Rivers – How Do They Work?

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What makes up a Drainage basin?

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river and all of it’s tributaries.ainage basins are separated by regions high elevation, know as divides.

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Rivers in the east flow....

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to the Atlantic Ocean.

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Rivers in the mid-continent region....

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eventually merge with the Mississippi River and flow to the Gulf of Mexico.

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Rivers in the west flow...

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to the Pacific Ocean.

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Continental Divide...

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is the drainage divide between the Mississippi
drainage basin and rivers that flow to the Pacific Ocean.

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drainage pattern is

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the pattern that rivers
make.

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Drainage patterns are controlled by

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the underlying bedrock structures.

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In regions of nearly horizontal strata

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the drainage that forms is dendritic (like
the veins of a leaf).

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Isolated conical mountains will result in

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radial drainage.

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In regions where the bedrock strata has been folded and eroded

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the presence of rock formations that are resistant to erosion form long ridges. Rivers and streams
will follow the long narrow valleys between the ridges resulting in trellis drainage.

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What causes erosion?

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Water flow.. Erosion in a river occurs through hydraulic action, abrasion
and dissolution. Hydraulic action removes rocks and sediment by high velocity currents.
Abrasion occurs like sandpaper action. Hard sedimentary grains of various sizes ranging
from sand to gravel, tumble and bounce along the river bottom. Each time these grains
impact the river bed, small pieces of rock will be broken off. This is the process of
abrasion that modifies the shape of rivers. Finally, some minerals in rocks are susceptible
to water dissolution and carried away.

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water flow in rivers will erode them at different rates

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Because rock formations have different hardness.

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undercutting occurs

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Where weak rock formations are eroded rapidly, Resistant rock formations will eventually break off,
crumble into smaller pieces and get carried away by
erosion.

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As river water flows,

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vertical eddies carrying rocks will act like a drill and grind deep cylindrical holes into the river bed. These holes are known as river bed potholes.

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Sediment Load.

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The amount of sediment moved by river water

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The sediment load occurs as
three types:

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1. Bed load; 2. Suspended load; and 3. Dissolved load.

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The bed load consists of

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particles
that slide, roll and bounce along the river bottom. Particles that bounce move by a process known as
saltation, as shown in the diagram above. The suspended load consists of smaller particles that remain
suspended in the water.

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bars are formed....

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Sediment deposited along the banks or within the river. typically made up of heavy particles that move slowly along the river, such as sand and gravel.

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Sediment bars are

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continually moving and changing as the
river water flows. During flood stage, the bars will migrate and accumulate, and during non-flood stage flow, the bars will be exposed,

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Cut-Bank is formed when

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Fast moving water will erodes the river bank

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a point bar...

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is caused by slow moving water that will deposit
sediment.

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As water flows around the bends in the river, the angular momentum causes the water on the outside of the bend to

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speed up, and therefore
erode the bank, while the water
on the inside of the bend will
slow down and deposit the
sediment to form the point bar.

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The gradient of a river will influence

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he water velocity, and ultimately influence the geomorphology (shape) of the river and it’s valley.

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as the river flows closer to the ocean

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it will cross the open Plain, have a very
low gradient, the water will slow down to form a meandering channel,

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Factors Affecting Erosion and Deposition
Channel shape and roughness
will influence

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water velocity in a river and therefore influence erosion and deposition

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when the channel has less surface area...

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the water velocity is higher. As well, when the channel is filled with debris and rough, the water velocity is lower.

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Discharge

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is the amount of water that flows through a river channel over a given time period.

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What influence erosion and deposition?

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The amount of water. Higher discharge
leads to greater erosion and lower discharge results in deposition.

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Discharge is reported

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as cubic feet per second (cfs) or cubic meters per second.

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To compute discharge (Q),

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The average water velocity is multiplied by the cross sectional area of the river channel.

Q = v x A Discharge (m3/s) Average velocity (m/s) Cross sectional area of stream (A = width* depth) (m2)

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a stream gauge is installed...

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To measure the average water velocity. will include a device that can measure water velocity across the river channel. The water level of the river is also monitored with a pipe that is inserted along the river bank. This makes it possible for the hydrogeologist to measure the river level very accurately, and safely during flood stage.

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Rivers can have an array of morphologies, however, most can be described as

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straight (A),
meandering (B) or
braided (C).
As before, the shape of the river is
greatly influenced by the amount of sediment that
the river can carry, and the ability to erode the
banks. These two factors are controlled by water
velocity, river gradient and discharge.

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Braided river systems form

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where there is too much bed load for river to carry, and are usually associated with rivers draining glaciers and/or tectonically active areas.

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The meandering river form

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where there is a dynamic balance between
erosion and deposition. Typical of rivers with low gradients, wide flood plains, and easily erodable floodplain and channel deposits, will develop meanders. The Mississippi River is an excellent example of a meandering river.

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Extreme meandering will form

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necks along the river. The narrow strips of ground will eventually erode connecting the river channel and cutting off the meander loop.

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Oxbow Lake

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two meander loops migrate into each other. The lake will form in the abandon channel.