558 KEPORT — 1887. 



the present conditions of the valleys, and which, as already stated, was 

 explained by Dr. Thomson. From the source of the river as the rain-water 

 acquires the velocity, it charges itself with deposit, which charge it 

 maintains with continual taxes and drawbacks until it reaches the ocean 

 or lake, when its water in again losing its velocity deposits its charge, 

 continually carrying forward the bar and extending its delta. 



In non- tidal rivers, whether large or small, fast or slow, the characters 

 of these actions are invariable, however much they may differ in intensity. 

 The case of tidal estuaries is, however, by no means so simple. Here we 

 have not, as in a river, a continuous progression of the same character 

 of action at the same point. On the contrary, at every point the action 

 is changed twice a day. For the change in the tidal current does not 

 merely change or reverse the direction of the sand-drift at each part of 

 the bed, but it changes and often reverses the character of this drift, 

 changing what has been a scouring drift during the ebb-tide into a 

 depositing drift during the flood ; so that the questioii as to whether the 

 regime is stable, depositing, or scouring is not simjjly a question as to 

 whether the current at this point is uniform, accelerated, or retarded, but 

 Avhether the action of the ebb to cause, say, scour is equal to, less than, or 

 greater than the action of the flood to cause deposit. 



As there is no likelihood that the resultant effect as regards the 

 general regime of two opposing influences will resemble what would have 

 been the simple effisct of either of the influences acting alone, this dual 

 control afibrds abundant reason Avhy the configuration of the beds of these 

 tidal estuaries should diS"er in character from the configuration of the 

 sand-beds of continuous streams. 



There is, however, another and an equally important difference 

 between the general motion of the water in rivers and tidal estuaries. 



The function of the estuary is by no means that of a simple channel 

 to conduct the tidal water up and down. It equally discharges the 

 function of a reservoir or basin, to be filled and emptied by each tide. 



In consequence of this action as a reservoir the dii'ections of the 

 motions of the water during flood and ebb, and particularly towards 

 the top of the flood and commencement of the ebb, are genei-ally very 

 different from what they would be were the estuary acting the simple 

 part of a channel conducting the water from somewhere to somewhere. 



When a vessel is filled hx a stream entering on one side the forward 

 motion of the water is stopped before reaching the opposite side. But if, 

 as is always the case, the motion which the water has on entering is more 

 than sufficient to carry it as far as is necessary, the remaining momentum 

 is spent in setting up eddies, or a general circulation in the water, so that 

 when the vessel is full the water within it is not by any means at rest, 

 but may be circulating round or have any other motion. If, then, the 

 water is allowed to flow out the initial motion will not be a steady move- 

 ment towards the outlet from all parts of the vessel, but those portions 

 of the water which are moving towards the outlet will have their motion 

 accelerated, while those which are moving in the opposite direction will 

 have first to be stopped before they begin to approach the outlet. And 

 thus the ebb will begin earlier at some points in the vessel than at 

 others. 



It was the observation of such an effect as this in one of our largest 

 estuaries that first directed my attention to the subject of this j^aper. 



Having investigated this point sufficiently for my own satisfaction 



