276 TRANSPORTING POWER OF WATER. 
larly formed concave channel, lined with sand, and reducing 
the depth of water, in some places, from five or six feet to 
fifteen or eighteen inches. Observing this deposit after the 
river had subsided and become so clear that the bottom could 
be seen, I supposed that the next flood would produce an ex- 
traordinary erosion of the banks and some permanent changes 
in the channel of the stream, in consequence of the elevation 
of the bed and the filling up of the spaces between the stones 
through which formerly much water had flowed; but no such 
result followed. The spring freshet of the next year entirely 
washed out the sand its predecessor had left, deposited some of 
it in ponds and still-water reaches below, carried the residue be- 
yond the reach of observation, and left the bed of the river 
almost precisely in its former condition, though, of course, with 
the displacement of the pebbles which every flood produces in 
the channels of such streams. The pond, though often pre- 
viously discharged by the breakage of the dam, had then been 
undisturbed for about twenty-five years, and its contents 
consisted almost entirely of sand, the rapidity of the cur- 
rent in floods being such that it would let fall little lighter 
sediment, even above an obstruction like a dam. The quantity 
I have mentioned evidently bears a very inconsiderable propor- 
tion to the total erosion of the stream during that period, be- 
cause the wash of the banks consists chiefly of fine earth rather 
than of sand, and after the pond was once filled, or nearly so, 
even this material could no longer be deposited in it. The fact 
of the complete removal of the deposit I have described be- 
tween the two dams in a single freshet, shows that, in spite of 
considerable obstruction from roughness of bed, large quantities 
of sand may be taken up and carried off by streams of no great 
rapidity of inclination; for the whole descent of the bed of the 
river between the two dams—a distance of four miles—is but 
sixty feet, or fifteen feet to the mile.* 
* Tn a sheet-iron siphon, 1,000 feet long, with a diameter of four inches, 
having the entrance 18 feet, the orifice of discharge 40 feet below the summit 
of the curve, employed in draining a mine in California, the force of the current 
