Today’s blogpost is brought to you by Xander Kalna.
As discussed in a previous post on logjams (read about them here!), logjams cause water to move sediment (clay, silt, sand, or gravel) that then settles upstream or downstream of the logjam. This process of sediment settling out of the water is called deposition. In the image from the previous post, you see how there are a few logs across the stream. These logs block the water, causing it to slow down in some areas and speed up in others. In these slow areas, the logjams cause deposition, which is how the gravel bar we were standing on was created. Bars are elevated areas in rivers and streams usually made up of sand or gravel. The water was moving quickly enough to move gravel and sand until the logjam disrupted the flow and slowed down the water. This caused the sand and gravel that was being carried by the water to be dropped, eventually creating a sand or gravel bar. See how water moves sediment here!
The logjams can also cause areas around them to erode. Erosion occurs when earth or materials are worn down and transported away by water, wind, or other natural forces. While the logjam slows down water in some areas resulting in deposition, in other areas the logjam forces the water to move faster, causing erosion. This is most easily seen by looking at the banks of a stream or river, especially if there are plants growing on the bank. If you see exposed roots, this means that areas that were previously covered with sediment are now exposed. This is evidence of stream caused erosion and can be seen in the photo posted this week. Here is a video demonstrating how logjams can cause deposition and erosion.
The erosion and deposition caused by logjams and other natural causes can change the shape of a stream or river. The changing elevation of the stream bed and the changing geometry of the stream banks can create various types of habitats and forms in streams such as pools, riffles, glides, and runs and cause meanders and oxbow lakes! Read all about these stream and landscape features next time!
Bill Nye Erosion: https://www.youtube.com/watch?v=ZTVuA1g0Uzw (play at 0.75 playback speed)
National Park Service Fluvial Landforms: https://www.nps.gov/subjects/geology/fluvial-landforms.htm
How logjams can form (flash flood): https://www.youtube.com/watch?v=mHJmfySkgMw
Erosion and deposition video: https://serc.carleton.edu/details/files/19075.html