As you can see from the above diagram without the reservoir removing the threat of flooding an example of a nearby settlement would be at great risk. The diversion of the floodwater helps prevent the flooding from occurring. This type of defence is most present towards the beginning of the Mississippi as this can only deal with small-scale base level increases. But the use of reservoirs isn’t terribly cost effective as the reservoirs face major risk of silting up. Using the Hjulstrom curve to back up this theory we can see how this will occur.
This theory makes sense to how silting up could occur. The reservoirs are often the results of damming, mentioned later. The trouble with the reservoirs is they reduce the velocity of the rivers flow. This means that, according to Hjulstroms curve there wont even be enough velocity to transport the finest of sediment particles. As the velocity of the river decreases the deposition rate increases. This means there is a great chance of reservoirs silting up. Referring back to the diagram above we can see how the velocity would change causing the deposition.
Lower down the river where the discharge and river width is greater there is a different kind of management. This is where levees are present. Levees are low ridges of earthen embankments of soil, sand or clay. Although they occur naturally the majority of the levees present along the Mississippi are man made. At present there are 1607 miles of levees on the banks of the Mississippi river, only a small % of these are naturally found.
These man made levees consist of an impermeable concrete wall surrounded by earth materials, to give a natural feel to the environment. This protects the concrete structure against erosion.
The levees create barriers hopefully stopping flooding as the presence of banks means the water levels will have to rise higher than normal to overflow. These maintain the water in the river, however, the worst flood in recorded history occurred as the Mississippi breached the levees in 1993. This caused widespread damage from May until September. This meant the further construction of more levees and in some area raising the height of the levees to prevent the occurrence of this again.
“$15 billion of damage was created …. 50 fatalities were suffered and over 10 000 homes were completely destroyed.”
www.cnn.com /archive/17263 (Properties of Central News Network © 1993
Another major source of management in the area is the diversions that man has created on the Mississippi. Considerable shortening and realignment of the river didn't begin until the early 20th century. In one area, a stretch of river on the 'Greenville Reach' was reduced from 84km to 32km in the years of 1933-1955. This included the cut-off of a meander, which shortened the river by an astonishing 30km.
In the same time period the whole Mississippi was shortened by 243km. This meant that the rivers transportation was even better, and was commercially a success, but some suggest that this tampering with the river is a major cause of the flooding. In some areas the rivers course is now direct when it was previously full of meanders, a typical example of this can be seen below:
But some feel that this is wrong, as the river has carved its path through history and has settled at this position for a reason. Some have argued that giving the river such a direct path allows the river to build up more energy than is natural causing problems of erosion down river. The quote shows some peoples view of the mass and diversity of the Mississippi management will have adverse effects as the different schemes do no necessarily compliment one another.
“The different ways that humans change their environment may be at odds with one another and conflicts may result.”
p.37 in `Changing Environments`, S. Warn and M. Naish (editors) Longman 2000
Along with the levees towards the mouth of the river there are flow deflection structures, revetments over some stretches of the Mississippi and it's main tributary the Missouri in areas where high bank erosion was present. This created two advantages: Silting occurred between the revetments reducing the width of the channel. This subsequently deepened the channel (with dredging), playing as an advantage to navigation. As the Mississippi has always been used for travel, communication and transport it was an important gain.
There are many diversion channels around the major cities in the Mississippi’s path. These are deep channels that are capable of diverting the river flow in times of high rainfall and river discharge. The reasons of the diversion channels are to prevent flooding as:
“River flooding occurs when the discharge of a river becomes too great for the capacity of the river channel”
p.30 in `Changing Environments`, S. Warn and M. Naish (editors) Longman 2000
The creation of a second “river channel” would mean double the amount of “discharge” would be needed to cause a flood. Diversions channels are constructed adjacent to the river along some of its length to cope with excess discharge. The water runs into the channel effectively like a second river, doubling the capacity of the original channel, a very effective way of coping with possible flooding situations. The actual impact to the rivers balance of water and processes is very low, so environmentally this is a very good scheme. The diversion channels are something that could plausibly happen in natural when the course of the river is diverted. This means that the presence of man made diversion channels is less than the impact of a hard structure would be.
The river is managed near the mouth of the Mississippi into a large diversion channel around the low - lying city of New Orleans, which is actually below sea level. A channel is designed to divert much of the Mississippi river flow around the city and into the Gulf of Mexico in times of high discharge, which could result in flooding.
There are also some dams present on the Mississippi and its tributaries. The Mississippi River pioneered the use of Hydro Electric Power. When the Keokuk Dam at Keokuk, Iowa, began operation in 1913, it was the largest hydroelectric dam in the world. By 1920 hydroelectric power plants accounted for 40 percent of the electric power produced in the United States. The photo shown below shows the Perrot State Park in Wisconsin. This has a reservoir along with a large dam and lock. This feature protects the economically valuable states of Michigan and Wisconsin. This not only prevents flooding but provides a tourist attraction for the people. Along with these social reasons the dams are used for other reasons. Many of the dams are built to raise river levels to create better transportation paths. It effectively creates barge canals.
The dam at the Perrot state park is very important to the water management of the area. It provides a water source for the local area. It also prevents flooding downstream as using electronic technology it limits flow based on discharge downstream to keep a constant flow along the river. These way problems are averted, and equilibrium is maintained.
But are “Dams – A blessing or a curse?”
p.34 in `AS Geogrpahy`, Phillip Allen Updates (Publisher) 2002
This is because these many dams can affect wetlands. The many dams on the Mississippi-Missouri Rivers have deprived wetlands in the Gulf of Mexico of sediment, exposing coastal Louisiana to devastating hurricanes. The environments of animals in the area are sacrificed for the dam’s construction. Fish are unable to migrate and forced to stay as vulnerable prey in the dam’s reservoir, as many bird habitats re created in the area. Some take an even more negative approach to dam building:
“In a way all dam-based schemes are ultimately self-defeating…”
From `River Management schemes` by John Pallister; Geofile 399. April 2001
People believe that in the long run dams are self-defeating, as the reasons they are created to prevent get worse. An easy example is HEP. Supposedly a clean and eco friendly power source, but to create the dam in the first place many hectares of forest was removed along Mississippi to create the Dams in the first place. Preventing the natural flooding of the drainage basin will produce infertile soils. So we can see that in the short term the dams seem to be the ideal solution for safety and economic gains but in the future these gains are lost.
To summarise the many features of management: levees, reservoirs, channel diversions, dams, deflection structures and revetments all create a different range of effects on Americas third largest river.
Bibliography:
Pictures: \media
Levee Diagram: Books Coleman
AS Geography Letts Revision
Dam it! Geo File
January 2000 Geo File