Today’s guest post is from Erin Kelly, a GIS Library Technician in the Geography and Map Division. A native of the Baltimore, Maryland area, Erin came to the Library of Congress as a recent graduate of Towson University.
Do you ever look out of an airplane window and admire the natural beauty that is below you? If so, you have most definitely seen a meandering river. For me, it doesn’t matter whether I’m looking at a map, a picture, or even out an airplane window, meandering rivers always grab my attention. A meander is formed when the flow of water slowly erodes the outer river bank and widens the other side. This change in the flow of the water makes the inner side of the river slowly fill with sediment and in turn, makes a nearly straight river become “wavy.”
While working on the Dynamic Indexing Project, I became amazed with all the maps that had these meandering rivers. Naturally, thinking of one of the longest rivers in the U.S., I decided to see just how much the Mississippi River meanders and how it has changed course over the years. I came across the “Lower Mississippi River Early Stream Channels,” a series of maps created by the U.S. Army’s Mississippi River Commission in 1938. These maps track the river as it changes over the course of 167 years.
Although the Mississippi River is much longer, the map series only categorizes the river’s north-to-south trajectory south of 37°N latitude. The different colors represent the path of the river on specific dates, as compiled from previous surveys. Between 1765 and 1932, the Mississippi had meandered from its original path, in some places cutting off part of the river. This section of the river that was cut off forms what is known as an “oxbow lake.” Just like a human, if a river sees a short cut, it will take it. The outside banks of two meanders come together and force the river to change its course of direction. The “left over” water that does not flow with the river change eventually becomes a lake. “Old River Lake Or Lake Mary” is a prominent example of an oxbow lake seen in the map below.
It’s exciting to actually see how topography in general changes over time. While working on maps in the Dynamic Indexing Project, it’s quite easy to think that something went wrong during the process, but really a lake or river from the 1800’s has shrunk or completely been dried up. Next time you look at a topographic map, look at the date and compare it to a recently made map. You may just find a feature that no longer exists today.