FILLING UP OF LAKES. 161 



streams, but that valleys have been in part filled up by the accumu- 

 lations brought by their own rivers ; and that their present smooth- 

 ness and uniformity is really the result of modifying powers of the 

 sea, atmosphere, local influences, and the river combined, exerted 

 through long time upon a ruder channel, left by more violent marine 

 agents. 



Rivers with Lakes. Let us now see what peculiarities in the 

 effects of rivers are occasioned by the circumstance of their traversing 

 quiet lakes. Two things are here to be attended to. First, the lake 

 causes, according to its extent, a more complete stagnation of the river 

 movement, and consequent deposition of the sediment brought by the 

 rivers, than is occasioned by the most level area of a river-valley ; 

 secondly, the materials dropped in the lake are regulated by somewhat 

 different laws from those which direct their accumulation on an 

 ordinary surface. 



When a river charged with sediment expands into the waters of 

 a lake, its motion, communicated to that large area in directions 

 radiating from the place of entr}^ is checked and almost lost, and the 

 sediment which it brought to the lake is gradually, and at last wholly, 

 deposited ; and the purified stream issues from the lower extremity 

 without a taint of its stormy origin, unless it be the colour, due to moun- 

 tain-peat, or some other substance held in chemical solution. Like 

 the lake from which it escapes, or the ocean far from shore, it gene- 

 rally assumes the purest ethereal hue, its native tint of blue or green, 

 but soon in its onward course again becomes turbid with sediment. 

 Every lake in Switzerland exhibits these effects upon the rivers, which 

 commonly enter turbid, and issue of a transparent green, though the 

 waters of the Ehone are pre-eminently blue. These lakes are filling 

 and contracting at their upper ends with the sediment which they 

 gather from the rivers ; and the process, though historically slow, is 

 monumentally impressive, since we find large tracts of level 

 meadows, cultivated, covered with trees, and supporting ancient and 

 modern towns, where formerly flowed the deep waters of the lake. 

 Thus the Roman town Portus Valesise, originally on the water's edge, 

 is now nearly two miles inland, owing to the delta of the Rhone 

 having encroached to that extent on the lake in the last 800 years, 

 and behind Port Vallais the delta extends inland for five or six miles 

 as an alluvial plain, which has displaced the waters of the lake. 



All this new land was formed from the spoils and waste of tho 

 upper countries drained by the river, and is a measure of the effect 

 of atmospheric and local influences in weathering the face of the 

 hills, and of rivers in carrying away the materials thus prepared 

 for them since the earliest period when the streams began to floio 

 down the valley of the Rhone. 



Arrangement of Materials in Lakes. The second thing to be 

 attended to in considering the effects of lakes on the line of rivers, is 

 the arrangement of the materials which they receive. It is known to 

 practical engineers that loose earth will remain at rest if it be placed 

 at an angle not exceeding 45 with the horizon, and when loose, earthy 

 VOL. i. L 



