watersheds.org the world in your watershed search
homewhat's newabout ussite mapcontact us


Earth Geology Erosion Field and Slope Erosion

Field and Slope Erosion 

When a raindrop falls it is usually absorbed into the pore spaces of the soil. However, when these pore spaces become saturated (full) the raindrops will either stand on the surface as a puddle or flow downhill. As the water flows downhill it will carry with it soil particles and other material. This is the start of erosion. The heavier the rainfall, the more water will run off and the more force it will have to move material. The steeper the slope, the faster the runoff flow and the more force it will have to move material. The less vegetative (plant) cover and leaf material on the ground, the more open the soil will be to the flowing runoff water. Four things cause slope erosion: the amount and rate of rainfall, the steepness or gradient of the slope, the amount and nature of plant cover, and the type of soil and bedrock underneath. 

Humans cause most soil erosion when we strip the natural vegetation from the steeper slopes and don't put back a cover of healthy grass or other vegetation. That causes the worst erosion. When a slope must be cleared it is very important to replant some vegetative cover. It's important also to make the slope into terraces. This will give the runoff water a longer and slower route to the bottom of the hill. The slower the runoff flow, the less force the water will have to erode the hillside. 

Heavy rain on a fairly even slope creates "sheet runoff." The water flows downslope as an even sheet. Any dips in the slope will collect more runoff water, which will be able to erode more strongly. If the force of this water is not stopped it will produce a gully (Figure 1). The gully will collect more and more water and cause even more erosion. In time it will remove the soil down to solid bedrock. 

Gully development in soft, easily eroded
Northview shale in a steep roadcut along
Highway 60 east of Cedar Gap.

Gullies often come together downslope to form even larger gullies. Larger gullies can carry even larger volumes of water resulting in even greater erosion. These larger gullies may become "intermittent stream" valleys. An intermittent stream is one that flows only during and after heavy rainfall. 

The chief difference between gullies and small stream valleys is time. Gullies can and do develop over a short time, even in a single heavy rain storm. They can only go as far as solid bedrock. Then it will take time for erosion to dig deeper. If bedrock becomes exposed to the air through erosion it will begin slowly to begin weathering. The slow breaking down of the bedrock will let it erode slowly to become deeper. This will produce a valley over a long period of time.