SCOOPING OUT AND FILLING UP OF EIVER BEDS. 391 
In the inundation of 1857, the Ardeche destroyed a stone 
bridge near La Beaume, which had been built about eighty 
years before. The resistance of the 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 sup- 
port, 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 a flood of the 
Escontay, a tributary of the Rhone, in 1846, piles driven six¬ 
teen 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 higher than it was before the flood, by new de¬ 
posits of sand and gravel, while the cut stones of the half-built 
pier were found buried to a great depth in the excavation 
wdiich 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 inundations are im¬ 
mensely diminished by this reduction of large stones to minute 
fragments. If the blocks hurled down from the cliffs were 
transported unbroken to the channels of large rivers, the me¬ 
chanical force of their movement would be irresistible. They 
would overthrow the strongest barriers, spread themselves 
of the rocks on which the abutments of this and some other similar struc¬ 
tures are founded, and of the channels of the rivers they cross, shows that 
the beds of the streams cannot have been much elevated or depressed since 
the bridges were built. In other cases, as at the outlet of the Val Tour- 
nanclie 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. 
