CONTROLLING EROSION

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On flat land, the puddles loiter around, grow bigger, and form temporary ponds. The soil structure is damaged somewhat, but there is no real erosion.

On slopes, however, the water flows downhill over the surface of the ground, evenly, like a sheet. It carries dirt particles dislodged from the tops of hills and deposits them below, creating what is known as sheet erosion .

Probably the worst thing that can happen at this point is that the flow of water becomes channelized, either because of the topography of the land or because of an accidental occurrence like a furrow, a tire rut, or a cow path running downhill. The water gathers speed and the particles of dirt act like sandpaper. The water soon cuts a small trench, or rill, which it may eventually widen and deepen into a gully.

As you can see, a gully is really the result of erosion — not the cause. Yet once the gully gets established, it brings about many severe problems. With each rainstorm, it gets deeper and deeper until it may even cut below the level of the groundwater, draining it and lowering the water table.

We now have the beginning of a vicious cycle. As you probably know, much deep-rooted vegetation depends more on groundwater than on surface water from the rain. As the water table is lowered — both from lack of rainwater penetration and from the draining action of gullies — vegetation over the watershed becomes more meager and scruffier. In some places fields of thick grasses are replaced entirely by sagebrush and chaparral, with scraggly growth and much exposed soil. Less groundwater leads to scruffy vegetation, which leads to more bare soil, which leads to more splatter, more soil clogging, less water penetration, more runoff, and a further deepening of the gully. As the gully deepens, it drains the water table still more, producing a further loss of vegetation, more exposed soil, more splatter, and so on for another downward cycle.

Meanwhile, as the gully gets deeper, the earth along its banks begins to cave in. Soon the gully sends out fingers that spread over the meadow, eating steadily away at the soil.

Within a few years, thousands of tons of topsoil are washed away, along with thousands of tons of subsoil. Where does it all go? Eventually, the gully probably drains into a stream. On a healthy watershed, a good cover of vegetation absorbs water, holds it like a sponge, and releases it gradually into the stream. The stream runs steadily and cleanly. But on an eroding watershed, the water runs off the surface with a heavy load of suspended silt, swoops through the gullies, and flushes out into the stream after every rainstorm. Instead of a clear, even-flowing stream, there is now an intermittent dry creek given over to flash floods. The silt kills whatever life there is in the stream and acts like sandpaper to cut into the stream bed and banks, causing further damage.

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