9.4: How Do Streams Erode?

How Do Streams Erode?

Rivers can erode laterally (sideways), vertically (downcutting) or back into the upland area (headward erosion), all of which result in the river becoming wider, deeper, and longer (Figure 9.8). Headward erosion Links to an external site. occurs when the river erodes in an upstream direction, lengthening the river valley. Downcutting Links to an external site., or vertical erosion, occurs as the river erodes downwards, deepening the river channel. Lateral erosion occurs along the edges (banks) of the river, widening the river valley.

Stream gradient Links to an external site.refers to the slope (rise over run) of the stream’s channel. It is the vertical drop of the stream over a horizontal distance. Gradient can be calculated by using the following equation: Gradient = (change in elevation) / distance. Stream gradients tend to be higher at a stream’s source, or the headwaters Links to an external site., and lower at the mouth Links to an external site..

Examine Figure 9.9. We want to determine the gradient from A to B. The elevation of the stream at A is 980’, and the elevation of the stream at B is 920’. Referencing the scale bar, the distance from A to B is about 2 miles. Using the equation above: Gradient = (980’-920’) / 2 miles, or 30 feet/mile (read 30 feet per mile).