2.1-Stream Erosion,Deposition and Valley Development
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Leticia Pollard-Torres
PHY 103-Lynda Folts
2-1 Assignment: Stream Erosion, Deposition, and Valley Development
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.
What will you find, including details of origin and relation to
landscape alteration
?
As can be seen under a microscope, the water sample contains microscopic sediment grains that
have been carried downstream by the stream. Due to natural processes such as lateral erosion and
downcutting, larger materials such as sand and rocks will be moved further down. This will
result in the modification of the terrain. This process of sediment transport is a gradual one, but
with enough time, it can result in significant changes to the terrain, such as a deepening of the
riverbed or the formation of new channels. Additionally, as the sediment is moved downstream,
it can cause the banks of the river to become steeper and more unstable, leading to further
erosion.
Next you let a sample of the water evaporate. What might you find in the
collection dish
once the water is gone? Why?
By analyzing the sample that has now evaporated, we can determine what remains behind.
Dissolution of the bedrock occurs because of water erosion. In some cases, bedrock consists of
soluble limestone, which is composed of calcium carbonate (CaCO3). In the presence of
dissolved CO2, this molecule reacts with water to produce carbonic acid, which is accompanied
by bicarbonate ions. In water, calcium bicarbonate ions dissolve and are carried away.
You also note that sand and gravel form the inside riverbed of the stream meanders.
Clarify where the
sand
came from and why it is found where it is.
It is possible that the sand came from an area of the river called headwaters where most of the
erosion takes place. The river carries the sediment downstream to the main trunk of the river with
tiny amounts of erosion and deposition. During the rainy season, the streams and floods of the
river wears down the sediment over time. This area of the river is called a cut bank. Once the
larger and heavier grains can no longer support their weight, they are released into the stream,
which causes the meander to gain energy and flow faster. This increased speed causes the
sediment to be suspended in the water, and when it slows down, the sediment is deposited in the
floodplain. Over time, this cycle is repeated, and the floodplain accumulates more sediment,
eventually leading to the formation of point bars or sandbars in the inside of the meanders.
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.
A process called saltation of sand occurs in this section of the river when the sand sinks to the
bottom and skips over pebbles and cobblestones. They are all part of the bed load of the river. As
a result of the force of the stream of water, they are dragged and rolled downstream. Streams are
deepest in the middle, where the water appears less turbulent, but in fact it is moving quite
rapidly under the surface, which is why boulders and even boulders can be transported
downstream. Due to the increase in the velocity and volume of water, the riverbanks are
subjected to greater erosion.
II. Stream Deposition:
You are at a second location, near the coast in Louisiana.
Explain what the ocean represents relative to the river and why and how
erosion
from the
river will not extend past the ocean level.
A river cannot erode indefinitely. There is always an end point to a process. The base level is the
lowest level at which a stream can erode. The sea level is considered the ultimate base level since
it is the lowest level at which stream erosion can cause land to be lowered. The stream's base
level is reached when it enters the ocean, a lake, or another river. After entering a large body of
water, the velocity of the stream decreases and the capacity for erosion is slowed down.
Detail how and why the river
channel breaks
into many near the coast.
As the river enters a larger body of water, such as an ocean or a lake, the forward motion slows
down, and sediments are deposited by a dying current. As the river seeks a faster route to its base
level, its main channel begins to divide into smaller ones called distributaries. Unlike tributaries,
distributaries carry water away from the main channel instead of feeding it. The map below
shows the Mississippi delta and its distributaries, the best example of a channel breaking into
smaller channels called sub-deltas forming a bird foot shape delta.
Explain why you are standing on a layer of very
fine soil
, yet just beneath you are layers
of coarse alluvium.
Flowing into the valley, the alluvial channels widen the floor and cause erosion of the riverbank.
As a result of floods in the area, clay and silt are deposited in the adjacent channels. It is likely
that you are standing on a layer of fine soil even though underneath there are layers of greater
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material. This is due to the creation of natural levees. Natural levees are built by years of
successive floods (Lutgens et al., 2021). On the valley floor, a thin layer of fine soil (silt and
clay) is left by the river as it expands across the floodplain. Natural levees have uneven
distributions, and there will be slopes in some areas.
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.
Upon entering the large body of water, the river deposits high concentrations of dissolved
sediments which extend beyond its mouth. During the slow movement of this sediment offshore,
fine sediments can be deposited there, such as clay and silt. For example, the Mississippi River
has been depositing sediment in the Gulf of Mexico since the last Ice Age, forming a subaqueous
delta that stretches for hundreds of miles offshore.
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?
The deltas and the levees which are formed over time transform the landscape, and as the deltas
extend downward, the forset beds, which are composed of coarse particles, continue to
accumulate sediments as is the case with the Mississippi delta on the coast of Louisiana.
Mississippi delta that has been built over the past 6000 years (Lutgens et al., 2021).
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?
In size, the Mississippi delta and the Nile River delta are equivalent, and they extend into the
larger body of water at the same distance from the coast. Mississippi carries a large amount of
sediment, similar to the Nile river. Due to the size of this river, it erodes the land that carries
them all the way to the Gulf of Mexico. The delta is formed by waves, high tides, and the flow of
water that forms the river. It is important to remember that when a stream enters a larger body of
water, it tends to slow down and deposits sediments that, over time, build up and take the shape
of either a "fan" or a "bird-feet.". Overall, the Nile and Mississippi deltas are remarkably similar
in size and shape.
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 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.
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.
On my arrival, I see a great valley with a river running in the middle. The landscape is alien and
beautiful at the same time. Hills separating parallel to the channel on either side are a clear
indication of water carving. As streams become more turbulent with increasing velocity, their
channels are eroded. Erosion of streams is a result of the action of turbulent streams that lift
sediment from the streambed. Erosion is a major factor in the transport of sediment from higher
elevations to lower elevations. It is caused by the flow of water, wind, and ice, which break down
rock, soil particles, and transport them downstream. The transformed landscape was a process
from thousands of years and the valley has deepened in a few spots and wide transforming the
area. After the stream has cut down the channel to the base level downward the erosion becomes
less dominant. Rivers cut away at one bank and then the other, resulting in a widening of the
valley. Later erosion widens the stream's meanders and flattens the valley floor covered with
alluvium. A floodplain is an area where sediments accumulate. Eventually, the floodplains will
widen the valley to the point of eroding it more and more. Seeing and comparing images from
previous visits to this area before my second visit was a fascinating experience.
References:
Lutgens, F. K., Tarbuck, E. J., & Tasa, D. G. (2021). Foundations of Earth Science (9th
ed.). Pearson Education (US).
https://mbsdirect.vitalsource.com/books/9780135851616
Guertin, L. (2007, January 1).
Lesson 4: The Nile River - An Overview
.
https://courseware.e-education.psu.edu/courses/earth105new/content/lesson04/09.html
Wendy Van Norden. (2012, July 26).
River Erosion, Transport and Deposition.mov
[Video]. YouTube.
https://www.youtube.com/watch?v=E6sWiPAu708
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