At times of flood, rivers are fed chiefly by the run-off; at times of low water, largely or even wholly by springs.

How, then, will the water of streams differ at these times in turbidity and in the relative amount of solids carried in solution?

In parts of England streams have been known to continue flowing after eighteen months of local drought, so great is the volume of water which in humid climates is stored in the rocks above the drainage level, and so slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter and a “spring freshet” follows the melting of the winter’s snows.  A “June rise” is produced by the heavy rains of early summer.  Low water follows in July and August, and streams are again swollen to a moderate degree under the rains of autumn.

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 of pools, or go wholly dry, while at long intervals rains fill their dusty beds with sudden raging torrents.  Desert rivers therefore periodically shorten and lengthen their courses, withering back at times of drought for scores of miles, or even for a hundred miles from the point reached by their waters during seasons of rain.

The geological work of streams. The work of streams is of three kinds,—­transportation, erosion, and deposition.  Streams transport the waste of the land; they wear, or erode, their channels both on bed and banks; and they deposit portions of their load from time to time along their courses, finally laying it down in the sea.  Most of the work of streams is done at times of flood.