Managing Semi-Arid Watersheds: Watershed Basics - What is Erosion?

Erosion is the removal and transport of soil by wind, water or mechanical means. This can happen naturally in three ways: surface erosion, gully erosion, and soil mass movement. Surface erosion is caused primarily by the action of raindrops and wind across the soil surface. Raindrops are the primary agents of erosion. They have high energy and initially start the erosion process by splashing and loosening soil particles. Surface erosion often occurs in range land and agricultural production areas where there is little or no vegetation.

Gully erosion occurs in well defined channels. Gullies can be as shallow as a few inches or as deep as the Grand Canyon. Relatively clean water has a high capacity to transport sediments. A good example is the dam at Page, Arizona, which eliminates most sediment coming down the Colorado River. Therefore, "clean" water below the dam is "hungry" and taking away the beaches and sand bars.

Soil mass movement occurs when large masses of soil move at once. This can happen rapidly, such as a landslide, or more slowly over time.

In addition to natural soil processes, human activities, such as building construction, road construction, timber harvest, grazing, and agriculture can accelerate these processes. Surface runoff occurs when the rate of rainfall or snowmelt exceeds the ability of the soil to absorb water. Sites that have been scraped bare and are sloped can have dramatic soil loss in a single rainfall event. Consider this: it takes many thousands of years for even an inch of soil to be "made" and human activities can eliminate it in a single day. That's a high price to pay for the loss of a valuable natural resource.