River Processes
A river is a body of water that flows downslope in
the general direction of the nearest ocean and in the process, transports
particles of sediment and dissolved substances.
5 Basic factors control a stream's behavior:
1. Width and depth of the channel.
2. Channel gradient
3. Velocity of the water
4. The discharge- quantity of water passing a point
on a stream bank during a given interval of time
5. Sediment load
When one of these factors changes one or more of the other
factors will also change. One example of this is if the discharge is increased,
the velocity also typically increases. This can cause the stream to erode
and enlarge its channel. This erosion continues until the increased discharge
can be accommodated in a larger channel and by faster flow.
Every
river varies in size and shape. Some rivers meander and create loop-like
bends in the river channel. One example is the Mississippi River. In meandering
rivers, the velocity of flowing water is not uniform. Velocity is lowest
along the bed and walls of the channel, because the water encounters more
resistance to the flow. Along a straight segment the maximum velocity is
found near the surface in midchannel. Wherever the water rounds a bend,
the maximum velocity swings toward the outside of the channel.
The continuous shift of a meander is accomplished by
erosion on the outer banks of the loops. Along the inside of the loop,
where water is shallowest and velocity is lowest, coarse sediment accumulates.
Collapse of the river bank occurs more frequently along the outside of
the meander bend where the highest velocity occurs, causing erosion and
undercutting.
A stream's sediment load consists of two parts. The
first part is the coarse particulate material that moves along the river
bed. The second is the fine particulate material that is suspended in the
water. In addition, streams carry dissolved substances that are a product
of chemical decomposition of exposed rock and floating organic debris.
The size of the particles a stream can transport is related mainly to velocity.
Sediments normally decrease in coarseness downstream.
Most rivers ultimately flow into the sea or large lakes.
A river flowing into standing water builds a delta or body of sediments
deposited at the mouth of a river. Deltas contain small channels that carry
water away from the main river and distributes it over the surface of the
delta. Sediments deposited at the end of a distributary block water flow
which shifts deposition.
The rate of sedimentation, tectonic subsidence and
compaction all effect delta preservation. The Mississippi River delta is
sinking. This is due to sediment build up behind upstream dams and the
construction of levees which in reduce the delta's supply. Also, there
is an increase in oil and gas extraction from beneath the delta. This causes
subsidence and compaction to occur.
This
is an example of a home in Kenner, LA. The land is subsiding and sinking
away. Homeowners in this area are constantly fighting against the work
of nature to stop their land from disappearing. They bring in piles of
dirt to try and build up their property. Doing this only makes the problem
worst. It cause the soil to compact and subside to occur.
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