Stream Erosion

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Geography

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Dec 6, 2023

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I. Stream Erosion: Imagine you are on location in Minnesota. You note that the water in the stream you are analyzing is clear. You take a sample of the water and look at it with a microscope. A. What will you find, including details of origin and relation to landscape alteration ? If I were take a sample of water from a stream at location in Minnesota and put it under a microscope, I am going to find a few different things. Sand will be obviously present in the water under the scope, as well as a number of different sediments, grains of rock, that came from the upstream. The presence of these materials coincides with the change in landscape due to headward erosion and downcutting. This also changes the movement in the stream we are looking at. B. Next you let a sample of the water evaporate. What might you find in the collection dish once the water is gone? Why? After I let the sample of the water evaporate from my collection dish, I will find different minerals that were not dissolvable when in the water or during evaporation of the water. Water is capable of holding dissolved minerals and other materials but will eventually reach a point where it cannot hold anymore. The minerals and materials that the water cannot hold will become solids (why it’s not dissolvable when water is at its maximum capacity of holding) and these solids will fall to the bottom of the dish. C. You also note that sand and gravel forms the inside riverbed of the stream meanders. Clarify where the sand came from and why it is found where it is.

The sand that forms the inside riverbed of the stream meanders came from the tiny particles upstream that made their way down during movement. The weight of the sand (which sand is the heaviest sediment after rocks and gravel) during this movement causes it to sink down and no longer be part of the moving stream. The particles mentioned also come from the process of erosion in this area. D. Next you find that the deepest portion of the stream is near the middle. Cobblestones occupy the bed there. Explain how that is possible and why the stream is deepest in the center. I found that the deepest portion of the stream is near the middle and that cobblestones occupy the bed there. This is possible because the weight of the cobblestones not being able to be carried all the way down the stream. The middle of the stream also has the fastest water movements, or highest velocity, giving it the ability to move the cobblestones until right after this point. Also, the deepest part of the stream is where the highest velocity occurs; depth and width will increase downstream because discharge will always increase downstream and cross section shape will change making the stream deeper and wider. II. Stream Deposition: You are at a second location, near the coast in Louisiana. A. Explain what the ocean represents relative to the river and why and how erosion from the river will not extend past the ocean level. The second location, near the coast in Louisiana, we are at the ocean and what the ocean represents relative to the river is that the ocean is where the river dumps into. Streams and rivers will eventually lead to a larger river, lake, or ocean in this case. When the river

goes through all the processes during movement, it eventually loses much of the energy used to move, so when it reaches the ocean, erosion will not occur past this point. What we see is large amounts of sediments that are then, depending on the strength of the current, deposited from the mouth of the river into the ocean. B. Detail how and why the river channel breaks into many near the coast. At this point, the river channel breaks into many other channels near the coast. As mentioned above, the large sediments that are deposited from the mouth of the river into the ocean will cause deltas to form and create the different channels. Deltas form because the river is moving slowly at the mouth, where the river is the shallowest, and the sediments then settle and build up. C. Explain why you are standing on a layer of very fine soil , yet just beneath you are layers of coarse alluvium. I am standing in the river, on a layer of very fine soil, and beneath me are layers of coarse alluvium. The layers of coarse alluvium are there, under very fine soil, because of two different processes: Transgression and Regression. Transgression is basically the landward migration of the shoreline, when the rate of sea level rise landward exceeds the rate number of sediments that are deposited, and causes overflow of water, flooding dry lands. Regression is generally the opposite of this, it’s the oceanward migration of the shoreline, causing draining of the mentioned flooded lands. It would be safe to assume

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that the water rising from transgression caused alluvium deposits and then the regression causes the area to be covered by soil again. D. Discuss why beds of very fine sediment are found far offshore and where these will be if the current landform extends farther out to sea over time. We will find beds of very fine sediment far offshore because deeper waters have less movement, or lower energy like we talked about before, so they only move much smaller sediments, like these very fine sediments, thus why we find them. Now if these sediments were found if the current landform extends farther out to sea over time, they would be found where they came from, which is in the deeper areas of the water where the energy is the lowest. E. Integrate all of the concepts above to fully explain the surrounding landform , including the surface and extending downward into the sedimentary layers. How has the landform changed over time and why? With all the concepts we have covered being taken into account, the surrounding landform can be explained by using what we know of the river, ocean, and stream processes. We know that the river has met the ocean at the mouth of the river, where large sediments are deposited and collected, creating deltas. Then these deltas create the newly formed channels going in different directions, not unlike roots of a tree visually. Now we see the effects of transgression and regression, where the rising sea levels deposit sediments and the water receding will show us the fine soil particles left behind. The particles will leave areas of the river that have high energy, like mentioned before, and

stay in areas of low energy. We will see the changes in landforms from the results of erosion, which is seen over short to long periods of time. F. Finally, compare the present landform of this river to a similar but vastly different landform associated with the Nile River in Egypt. How are the landforms similar? Different? Why? We are comparing the similarities and differences of the landform of this river in Louisiana to the landform associated with the Nile River in Egypt. It would be safe to assume that the Nile River in Egypt is vastly larger than this river in Louisiana, since the Nile River is longest river in the world. The river I have been investigating has sand, cobblestones, likely mud and clay as well. The Nile River has sediments that include gravel, sand, and mud like my river, but it also has heavy metals from pollution. The Nile River will also have more deltas due to it’s size and amount of sediments being moved, as well as experiencing more erosion. Both rivers experience the natural processes of transgression and regression. III. Valley Development: You have control of a time machine. Your journey into the far future begins in a deep, rocky canyon, inhabited by a small stream in the center. Your journey ends in the same location, but the landscape has changed to one that is flat in every direction for as far as you can see. The stream is present, lies generally above the surrounding landscape, and is separated by hills that parallel the channel on either side. You note that the stream is much larger than the one you left in the past. A. Fully detail and discuss the evolution of the landscape from a high gradient one to that of a low gradient, well-developed, and wide floodplain over time. Be sure to list and detail all relative landforms and stream characteristics

(meanders, oxbow lakes, Yazoo streams, natural levees, etc.) that result as the landscape alters with each phase over time. We are going to describe the processes of this landscape through it’s evolution from a high gradient one to a low gradient over time. We start with a new system formation for drainage, where the river is going through a process of getting deeper and wider. Then the river spills into the ocean creating deltas during this stage. Yazoo streams, which are higher gradient streams, are present at this time and run parallel to lower gradient rivers and then connect to the developed river. Erosion will cause the evolution of this stream to become more fully developed. Oxbow lake can be formed, which are the remains of the bend in the river. Overtime, the landscape is altered by these processes and more, which is visibly obvious in comparison. Resources: LUTGENS, F. K. (2021). Foundations of Earth Science . PRENTICE HALL. U.S. Department of the Interior. (n.d.-b). Fluvial features-meandering stream (U.S. National Park Service) . National Parks Service. https://www.nps.gov/articles/meandering- stream.htm U.S. Department of the Interior. (n.d.-d). River systems and fluvial landforms . National Parks Service. https://www.nps.gov/subjects/geology/fluvial-landforms.htm#:~:text=Fluvial %20systems%20are%20dominated%20by,are%20dependent%20on%20fluvial %20systems. YouTube. (2007, January 31). Brief soil intro . YouTube. https://www.youtube.com/watch? v=XNJFksa9sJc YouTube. (2011, February 6). How to read a geologic map (3/3) . YouTube. https://www.youtube.com/watch?v=5EZbHCxv0NY

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