Running Water Erodes the Land

Water from rain or melted snow may soak into the ground, evaporate, or flow over the surface as runoff. Erosion begins when runoff carries small particles of soil downhill. Runoff gradually forms small channels in the soil. These channels join together to form larger channels called gullies. As water flows down a hillside through gullies, the water can erode large quantities of soil. Gullies, in turn, flow together to form streams. Unlike gullies, streams flow year-round, except in dry areas or during droughts.

Streams continue the process of erosion by transporting sediment, as shown in Figure 13. Most of the sediment in streams is carried in suspension, in which tiny sediment grains move along with the water. Larger particles of sediment slide, roll, or bounce along the bottom of a stream. The process of particles bouncing along a stream bottom is called saltation. During floods, a fast-moving river can move boulders the size of a small car. A large amount of sediment is also carried in solution. Faster streams carry more sediment, which increases abrasion.