Old Channel of the Black Warrior River Earthcache #2
Rivers are dynamic features that periodically change their courses by abandoning old channels and creating new ones. Where you are standing, at GZ of this EarthCache, is an old abandoned channel of the Black Warrior River. Today, the channel of the Black Warrior is about a half mile to the south. This is an Oxbow Lake that was cut off from the river.
The path a river takes from its headwaters to the ocean is affected by a number of factors. They range from precipitation rates, bedrock geology, topography, vegetation, tectonic movement, just to name a few. Over time these factors will change and the flow of the river changes with them. Sometimes the entire course of the river will change. The frequency and magnitude of these changes are of interest to planners and builders because they need to know the likelihood of a nearby river changing its course again, threatening cities and the type of soils they will be building on.
Tuscaloosa is located at of the fall line of the Black Warrior River. That is, above Tuscaloosa, the Black Warrior River flows through steep sided hills with exposed bedrock on the channel flanks that limit the lateral movement of the river channel. Below Tuscaloosa, the Black Warrior River flows through a flat broad plain with limited exposed rock formations allowing the river to meander or change its channel laterally. Generally speaking, rivers erode their banks on the outside of a river bend and deposit sediment on the inside of the curve of the river bend. Over time, this eroding of the outside of the river bends and depositing of material on the inside of the curves causes a river to meander or constantly change its channel course. Periodically, whole sections of the river channel are cut off and abandoned. This is what happened to create is swamp looking area here that was once the main channel of the Black Warrior River long ago.
A river channel is the depression through which the water of a river flows during non-flood conditions. When the flow of a river changes to a new channel the old channel is considered relict. Over time these abandoned channels are filled in by flooding and dead plant material. Even long after the old channels have been abandoned, they can be identified by the soils that formed in them, the change in vegetation that grows in them, and the different infiltration rates through them.
Thus, one of the easiest ways to identify old abandoned channels is on aerial photographs. On aerial photos, the vegetation and water content of the old channels stand out. Below is the aerial photo of the area. The red X marks your location on the Aerial Photo and the boundary of the Old Oxbow Lake is also marked in red as is the flow of the present day Black Warrior River with red arrows.
You can determine the relative age of a relic channel by looking at what remains of that channel. If the relic channel is still deep, too deep for trees to grow, then the relic channel is relatively young. If the relic channel has had deposition occurring for a long period of time and has become shallow enough for trees like the Bald Cypress and others to grow in the relic channel, then the channel is relatively old. If the relic channel has become more of a shallow swamp or marsh then the old channel is very old.
To claim credit for a “Find” of this EarthCache, send me an email with the answers for the following questions. At the top of the email please write the title: Old Channel / Oxbow Lake
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Estimate how wide is the abandoned relic channel here.
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Do you think that there is water in the old channel year round. What evidence is there for your answer?
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What type of trees are growing in abundance in the channel?
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See the reading above. Relatively, how old do you think this old channel is? Give evidence for your answer.
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Optional: Take a pic of yourself at GZ and post the pic on the cache page.
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