Lab 8: Mapping - Landforms on Contour Maps - Creeks and Valleys

 

And last, but not least of the common landforms on contour maps, we have the creek, and its associated valley. The form of the contours crossing a creek is a bit like an inverse nose, except much sharper, usually making a "V." The sharp point of turning of a contour, at the "V," is the point where the contour crosses the creek or river.

 

This map is cropped from a topographic map of a creek in southeast Texas:

 

 

There is a main creek flowing west to east (left to right) across the top, and there are several tributary creeks flowing into it from the south. Contour lines in the main creek and the tributary at left are so closely spaced (showing steep creek banks) that it is difficult to see the V made by each contour as it crosses the creek channel. There is a smaller tributary in the lower right area where it is easier to see the V shape. Find the exact bottom center of the map, just to the right of the point where the PIPELINE crosses the bottom edge of the map. You should see the dashed line marking the path of the tributary extending from the bottom center edge of the map, from the point just right of the pipeline, curving up and to the right, and then turning into the main creek. Along this tributary you see the V pattern of the contours more clearly. You can also see a small drainage in the bottom right corner, starting along the bottom edge where the 250 foot contour is labeled. The contour lines make a V pattern here, but there is no actual creek labeled -- this is the usual case for such small drainages -- if the paths of all little tributaries were shown, the map would become too cluttered.

 

Let's take a look at a computer model of this topographic surface. The data for this area, and most areas of the United States, can be downloaded from the United States Geological Survey (USGS) website, or state websites. The data comes in the form of a grid of xyz locations, called a digital elevation model (DEM). Software such as Blender can render nice 3D views of the z value surface (topographic surface):

 

 

The plane shown is a horizontal plane -- a plane of equal elevation. If we raise this plane up, it will intersect the topographic surface, first in the lower areas of the creek and tributaries. As we raise it up more, we'll see the intersection climb up the sides of the creek and tributaries, marking successive contour lines -- remember, that's what contour lines are -- lines of equal something, in this case, elevation. In the next few images, the horizontal plane will be raised a bit each time:

 

 

 

 

 

 

Do you see the V made by the contour lines as they cross the creeks and tributaries on the perspective images above?

 

The following animation shows the incremental raising of the horizontal plane through all elevations in this map area:

 

 

And, in map view (bird's eye view):

 

 

If we mark contour lines on the perspective view where the horizontal plane intersects the land surface, we get this "map":

 

 

To see these contours appear sequentially, a final animation: