Water Erosion

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The power of water is amazing. Water is heavy and when it's moving quickly, it can easily knock you off your feet. This is why people are warned against trying to cross a raging stream during a heavy rain or flash flood. Entire cars, trucks, and homes can be washed away as easily as a leaf into a sewer drain.

Water impacts soil in a variety of ways. Most of them are good, like watering plants and filling lakes and rivers, but some are destructive.

Rills are formed when water first washes soil grains away through the same downhill grooves over and over again. From the air, they look like fingers extending down a slope from higher elevation.

When water erosion takes place in the same areas over time, rills get big enough to impact roads. When this happens, they become gullies.

Large volumes of high-velocity runoff in large rills wash away huge amounts of soil. This cuts deep gullies along depressions and drainage ditches. It can also be a problem when a road is cut along one side of a steep mountain. When there is enough rock and soil washed out from under the roadbed, it collapses. Although it can be fixed, ongoing erosion will probably create the same problem again.

When topsoil is washed away by swift surface water, it creates deep and wide gullies of two kinds: scour and headward erosion gullies.

In scour gullies, rill runoff water takes away soil particles through sluicing. Sluicing is the process used by Gold Rush miners of the mid-1800s. They built long, thin, wooden boxes through which running water poured. River current was often directed through sluices so that gold-laden gravel could be sieved faster than using the time-consuming panning method. The moving water washed lighter soil particles away from the heavier gold nuggets (with luck!). The eroded soil was usually the size of fine-to-medium sand. Scour gullies are often found in low, rolling hills.

In headward erosion the gully extends upstream as a result of waterfall undercutting and gravitational pull at the gully head. This is often seen in steeper areas. Gullies get bigger through sideways erosion, while undercutting water causes the gully's sides to fall in.

The problem with either of these types of gully erosion is that greater amounts of soil are lost in a shorter time period than with wind. Wide, steep gullies, often nearly 30 meters deep, severely limit the use of the land. Bigger gullies also disrupt farm work, causing access problems for vehicles and livestock. In already ravaged regions of the world this could mean the difference between planting crops or going hungry.

Runoff water flowing over the sides of gullies as well as that pouring down the gully head can cause lateral gullies, also known as branching gullies. Buildup of eroded rock and soil at the bottom of slopes or along fencelines is a common sign of branching erosion. The gully keeps getting bigger as additional water rushes down its slopes. When environmentalists are rating gully erosion, small unconnected gullies are not a big deal (minor erosion). Continuous gullies need to be watched (moderate erosion). However, expanding and deeply branching gullies are a sign of severe erosion.

Some of the things to watch for when checking erosion is the formation of nick points (rabbit holes, empty root holes, and livestock and/or vehicle ruts) on slopes or drainage areas. These seemingly unimportant grooves in the land can allow water erosion to begin. High water runoff in existing rills, sloped agricultural land, or hillsides can allow water erosion to intensify.

To head off erosion, farmers can reduce or divert some water from reaching the gully. This can be done by increasing regional water use for crops when the rain falls, which reduces runoff. Proper crop maintenance creates a natural way to hold soil against water erosion. Runoff water may also be controlled through creative containment in local holding ponds and regional dams.

The good side of erosion issues is that many solutions can be found by individuals at local levels. People don't have to wait for national mandates and expensive environmental testing equipment to see a problem and take steps to fix it.

Quiz

1. Mass wasting is a combination of

(a) weathering plus centrifugal force

(b) gravity plus magma

(c) weathering plus gravity

(d) erosion plus pyroclastic flow

2. Frost wedging is caused by

(a) Jack Frost

(b) the repeated freeze-thaw cycle of water in extreme climates

(c) mudflows

(d) lava backup

3. Wide destruction of native vegetation across the planet has increased

(a) soil exposure and erosion

(b) the number of scenic vacation spots

(c) oxygen levels in homes

(d) volcanic eruptions

4. The soil found in dry or semi-arid climates with little organic matter and little to no leaching of minerals is called

(a) pedocal

(b) pedalfer

(c) laterite

5. Wind erosion blows sand grains in a series of skipping movements called

(b) vegetation

(c) blasting

(d) saltation

6. When rock disintegrates and is removed from the surface of continents, it is called

(a) an eruption

(b) denudation

(c) shedding

(d) lithification

7. The process that Gold Rush miners of the mid-1800s used to separate rock samples is called

(a) dipping

(b) panning

(c) rolling

(d) sluicing

8. Chemical weathering happens in all but one of the following ways

(a) acid action

(b) hydrolysis

(c) intoxication

(d) oxidation

9. When plant roots grow into rock fissures to reach collected soil and moisture, it is known as

(a) recreational weathering

(b) biological weathering

(c) zoological weathering

(d) desertification

10. To slow or stop erosion, farmers use all of the following methods except

(a) water diversion and control

(b) proper crop maintenance and rotation

(c) overgrazing and frequent tilling

(d) local holding ponds and dams r i i

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