them away is therefore far in excess of that just stated.

The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to
the depth of four feet. At what rate is the Potomac basin being
lowered from this cause alone?

It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years.
Why so much faster than the Potomac and the Mississippi?

HOW STREAMS GET THEIR LOADS. The load of streams is derived from a
number of sources, the larger part being supplied by the
weathering of valley slopes. We have noticed how the mantle of
waste creeps and washes to the stream ways. Watching the run-off
during a rain, as it hurries muddy with waste along the gutter or
washes down the hillside, we may see the beginning of the route by
which the larger part of their load is delivered to rivers.
Streams also secure some of their load by wearing it from their
beds and banks,--a process called erosion.

EROSION

Streams erode their beds chiefly by means of their bottom load,--
the stones of various sizes and the sand and even the fine mud
which they sweep along. With these tools they smooth, grind, and
rasp the rock of their beds, using them in much the fashion of
sandpaper or a file.

WEATHERING OF RIVER BEDS. The erosion of stream beds is greatly