Mississippi River Case Study
Photograph 1: The Upper Mississippi
Physical Characteristics
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The Mississippi river is the fourth largest river in the world.
It is 3,705 kilometres in length.
Its drainage basin covers 41% of the area of the United States (3.2 million
square kilometres).
It rises from Lake Itasca, NW Minnesota, a small glacial lake.
At source it is 450m above sea level.
Nearly half the drop in elevation occurs in Minnesota State.
The average discharge into the Gulf of Mexico is 16,200m 3/s.
The mouth of the river enters the Gulf of Mexico.
At the headwater of the Mississippi it is approximately 1m deep.
At the mouth of the Mississippi it is approximately 67m deep.
Each day an average of 436,000 tons of sediment is transported.
Over the course of a year 159 million tons of sediment is transported.
The river is divided into 2 sections:
1. The Upper Mississippi, from the source south to the confluence of
the Ohio river with the Mississippi.
2. The Lower Mississippi, from the Ohio to the mouth 160km south of
New Orleans.
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The river has under gone a series of natural shifts in its course, two major
earthquakes in 1811 and 1812 (estimated 8 on the Richter Scale) are
thought to have temporarily reversed the course.
Map 1: Mississippi River Watershed
Ecosystem Characteristics
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Home to at least 260 different species of fish, 25% of all North American
fish species.
40% of migratory birds in the United States use the river corridor during
their Autumn and Spring migrations.
60% of all North American migratory birds use the river basin as part of
their flyway.
The Upper Mississippi is home to 50 species of mammals.
At least 145 species of amphibians and reptiles are found in the Upper
Mississippi.
38 species of mussel are found in the upper reaches of the river.
In the Lower Mississippi there are thought to be 60 separate species of
mussel.
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Seasonal flooding drives a highly productive and diverse ecosystem.
Fire was once a dominant force maintaining floodplain grassland-savanna
landscapes.
Ice flows, tree falls, and log jams are all natural occurrences that help
define local habitats and maintain high habitat diversity.
Biological disturbances (e.g., beavers) are important in the development of
floodplain landscapes.
Diagram 1: Pre-settlement vegetation of Upper Mississippi River valley
(J. Nelson and L. DeHaan)
Table 1. Ecological Disturbances
Natural
Man Made
Flood
Drought
Sedimentation
Channel migration
Sediment resuspension
Fire
Ice shear
Tree wind-throw
Log jam
Beavers
Water level regulation
Dredging and dredged
material disposal
Channel training structures
Boat generated waves
Levee construction
Agriculture
Nutrient enrichment
Logging
Urban development
Contaminants
HNA Summary Report, USGS.
Human Influence
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Population of the Mississippi corridor is 12 million people.
It flows through or borders 10 states; Minnesota, Wisconsin, Iowa, Illinois,
Missouri, Kentucky, Arkansas, Tennessee and Mississippi.
27 locks were built on the Upper Mississippi in the 1930s designed
primarily to maintain a channel for commercial barge traffic, this created a
series of lakes used mainly for boating and fishing.
Although the dams make the river deeper and wider no flood control is
intended.
During periods of high flow the dams can be submersed and opened,
ceasing to function.
Below St. Louis, the Mississippi is relatively free flowing, although it is
constrained by a series of levees and directed by multiple wing dams.
Davenport, Iowa, is the only town over 20,000 people in the Upper
Mississippi to not be protected by a permanent floodwall or levee.
Impoundment and river regulation for navigation have significantly
modified the hydrologic regime and the pattern of sedimentation.
Impoundment, water level regulation, channelization, levee construction,
logging, and urban and agricultural development are the dominant human
activities affecting river habitats on the Upper Mississippi River System
(UMRS).
Portions of the floodplain were permanently flooded by the dams and
backwater area increased significantly in some river reaches (Fig. 1).
Rock wing dams, closing dams, and bank revetments are used to maintain
the navigation channel and to reduce dredging requirements, these
structures reduce bank erosion and force flow into the main channel.
In the Open River reach, channel training structures have greatly reduced
the number and quality of secondary channels (Fig. 2), there has also
been loss of channel area as sediment filled the area between wing dams.
Over 1.1 million acres, mostly agricultural land, are protected from
moderate floods by levees in the UMRS.
Logging has caused significant habitat degradation throughout the river
floodplains and northern parts of the basin.
Deforestation and agricultural conversion throughout the basin increased
sediment delivery to the main rivers in the watershed.
Urban development displaced native habitats, but also caused indirect
impacts; sewage and industrial pollution caused significant water quality
problems that eradicated sensitive species downstream of large cities. The
problem has subsided since the 1970s.
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The separation of the river from its original floodplain and the loss of
millions of acres of wetlands, many drained for agricultural purposes, have
limited the river's ability to absorb and protect against spring floodwaters.
Although billions have been spent to tame the river and reduce flood
damages, recent floods have cost taxpayers billions and resulted in
significant loss of life.
Nutrients discharged from the river system into the Gulf of Mexico have
contributed to a "dead zone."
Strategies to improve and increase wetland area (Fig. 3) in the lower
reaches of the Mississippi would also help reduce the problem of nutrient
discharge due to uptake by plants in these areas leading to a potential
reduction in size of the “dead zone” in the Gulf of Mexico.
Figure 1: Before dams were
constructed (ca. 1890), the
river near La Crosse,
Wisconsin had many
channels, islands, forests
and marshes. Lock and Dam
8 permanently inundated
these features in the
downstream half of the pool.
HNA Summary Report, USGS.
Figure 2: An example of
side channel loss south
of St. Louis.
Figure 3: Coastal wetlands project in Louisiana
Photograph 2: Mississippi Sediment Plume, 05/03/2001 (Earth Observatory, NASA)
References and Bibliography
Earth Observatory, NASA.
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=4720
Earth Trends, The Environmental Information Portal
http://earthtrends.wri.org/maps_spatial/maps_detail_static.cfm?map_select=390&theme=2
HNA Summary Report The Role of Disturbance in the UMRS Ecosystem
www.umesc.usgs.gov/.../ disturbance.html
http://indystar.gannettonline.com/gns/wetlands/wetplan50K.gif
Mississippi National River and Recreation Area
http://www.nps.gov/miss/features/factoids/
Nelson, J. and DeHaan, L. Presettlement and Contemporary Vegetation Patterns
Along Two Navigation Reaches of the Upper Mississippi River.
biology.usgs.gov/ luhna/chap7.html
http://rathe.medinfo.ufl.edu/personal/bikecentennial/images/35-mississippi-river.jpg
United States Environmental Protection Agency. http://www.epa.gov/msbasin/
Wikipedia. http://en.wikipedia.org/wiki/Mississippi_River