Why do rivers meander? As is the case with a lot of natural phenomena, water is looking for the path of least resistance to the ocean. Meandering rivers are often found in low-lying, flat landscapes with low discharge. When the river has a bend in the channel because it has tried to find the path of least resistance, it begins to erode on one side of the channel because water is flowing faster. Think about going down a water slide, as you go around a curve you will go up the side where water is moving faster. Where the water is moving fastest is called the Thalweg. The thalweg is technically a line connecting the lowest points along the entire length of a river bed defining its deepest channel (Wikipedia). On the other side of the curve the water isn't flowing as fast, so in a river, we tend to get deposition. This bend is where you will find sand bars. With erosion on one side of a bend and deposition on the other, it is safe to say that river channels move laterally.
As a river continues to erode on one side of the channel, the meander begins to pinch off. At some point, the river decides that the path of least resistance is now not through the loop, but in a more straight path. When this occurs an oxbow lake is formed. Remnants of oxbow lakes that have filled or dried up can be seen as meander scars. A meander scar is identified typically by its crescentic morphology. Below is an image of the Mississippi River that shows the features I have described today.
I, of course, want to note that this is a simplified description of a fluvial system. In the future, I think that I will dig deeper into the geology of the sand bars describing what sedimentary features are formed and the pattern of sedimentary features. It is these sedimentary structures and patterns that allow geologists to identify fluvial systems in the rock record.
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| Image of the Mississippi River (wide) and another river joining together. The red dotted line represents approximate location of the Thalweg. |
