chap11 - Learning Services Home

WaterYou probably have seen muddy water streamingdown a street after a heavy rain. You might evenhave taken off your shoes and waded through thewater. Water that flows over Earth’s surface is calledrunoff. Runoff is an important agent of erosion,especially if the water is moving fast. The morespeed water has, the more material it can carry withit. Water can flow over Earth’s surface in several differentways, as you will soon discover.Sheet Flow As raindrops land on Earth’s surface, they breakup clumps of soil and loosen small grains of sediment. If theseraindrops are falling on a sloped land surface, a thin sheet ofwater might begin to move downhill. You have observed somethingsimilar if you’ve ever washed a car and seen sheets of waterflowing over the hood, as shown in Figure 16. When water flowsdownhill as a thin sheet, it is called sheet flow. This thin sheet ofwater can carry loose sediment grains with it, causing erosion ofthe land. This erosion is called sheet erosion.Figure 16 Water flows overthe hood of a car as a thin sheet.Describe how this is similar tosheet flow on Earth’s surface.Can evidence of sheet erosion be seen in a farm field?If you’ve ever traveled through parts ofyour state where there are farms, youmight have seen bare, recently cultivatedfields. Perhaps the soil was prepared forplanting a crop of corn, oats, or soybeans.Do you think sheet erosion can visiblyaffect the soil in farm fields?Identifying the ProblemThe top layer of most soils is muchdarker than layers beneath because it containsmore organic matter. This layer isthe first to be removed from a slope bysheet flow. How does the photo show evidenceof sheet erosion?Solving the Problem1. Observe the photo and write a descriptionof it in your Science Journal.2. Infer why some areas of the field aredarker colored than others are. Wheredo you think the highest point(s) are inthis field?3. Make a generalization about the darkerareas of the field.SECTION 2 Erosion of Earth’s Surface 329(t)KS Studios, (b)Tess & David Young/Tom Stack & Assoc.