GEOL-1010 Lecture Notes - Lecture 15: Braided River, Point Bar, Catawba River

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5 Feb 2018
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Lecture 15: Rivers and Streams
Major agent of weathering and erosion
Why do we care?
Lead to a lot of sediment being transported
Major source of water for recreation, drinking etc.
Groundwater supply is much larger than surface water supply
When there is too much water present it results in flooding
o This is by far the most common and most dangerous form of natural disasters
Forming a River
Drainage Basin- the total area where you have runoff water filtering down into a
particular stream
o In diagram it starts on the top of the hill and goes over to the top of the next hill.
All of the land in between is one drainage basin
o It does not include the other side of the hills
Channels- the pathway that has been carved out by the movement of water. It is where
most of the water stays contained
o Water will always follow the path of least resistance
V-shaped valley-
o Glaciers have more of a u- shaped where the walls are steep and the bottom is
broad and rounded
Tributaries- very small streams that merge together and eventually flow together into one
major channel. Where the river originates
Different patterns
o 1. Branching/ dendrite pattern:
looks like branches/ veins, all coming together at very small angles, acute
angles in between them. This is very common
o 2. Radial Pattern:
you have an area on the map where you don’t see any streams at all but as
you move away you see a lot of streams. Some point of very high
elevation in the open spot
Good for helping you see where the highest point is/ elevation changes
o 3. Rectangular Pattern:
all of the tributaries join up at right angles
It gives a block or rectangular pattern
Tells you about below ground: happens when you have a lot of fractures
or cracks in the bedrock below the surface
Levee- an area right on the banks where the sediment is built up a little bit higher
compared to all of the surrounding area.
o Built up naturally during flood events
o When it floods water will flow out of the channel but also deposit sediment
o Overtime it builds up higher and higher
o (can also be manmade)
Floodplain- a flat broad plain and whenever the water rises, it’s the area that is going to
flood
o In a lot of areas, we tend to do a lot of development on flood plains
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o This is bad because we have to do a lot of clean up
o A lot of people seem caught off guard by the flooding- it’s literally called a flood
plain
Channels
Braided Streams- a bunch of smaller channels that are all mixed together, there isn’t one
main pathway for the water to flow through
o Usually have to have certain conditions:
Causes of Braided Streams
o Variable discharge
Monsoonal climate where it’s really dry than really wet. You have to have
new channels to handle the influx of water
o Large sediment supply- high amount of sediment that is trying to flown down
through the stream
o Easily erodible banks- if banks are really soft and easy to spread around,
eventually it will split. Every channel will spread and split until you have this
braided stream
Steep elevation gradients
o Looking at the change in elevation over distance
o Water is flowing downhill very rapidly giving it a high kinetic energy. Water just
slices things in its path and plows through any obstacle giving it a very straight
path
In areas of lower gradient channels start meandering
o Not just flowing from point a to point b. instead had a windy pathway
o Meanders- the big curve pattern
o Very indirect path
o Very low gradient so the water is flowing much slower so it doesn’t have enough
momentum to cut through as many obstacles. If it ran into something it would just
go around that obstacle instead
Meanders
o Cut bank- being cut by water (the outside water on the picture on the right cuts the
edge because its flowing faster, the right side of the river in that pic)
o Point bar- water flowing slowly enough to where it deposits sediment (the inside
of the river on the right pic)
Think of race car: the water that is going the longer distance has to move
faster to keep up
On the picture on the right side of the slide: cut bank switches from the
right side to the left side
o Meanders migrate over time
o Oxbow lakes (aka oxbows)
The part that has been cut off from meanders over time
Has a distinct horseshoe shape because it used to be part of the loop of a
meander
o Meander vs. Man
Meandering causes problems for people
They move, need to get water to stay where you want it to
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