THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the
drainage area, the texture of the rocks, and other factors. With
an annual rainfall of fifty inches in an open country, about fifty
per cent is discharged; while with a rainfall of twenty inches
only fifteen per cent is discharged, part of the remainder being
evaporated and part passing underground beyond the drainage area.
Thus the Ohio discharges thirty per cent of the rainfall of its
basin, while the Missouri carries away but fifteen per cent. A
number of the streams of the semi-arid lands of the West do not
discharge more than five per cent of the rainfall.

Other things being equal, which will afford the larger proportion
of run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen
or unfrozen ground? Will there be a larger proportion of run-off
after long rains or after a season of drought? after long and
gentle rains, or after the same amount of precipitation in a
violent rain? during the months of growing vegetation, from June
to August, or during the autumn months?

DESERT STREAMS. In arid regions the ground-water surface lies so
low that for the most part stream ways do not intersect it.
Streams therefore are not fed by springs, but instead lose volume
as their waters soak into the thirsty rocks over which they flow.
They contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they
wither at times of drought to a mere trickle of water, to a chain