266 The Scottish Naturalist. 



My interest became at once very much increased by observ- 

 ing several large pebble and gravel beds that had been or were 

 still being formed by the river. In some places they were piled 

 up eight feet at least above the level of the water. It was soon 

 seen that these pebble-beds corresponded to curves in the 

 river, and not to other beds on the opposite side, but in every 

 case to a steep bank, the pebble-bed generally dipping gently un- 

 der the stream. The steep bank always presents (unless arti- 

 ficially protected) a tumble-down appearance, the green turf 

 overhanging or breaking away in masses, — in other words, it is 

 being denuded. The pebble and gravel beds are being deposited. 



In this process of denudation and restoration the stream 

 follows a definite rule, that may be stated thus : The river de- 

 nudes the inner ba?ik of a curve and restores the outer. This is a 

 rule from the very nature of the phenomenon, but it is none the 

 less a rule, and it leads to curious, and indeed wonderful results. 

 If the river channel were perfectly straight to-day it would hardly 

 be so to-morrow, for the variable rate of the current causes the 

 material in the river bed constantly to change its position, — 

 hence an indirect course. Such a river, surrounded with, and 

 flowing over, loose detrital matter, cannot fail to act in what we 

 consider an eccentric manner. 



When a " spate" (that is, a flood) occurs, the power oft he 

 water is wonderfully increased, so that immense volumes of boul- 

 ders and pebbles are carried along, grinding each other smooth, 

 while the abraded material, and any lighter detritus, is carried 

 away to be deposited wherever the surplus water may find a 

 resting place. When once a heap is thrown up in the river bed 

 it becomes the centre of a general accumulation. Although 

 the unobstructed current carries gravel and even pebbles in its 

 course, its power of sustaining them is gone the instant it 

 receives a check. Any impediment in the bed of the stream, 

 especially during a "spate," is sure to receive constant showers 

 of debris, accummulating more and more as it becomes powerful 

 to obstruct. The obstruction necessarily deflects the water. 

 The consequence of this deflection is the abrasion of the invaded 

 bank. The rate of denudation increases with the development 

 of the pebble-bed, which reaches a maximum height of eight 

 feet above low-water. 



Bearing in mind the outer abrasion and the inner restoration 

 with the continued action of the river, the following will be seen 

 to be the effect: — The first or out-going curve is encroached 



