Pre-Visit Activities : River Erosion : Background Sixth - Eighth Grade Online Curriculum : Watersheds

Key Points
Key Points will give you the main information you should know to teach the activity.

• Streams and rivers have the ability to erode and transport sediment. Erosion is the picking up of sediment by wind or water. The higher the velocity of water, the better its ability to transport sediment.
• Because streams and rivers can erode and transport sediment, sediment is often carried by these bodies of water all the way from the mountains to the ocean through watersheds.
• Reading aerial infrared photographs 
  • These photos were taken from an airplane with an infrared camera in 1999 in different parts of South Carolina. 
  • The red areas on the photographs are forested places or other places with lots of vegetation. 
  • The greenish-blue areas are areas of low vegetation, such as salt marshes. 
  • The white and gray area are developed areas.
  • Infrared photographs show how much sediment is suspended in the water. Bodies of water that are dark blue in color have almost no sediment suspended in them. Bodies of water that are almost white have a great deal of sediment in them. Depending  on how blue or white the water shows up on the infrared photograph is an indication of how much sediment is in the water.
  • The photographs of the mouths of the Edisto River, the Pee Dee River and the Santee Rivers show the outlets of three watersheds in South Carolina. By looking at the coloration of the water in these photographs, students can get an idea of how much sediment is being transported in each watershed. Because the photographs of the Pee Dee and Santee Rivers have more white coloration in them than the Edisto River, this gives the indication that these bodies of water transport more sediment than the Edisto River. This makes sense if students examine a map and see that the watersheds of the Pee Dee and Santee are much larger than the watershed of the Edisto River, and therefore are receiving more water with more sediment.
  • The photograph of the convergence of the Saluda and Broad Rivers shows where two rivers in Columbia, SC come together to form the Congaree River. The Saluda River is dark blue in coloration, indicating very little sediment, while the Broad River is much lighter in color, indicating that it is carrying a lot more sediment. The differences in sediment load can be explained by looking at a South Carolina map. This section of the Saluda River is downstream of the Lake Murray dam and reservoir. Because the reservoir slows down the velocity of the Saluda River so much, all of its sediment will drop out, and the Saluda River downstream of the reservoir is carrying very little sediment.
  • All aerial photographs are courtesy of the Land, Water and Conservation Division of the South Carolina Department of Natural Resources. For information on attaining aerial photographs, topographic maps and other materials contact the SCDNR Map & Information Center at (803) 734-9108.

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Detailed Information
Detailed Information gives more in-depth background to increase your own knowledge, in case you want to expand upon the activity or you are asked detailed questions by students.

(Click on photo to enlarge) South Carolina is a state covered by numerous streams and rivers. These streams perform two major functions important for natural vitality. They remove all the excess water runoff from rainfalls and other precipitation. In the process of removing rainfall, they also erode, transport and deposit sediment. By moving sediment, streams are the major shaper of landforms on approximately 75% of the Earth’s land area.

To understand how sediment gets into rivers, one needs a basic understanding of weathering and erosion. Weathering is the processes by which rock is broken down into smaller particles either by physical or chemical processes. Erosion is the process by which these smaller particles are then transported. Transport of sediment can be conducted by wind, gravity, ice or water. When it rains, water that does not infiltrate the ground becomes runoff. This water will first collect in shallow depressions in the land. As these depressions fill, the excess water will be pulled by gravity downward along the slope of the land. This water moves in a slow disorganized fashion and is known as overland flow. As the overland flow moves downhill, it will begin to build in volume and velocity and this increase in power gives it the ability to erode away soil particles. This erosion is the beginning of a stream channel. If water continues to move to this channel and to erode away the sediment in it, it will eventually be deep enough to where it is below the water table, the boundary beneath which all of the ground is saturated with water. Being below the water table, allows groundwater to flow into the stream and gives it a source of continual water flow even during dry periods. As these streams move downhill they will eventually join together to form larger streams and rivers.

(Click on photo to enlarge) The ability of a stream to erode and transport sediment is dependent on many factors. Primarily among these is the force and friction produced by the flow of the water. The more velocity and volume water has, the more friction it will produce and the more sediment it can erode and transport.

The velocity of a stream is influenced by the gradient of the slope it is moving down and the volume of water flowing through it. The steeper the gradient, the faster the water will flow. So a stream in the Blue Ridge Mountains will flow faster than a stream of the same size in the Coastal Plain. Increased water volume will also increase velocity. As more water enters a stream the amount of water moving through the channel increases and the rate it travels will speed up. This is known as discharge and is measured in cubic feet per second of water that passes a given point. Velocity is not uniform across a stream, but tends to be faster the deeper the water is, because a larger volume of water is moved in the deeper parts.

Topography and climate both affect the velocity of streams. Mountainous area with steep gradients will have faster streams than flatland areas with gentle gradients. Flatland streams can be faster though if the area receives more rainfall than the mountain area, or if the stream in the flatland has a larger drainage basin than the stream in the mountains, as a larger volume of water will be in that stream. Rainfall amounts fluctuate throughout the year, so stream volumes and velocities will vary with this fluctuation. Generally, though, the larger the drainage basin of the stream or river, the higher that stream’s volume, velocity and discharge.

Vegetation also has an effect on the volume of water that reaches a stream. Plants are dependent on water to survive. As it rains, plants will intercept a large amount of the rainfall before it can become groundwater or surface runoff. The water not intercepted by the plants is more likely to become groundwater than surface runoff in these well-vegetated areas, as the soil under plants tends to be very porous. Because plants are intercepting rainwater, streams in these areas are unlikely to have sudden volume increases. The water is more likely to seep in the streams through the groundwater than to arrive as surface runoff. Erosion is low in these areas too, because the root systems of the plants hold the soil and sediment in place and prevent water from carrying it away.

(Click on photo to enlarge) Dry areas with low vegetation have much more surface runoff and are more likely to have flash floods than well vegetated areas. With no plants to intercept the water, and with dry soils tending to have low porosity, almost all of the water will run off the surface, building in volume and velocity. Because a high quantity of water can rush through these areas in a short amount of time, and because there is little vegetation roots to hold soils in place, these low vegetation areas are much more prone to erosion. This was a problem in the Piedmont region of South Carolina during the mid-century. Farming practices cleared large areas of land and with no vegetation on them, a great deal of erosion took place. An average of 12 inches of soil was lost to erosion at this time.

The volume and velocity of a stream create friction on the beds they travel over. The volume, velocity and friction create the energy that allows a stream to erode and transport sediment. When the stream has the energy necessary to dislodge sediment, it has reached critical erosion velocity. Different sediment sizes will have different critical erosion velocities, and it gets kind of tricky. Clays, the smallest sediment with particle sizes under .002 mm, has a very low velocity to transport and can be suspended in water indefinitely. Because of the tiny particle size, though, clay exhibits a great deal of cohesion, (it sticks together well). For this reason, it takes a high critical erosion velocity to erode clay, about the same velocity it takes to erode gravel, which has particle sizes more than 500 times that of clay (2.0 to 100 mm). Gravel needs a lot of energy to erode, not because of cohesion, but because of its large size. Unlike the much smaller clay, gravel also needs a lot of energy to transport it, almost as much velocity as it needs to erode it. Sand, sediment with particle sizes of .06 to 2.0 mm, requires less velocity to erode than both clay and gravel. This is because it is smaller than gravel but the particles do not have the cohesive property of clay. Because of size differences, though, it requires more velocity than clay to transport and less than gravel.

All sediment sizes also have a settling velocity. This is the velocity of water flow at which the water no longer has the energy necessary to transport a specific sediment size. If the water flow dips below that sediment size’s settling velocity, that sediment size will settle out. For clay, this velocity is very low. The water would practically have to stop flowing for the clay particles to settle out. As particle size increases, the settling velocity increases with it. Rivers can transport sand, but without maintaining a certain velocity, sand will settle out and deposit. Gravel has the largest particle size. For this reason, the water would really have to be ripping to have the power to allow gravel to be transported. This is why it is rare for small rocks to be seen riding the current downstream, though at the right velocity it could happen.

(Click on photo to enlarge) Because the volume and velocity of water is dependent on rainfall amounts, sediment will go through periods of being carried and dropped. During high rainfall times, more and larger sediment can be carried because more water is flowing through the rivers. When the rainfall is low, the river velocity and volume will also lessen and some of the larger sediment will settle out. Then when the rainfall is up again, the volume and velocity will increase and the sediment is picked up again and transported a little farther downstream, until the next dry period comes. This pulsing of stream flow, the regular changes in water volume and velocity, is a normal characteristic of natural streams.

Most streams and rivers dip below their settling velocity for almost all sediments when they flow into a large body of standing water such as a lake or ocean. This causes the sediments to settle out and creates the alluvial deltas of places such as the Mississippi River and the Santee River in South Carolina. The deltas are the sediment deposits of the rivers, generally in a fan-shaped pattern. Not all rivers carry enough sediment to create deltas, but they all transport sediment and when this reaches the sea, it provides some of the sand that replenishes the beaches.

The building of dams on the rivers that has created the many reservoirs in South Carolina, have resulted in large bodies of water that intercept the sediment on the way to the ocean. The river current is slowed down when it flows into these reservoirs and much of its sediment load is lost. This creates a problem on the coast as little sand from the mountains can now reach the ocean to replenish the eroding beaches.

For information on the watersheds of South Carolina and the sediment they carry, see the "Background Information" in the "What is a watershed?" activity.