Rivers

Precipitation, such as rainwater or snow, is the source of the water flowing in rivers. Rainwater can either return to the oceans as run-off, it can be evaporated directly from the surface from which it falls, or it can be passed into the soil and mantle rock. Water can reappear in three ways: (l) by evaporation from Earth's surface; (2) by transpiration from vegetation; (3) by exudation out of the earth, thereby forming a stream. The third way, by exudation, is of primary importance to the formation of rivers.

When a heavy rain falls on ground that is steeply sloped or is already saturated with water, water run-off trickles down Earth's surface, rather than being absorbed. Initially, the water runs in an evenly distributed, paper-thin sheet, called surface run-off. After it travels a short distance, the water begins to run in parallel rills and, at the same time, gathers turbulence. As these rills pass over fine soil or silt, they begin to dig shallow channels, called runnels. This is the first stage of erosion.

These parallel rills do not last very long, perhaps only a few yards. Fairly soon, the rills unite with one another, until enough of them merge to form a stream. After a number of rills converge, the resulting stream is a significant, continuously flowing body of water, called a brook. The brook now flows through what is termed a valley. As a brook gains sufficient volume from groundwater supplies, the volume of water it carries becomes more constant. Once the volume of water carried reaches a certain level, the brook becomes a river.