the surface has con- 

 siderably increased, and the conditions approximate to those of an 

 ordinary river. The bottom layers of the water then spring suddenly 

 into motion, the surf ace-^velocity diminishes steadily as the tidal waters 

 disappear, until it assumes the normal rate of the low-water flow. 



* This expression is doubtless somewhat too strong. Actual experiments 

 made in 1837, showed the presence of -oV^r part, by weight, of sediment in the 

 tidal water opposite Avonmouth, and ¥ y^ P ar * on the opposite coast (B. A. 

 Rep. Trans, of Sections, p. 76). Fresh determinations are doubtless needed. 



t Proc. Inst. Civil Engineers, vol. lxvi. p. 1. To this valuable paper is 

 appended the report of a no less valuable discussion. 



