SCOOPESTG OUT OF EIVER BEDS. 255 



In the inundation of 185Y, the Ard^che destroyed a stone 

 bridge near La Beaume, wliich had been built about eighty years 

 before. The resistance of tlie piers, which were erected on piles, 

 the channel at that point being of gravel, produced an eddying 

 current that washed away the bed of the river above them, and 

 the foundation, thus deprived of lateral support, yielded to the 

 weight of the bridge, and the piles and piers fell up-stream. 



By a curious law of compensation, the stream which, at flood, 

 scoops out cavities in its bed, often fills them up again as soon as 

 the diminished velocity of the current allows it to let fall the 

 sand and gravel with which it is charged, so that when the waters 

 return to their usual channel, the bottom shows no sign of having 

 been disturbed. In 1846, during a flood of the Escontay, a tribu- 

 tary of the Rhone, piles, driven sixteen feet into its gravelly bed 

 for the foundation of a pier, were torn up and carried off ; and 

 yet, when the river had fallen to low-water mark, the bottom at 

 that point appeared to have been raised, by new deposits of sand 

 and gravel, higher than it was before the flood, while the cut 

 stones of the half-built pier were found buried to a great depth 

 in the excavation which the water had first washed out. The 

 gravel with which rivers thus restore the level of their beds is 

 principally derived from the crushing of the rocks brought down 

 by the mountain torrents, and the destructive effects of inunda- 

 tions are irmnensely diminished by this reduction of large stones 



— and especially where they are fed by glaciers not overhung by crumbling 

 cliffs, the channel may remain almost unchanged for centuries. This is ob- 

 servable in many of the tributaries of the Dora Baltea, which drains the valley 

 of Aosta. Several of these small rivers are spanned by more or less perfect 

 Roman bridges — one of which, that over the Lys at Pont St. Martin, is stiU in 

 good repair and in constant use. An examination of the rocks on which the 

 abutments of this and some other similar structures are founded, and of the 

 channels of the rivers they cross, shows that the beds of the streams can not 

 have been much elevated or depressed since the bridges were built. In other 

 cases, as at the outlet of the Val Tom-nanche at Chatillon, where a single rib 

 of a Roman bridge still remains, there is nothing to forbid the supposition 

 that the deep excavation of the channel may have been partly effected at a 

 much later period. 



The Roman aqueduct known as the Pont du Gard, near Nismes, was built, 

 in all probability, nineteen centuries ago. The bed of the river Gardon, a 

 rather swift stream which flows beneath it, can have suffered but a slight 

 depression since the piers of the aqueduct were founded. 



