Stream Systems

Rain falls on the earth as a thin, relatively uniform sheet of water. Upon reaching the ground, some of this water will infiltrate into the soil or be taken up by plants, but much of it will accumulate on the surface. This thin layer of water will flow downhill in response to gravity, creating what is called "sheetwash". Because sheetwash is very thin, and very diffuse, the flowing water has very little energy and so does not modify the surface over which it flows.

As sheetwash flows downhill, it begins to concentrate. As it concentrates, it gains energy, and so the ability to erode the landscape. This flow incises small channels in the surface called "rills". Rills are normally no more than about an inch deep, and only contain water during a rainstorm. The water flowing down rills may eventually enter a gully, a larger type of channel, and from there into a small creek, to a big creek, to a small river, to a big river and eventually return to the ocean.

The entire network of channels, of all sizes, that drains an area is called a stream system. Look at your Cumberland map. It is covered with channels of various sizes. In order to see one of these systems more clearly, take a blue colored pencil and trace over Rush Run that is joined by another tributary and runs off the north of the map in the center. Be sure to include all tributary streams south of this junction, both perennial and intermittent. When you think you are done, check your work to see if you have found all of the drainage channels. This is a very important activity, because it will help you more clearly develop the EYE to SEE the topography shown by the contour lines. You will need this acquired skill in subsequent activities throughout the rest of the lessons and whenever you use topographic maps. TAKE THE TIME AND EFFORT TO DO THIS, DON'T JUST LOOK AT THE ANSWER.

Drainage Patterns

Stream systems will possess one of a number of different patterns depending on the underlying geology of the area. Your Cumberland map shows a mixture of two of these. A very common stream pattern is called "dendritic". This word means "tree-like", referring to the way that streams and their tributaries join together to appear like the branches on a tree. The stream you just traced has a dendritic pattern. Dendritic patterns usually form in areas where topography and the underlying geology exert relatively little influence on drainage.

Another important type of drainage pattern is "trellis". Trellis systems are created where the topography, usually in the form of ridges, force streams to flow parallel to each other, one on either side of a ridge. This is a common stream pattern here in the Appalachians, where the topography is characterized by alternating ridges and valleys. In some cases, large streams often have the energy to cut straight through these ridges. Look at the stream systems on this map. The main channel of Wills Creek cuts right through several ridges. Some tributaries are forced by the topography to flow parallel to smaller ridges, joining the main channel at nearly 90 degree intersections in a trellis pattern.

Watersheds and drainage divides

Stream systems are fed by the sheetwash flowing down the topography. The total area that feeds water to a stream is called the "watershed" or drainage basin. Every point on your Cumberland map is part of a watershed, since all of the rain falling on the land must drain somewhere. Watersheds can be any size, depending on your point of reference. If your point is on a huge river, the watershed will also be huge. If your point is on a small creek, your watershed will also be small. For examples, the watershed for the Mississippi river is about half of the United States. The watershed for the Coosa is about half of Alabama. Note how Rush Run on the Cumberland map has little watersheds contained within larger watersheds and how these would in turn be part of larger watersheds.

Watersheds are separated from each other by "drainage divides". Watersheds are shaped roughly like bowls. The water flows down the sides of the bowl and collects in the low point, that is, the stream system. Drainage divides are the rims of the watershed. In other words, drainage divides are the topographic highs, such as ridges and mountains, that force water to drain in separate directions into different watersheds.

Use a red pencil to draw the drainage divide around Rush Run on your Cumberland map. This line will follow the highest points of the local topography. As you draw these lines, periodically check yourself by making sure that the water on one side of the line will flow into Rush Run, that on the other side into a different system. Once you have drawn the drainage divides, the area contained within is the watershed. DO THIS ACTIVITY. You need to develop the skills in seeing these ridgelines. Then, check your work.

All of the water falling within this area will flow through the stream channel at the north edge of the map. Estimate, using the grid lines drawn on your map (round to square kilometers), the area of this watershed in square meters. If one inch of rain (.0254 meters) falls on this watershed, how much water (cubic meters) from this stream will pass through stream channel at the point where the stream runs off of this quadrangle? Would this be a good place to build a house? Answer

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