Effects of the Contraction and Expansion of Riven-. 27 



the volume of water, it remains to notice the variations in the 

 depth of the same stream, flowing in the same volume, which 

 are produced by differences in the form of its channel. 



The most important of these differences consists in the com- 

 parative width or narrowness of the channel. The quantity of 

 water being the same, a narrow channel, by increasing its depth, 

 adds in the same proportion to its carrying force ; whereas an 

 expanded channel, by diminishing its force, and diffusing its influ- 

 ence, causes it to strew its contents over every spot of ground 

 which it reaches. If the depth be sufficiently increased, it acts 

 with all the force of a solid body, but with the advantage of 

 continually changing its direction. It may therefore be regard- 

 ed as a lever, not only of immense power, but of infinite flexi- 

 bility. In passing through a narrow gorge it is continually re- 

 flected from the sides and bottom, and hence it assails every 

 loose mass of stone with impressions upon all sides, and carries 

 it in the direction in which there is the least resistance. 



Evei'y river exhibits expansions and contractions in continual 

 alternation : but they are most evident, and their respective ope- 

 rations most distinct, in elevated regions. I have never seen 

 the contrast more strikingly displayed than in the upper part 

 of the Valley of Bagnes, on coming to a nearly circular expan- 

 sion, succeeded by a narrow and lofty gorge. In this scene of 

 desolation we observe the plain, which is more than a mile in 

 diameter, covered with large pebbles and boulder-stones, many 

 of them 6, 8, or 10 feet thick. These were carried through 

 the gorge in a few hours by the debacle of 1818 ; and it is, I 

 conceive, only by taking into account the extraordinary height of 

 the water, that we can explain the passage of such a vast quan- 

 tity of materials, containing pieces of so great size, in so short 

 a time, through a channel, which is nowhere more than a few 

 yards wide, and which is full of anfractuosities and projections. 

 The effect can only have been produced by the impinging of 

 the water against the various unevennesses of the sides and bot- 

 tom, which occasioned its reverberation upwards and sideways, 

 and which, being exerted with a force proportioned to the depth, 

 kept rocks and stones of every size suspended ankj dancing in 

 the mighty ebullition. On escaping suddenly from the gorge. 



