SOIL EROSION AND SEDIMENT YIELD 

A.  SOIL EROSION

        1.  INTRODUCTION

                 Soil erosion is a worldwide problem that threatens an 
             important and non-renewable resource:  the farmland
             that is suitable for cultivation.   Soil is eroded by both
             wind and water, but the latter is more significant (see
             Figure 6.20 (a) and (b) on page 161 in your textbook).

             Besides removing a valuable resource, soil erosion
             leads to increased sediment input to nearby rivers. 

        2.  EROSION BY WATER  (Map)

             a.  Sheet erosion

                  Sheet erosion is accomplished by a thin layer (sheet)
                  of water moving over the land surface, as opposed to
                  a concentrated stream of water.   

             b.  Rill Erosion

                  Rill erosion occurs when the overland flow of water
                  (i.e., sheet flow) is concentrated into small channels
                  called rills.  

             c.  Gully Erosion

                  Gullies are enlarged rills created as the water from
                  several rills flows together, eroding a larger channel.

       3.  CONTROLLING FACTORS

            a.  Rainfall Intensity

                 The impact of raindrops on the land surface loosens
                 soil particles and makes them susceptible to erosion. 
                 All other things being equal, higher rainfall intensity
                 leads to greater erosion. 

 

            b.  Topography

                 There are two effects that topography has on erosion.
                 Obviously, steeper slopes favor greater erosion, but
                 the length of a slope also plays an important role.  In
                 general, the longer the slope, the greater the erosion,
                 because longer slopes favor higher water velocities. 

                 To counteract the effects of topography on erosion,
                 farmers are encouraged to employ soil conservation
                 measures, such as contour plowing and strip cropping
                 (see pages 187 to 188 in your textbook for additional
                 soil conservation measures).       Gays Mills, WI  
 

            c.  Land Use

                 The single condition that leads to maximum erosion is
                 exposed (bare) soil.  This is a regular occurrence in
                 agricultural areas, where farmers must plow the soil
                 to plant seeds.  It can also occur when vegetation is
                 removed for construction purposes (e.g., roads and
                 buildings).  The clear-cutting of forests is another way
                 is another way to expose soil to erosion.  Nature can
                 accomplish this same effect through forest fires.

B.  SEDIMENT YIELD

         1.  DEFINITION

               Sediment yield is the total sediment load that leaves a
               drainage basin (usually measured in tons/acre/year). 
             

               Coarse sediment is transported by streams as bed load,
               and fine sediment is transported as suspended load.

         2.  SOURCES

               a.  Soil erosion (usually carried as suspended load)

               b.  Material eroded from the stream channel (which is
                    transported as both suspended load and bed load).
 

         3.  CONTROLLING FACTORS

               a.  Soil Erosion Rate

               b.  Stream Capacity

               c.  Annual Precipitation

         4.  GRADED STREAMS

              A graded stream is one whose gradient is adjusted to
              balance sediment load with stream flow (discharge).

              Streams naturally evolve towards a graded condition. 
              When this condition is disturbed, streams adjust their
              gradients (slopes) accordingly:                       

              a.  If sediment load increases or discharge decreases, 
                   a stream will eventually increase its slope.  A stream
                   accomplishes this either by depositing sediment or
                   straightening (through cutoffs) its channel (or both).

              b.  If sediment load decreases or discharge increases, 
                   a stream will eventually decrease its slope.  A stream
                   accomplishes this either by eroding sediment or by 
                   meandering (or both).

         5.  EXAMPLES

              a.  Coon Valley, Wisconsin

                   Location of the nation's first soil conservation efforts

                   Effect on sediment yield to Mississippi River 

                   Subsequent stream adjustments

              b.  Chippewa River, Wisconsin

                   River adjustments