Management Approaches to the Ecological Restoration of the Mississippi Basin
John W. Day, Jr.1 and William J. Mitsch2
1
Department of Oceanography and Coastal Sciences, School of the Coast and
Environment, Louisiana State University, Baton Rouge, LA 70803, USA
2
Olentangy River Wetland Research Park, School of Natural Resources, The Ohio State
University, 352 W. Dodridge Street, Columbus, OH 43202, USA
Abstract
Ecological and hydrologic restoration of the Mississippi-Ohio-Missouri (MOM) Basin is
proposed to solve problems of habitat loss, hydrologic disruption, and water quality
deterioration. High nitrate-nitrogen, mainly from fertilizer use in the Midwest, leads to
water quality problems throughout the basin and in the Gulf of Mexico where there is a
large summer hypoxic zone. In the Midwest, the land has also been artificially drained
and 80 to 90 percent of the original wetlands have been lost. Restoration involves the
strategic creation and restoration of 2.1 to 2.6 million hectares of wetlands where in-field
wetlands intercept agricultural runoff and diversion wetlands are overflowed by river
flood waters. Case studies of a total of 46 wetland-years of data from Illinois, Ohio, and
Louisiana are summarized as the basis for the restoration area estimate. Benefits of this
restoration include solving the hypoxia problem, water quality improvement, habitat
creation, reduction of public health threats, and flood mitigation that will accrue
throughout the basin. Before the restoration begins, there is a need for a large-scale
research program in the basin.
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Introduction
Human activity has impacted the Mississippi-Ohio-Missouri (MOM) River Basin in a
variety of ways. There has been a large loss of wetlands. In some states of the Midwest,
over 90% of original wetlands have been lost. Most of the lower river is leveed and most
flood plain wetlands have been cleared, mainly for agriculture. Most of the length of the
main tributaries above St. Louisiana has been impounded by a series of dams. Due
mainly to high fertilizer runoff, there is poor water quality in much of the river system.
In the Mississippi there has been a drastic loss of wetlands and in the nearshore Gulf of
Mexico, there is an annual summertime hypoxia zone that covers thousands of square
kilometers (Mitsch et al. 2001, Day et al. 2000a, Scavia et al. 2003).
The subject of this paper is the ecological restoration of MOM to solve the
environmental problems of the basin. There are three general approaches for reducing
agriculturally derived nitrogen and restoring habitat in the basin (Mitsch et al. 2001): 1)
change farming practices to minimize nutrient loss; 2) reduction of nitrogen in surface
and subsurface runoff with created and restored wetlands; and 3) use of river diversion
backwaters along rivers and in the Mississippi River delta for interception of large fluxes
of nitrogen associated with flood events. This paper discusses the second and third
options as part of a major hydrologic restoration of the Basin and the resulting benefits
throughout the basin. This restoration should be based on what has been learned from
other large-scale restorations such as the Everglades and the Mississippi Delta. We also
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outline a research program that is needed to reduce uncertainties in the restoration
approaches before it is attempted on a large scale.
Causes of Environmental Problems in the Mississippi Basin
The area of the Mississippi Basin (Fig. 1) is about 3.2 million km2 or about 40% of
the lower 48 United States. The Ohio, Missouri, and Upper Mississippi Rivers contribute
90% of the nitrate-nitrogen flux to the Gulf of Mexico. Most comes from the upper
Mississippi (43%) and Ohio (34%) rivers of this flux, while the Missouri contributes only
13.2%. The region of highest nitrate-nitrogen flux, generally greater than1,000 kg-N km2
yr-1, is in the Midwest and stretches from southern Minnesota southward through Iowa
then eastward through Illinois and Indiana to central Ohio. This is the “corn belt”, one of
the most productive agricultural areas in the world. There was a dramatic increase in
fertilizer use in the corn belt beginning in the late 1950s and this led to increased flux of
nitrate to the Gulf of Mexico. Other major sources of nitrogen to waterways in the basin
include legume, especially soybean, nitrogen fixation, animal manure, soil
mineralization, atmospheric inputs, and municipal and industrial wastewater, but fertilizer
is the most important source (Goolsby et al. 1999).
There is a clear seasonal pattern of Nitrate-nitrogen concentrations in the streams of
the basin that results from the seasonal use of fertilizer, the wet season of January through
June in much of the Upper Midwest, and the in-stream degradation of nitrates during low
flow periods in the warm stream and river waters of summers and fall. Concentrations as
high as 14 to 18 mg-N/L occur in small agricultural streams in some states and average 4
to 8 mg-N/L in third and fourth order streams such as the Olentangy River in Ohio during
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the wet season and drop to 2 mg-N/L or less during the dry season. Concentrations of
nitrate-nitrogen in the Mississippi River discharging to the Gulf of Mexico average less
than 2 mg-N/L but this is sufficient to cause Gulf eutrophication.
The effect of increased use of fertilizer on flux of nitrate-nitrogen to the Gulf is
exacerbated by the increased drainage of about 30 million ha of land in the basin in the
20th century, much of which was wetlands (Mitsch et al. 2001). About 30 million ha of
land has been drained in the Mississippi River basin in the twentieth century (Mitsch et
al., 2001). The most intensively drained part of the basin corresponds to the area in the
Midwest of highest fertilizer runoff. Seven states in the upper half of the basin (Indiana,
Illinois, Iowa, Minnesota, Missouri, Ohio, and Wisconsin) accounted for about 19 million
ha of drainage and had a total of 14.1 million ha of wetlands lost over the past 200 years
(Dahl 1990). Thus, not only has the landscape has lost part of its ability to maintain a
biogeochemical balance by shortening the time that water spends on the land, but the
streams and rivers and ultimately the coastal areas are no longer buffered from upland
regions by wetlands and riparian forests. Added to the increased use of fertilizers, the
drainage systems in place in much of the Basin are the reasons for the hypoxia in the Gulf
of Mexico.
The ecological solution
In order to help solve the problem of hypoxia, the flux of nitrate-nitrogen to the Gulf of
Mexico must be significantly reduced. Since the source and rapid transport of the
nitrogen to the Gulf is due to a combination of two human actions—the excess use of
nitrogen fertilizer, and artificial drainage that accelerates the flow of fertilizer-rich waters
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to adjacent streams and rivers—the ecological solution suggests landscape changes that
both reduce nitrate-nitrogen from runoff and slows the speed of runoff. The agricultural
community is already responding by identifying and implementing management practices
such as changing cropping systems, reducing fertilizer application, controlled drainage,
and managing manure spreading and timing of nitrogen application, that may be able to
reduce nitrogen discharge to streams by up to 20% (Mitsch et al., 2001).
We recommended that in addition to these agronomic practices, there are two
fundamental approaches that are needed to allow agriculture to maintain its productivity:
1. Farm runoff wetlands—creation and restoration of wetlands and riparian buffers
between farms and adjacent streams and rivers; and
2. River diversion wetlands—diversion of river water into adjacent constructed and
restored wetlands along the main river channels and in the Mississippi delta
during flood periods.
Nitrate-nitrogen retention has been effective in both types of wetlands. These are
described here.
Farm runoff wetlands
The ability of natural and constructed wetlands to remove nutrients and organic loads
from farm runoff has been well demonstrated (e.g., Arheimer and Wittgren, 1995; Kadlec
and Knight, 1996; Comin et al., 1997; Peterson, 1998; Kovacic et al., 2000; Day et al.
2003, 2004). This wetland function is especially effective if the wetlands are located in
the headwaters of small watersheds.
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Four surface-flow wetlands were constructed in 1994 on the Embarras River
floodplain in east-central Illinois,USA, to intercept field drainage tiles that drained corn
and soybean fields (Kovicic et al., 2000; Larson et al., 2000; Hoagland et al., 2001).
These wetlands decreased nitrate-nitrogen flux from 32 to 66%, with an average a
retention of 44% and 23 g-N m-2 yr-1.
A multi-celled, 1.2 ha agricultural runoff wetland in Logan County, Ohio, USA,
receives drainage from a 17-ha watershed, 14.2 ha of which was used for intensive rowcrop agriculture. Surface inflow and groundwater discharge were about equal at multiple
locations within the site amounted to almost the same amount. Overall, the wetlands
retained 40% of nitrate-nitrite and retained a mass of 39 g-N m-2 yr-1. The overall design
of this wetland, with multiple cells and a watershed:wetland ratio of 14:1 appeared to be
well designed to buffer storm pulses of surface runoff coupled with more consistent yet
higher nitrate concentrations in groundwater seeps (Mitsch, personal communication).
River diversion wetlands
A river diversion wetland is a wetland on the adjacent floodplain or behind artificial
levees that receives water by pumping or gravity flow from the main channel of a river.
River diversion wetlands, both at a small scale in the upper half of the MOM Basin in
Illinois and Ohio, and large-scale on the Mississippi River itself in Louisiana illustrate the
potential of this approach for improving water quality. Each river diversion system was
the site of significant research with similar sampling and analytical methodologies for
several years. Each project involves diverting nutrient-laden river water into adjacent
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riparian wetlands. In all cases, some infrastructure (diversion gates, retention valves,
check values, and pumps) was used to control and measure flows into adjacent riparian
wetlands.
A series of experimental diversion wetlands, 1.8 to 3.8 ha, were contructed at the Des
Plaines River Wetlands in northeastern Illinois (Sanville and Mitsch, 1994, Hey et al.,
1994). All of the wetlands were sinks for nitrate-nitrogen, removing 78 – 84% of the
inflowing nitrate, by mass (Phipps and Crumption 1994). Over a 3-year period, retention
by concentration ranged from 46 to 95% and etention rates ranged from 2 to 43 g-N m-2
yr-1.
The ability of construced wetlands in the Olentangy River Wetland Research Park at
Ohio State University to retain nutrients has been studied extensively (Mitsch and
Wilson, 1996; Mitsch et al., 1998; Mitsch and Jørgensen, 2004). For 16-wetland years of
measurements, the wetlands retained an average of 34% of nitrate-nitrogen by
concentration and 33% by mass.
In Louisiana, nutrient uptake has been studied at a Mississippi River diversion where
river water is being reintroduced into a coastal estuarine system to restore deteriorating
wetlands (Day et al. 1997). The diversion structure delivers up to about 250 m3 sec-1 to a
500 km2 area of fresh and brackish wetlands. The Caernarvon wetland retained 39 to 72%
of nitrate by concentration, depending on the sampling location (Table 2) and 31 to 90 gN m-2-yr-1. Earlier studies at Caernarvon (1991-94) showed a long-term reduction of
nitrate of 76 to 95% nitrate reduction by concentration (Lane et al., 1999, 2004). In a
related study in Louisiana, Lane et al. (2002) reported that nitrate concentrations in the
7
Atchafalaya River outflow were reduced by 40-50% as the water flowed into the Gulf of
Mexico.
Strategies for the Restoration of the Mississippi Basin
Mitsch et al. (2001) developed relationship between loading and retention rates for
wetlands (Fig. 2) based on several wetland-years of data from several independent
wetland basins in the Basin. Assuming that 40 – 50 % N removal is maintained at a
median loading rate of 60 g-N m-2 yr-1, creation or restoration of 2.1 – 2.6 million ha of
wetlands will required to remove 40% of the total nitrogen discharging to the Gulf of
Mexico. This ecologically engineered nitrogen reduction, combined with nitrate
reduction from agronomic practices (Mitsch et al., 2001) would assure a sufficient
enough reduction in nitrates entering the Gulf of Mexico to ensure a decrease in the size
of the Gulf of Mexico hypoxia.
This ecological restoration would return the basin to a functioning that is more akin to
how the system functioned under natural conditions. Understanding river ecosystems has
evolved from the River Continuum Concept (Vannote et al., 1980) where the longitudinal
pattern changes from upstream to downstream are emphasized to the Flood Pulse
Concept (Junk et al. 1989; Junk, 1999; Tockner et al., 2000) where exchange between a
river and its floodplain is emphasized as the main factor determining the function of both
the river and its adjacent riparian floodplains. For major river deltas, ideas have changed
from a physical-based model of deltaic lobe formation forced mainly by mineral
sediments (e.g., Kolb and Van Lopik 1958) to the concept that deltas are sustained by a
8
hierarchical series of pulses that include tides, storms, and floods as they interact with
biological, chemical, and geological processes (Day et al. 1995, 1997, 2000, Roberts
1997). The restoration plan we describe reconnects the rivers with the floodplain and
delta through a series of pulsed introductions of water from runoff and flooding.
Benefits of restoration
The restoration of the Mississippi basin has a range of benefits. These occur mostly
in the Midwestern states, but also accrue throughout the basin.
A reduction in nitrate concentrations will result in public health benefits. High nitrate
concentrations are known to cause methemoglobinemia (“blue baby” disease that can
cause fatalities to babies less than 6 months old). Creating wetlands to control nitratenitrogen could reduce the number of nitrate water quality alerts in the Midwest and could
save treatment plants a great deal of chemical treatment costs.
Habitat restoration is another benefit of restoration. Most states in Midwest have lost
80 to 90% of their wetlands. Restored and created wetlands greatly enhance wildlife
benefits to the region. For example, Hickman (1994) found that the number of breeding
bird species increased from 37 to 54 at the Des Plaines River Wetland project after
wetland basins were created. The National Research Council (1992) described a goal for
wetland restoration for the United States for which this hydrologic restoration would
provide half of the goal, “A gain of 4 million hectares of wetlands by the year 2010,
largely through reconverting crop and pastureland and modifying or removing existing
water control structures.
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An advantage of creating and restoring wetlands is the mitigation of floods. In fact,
this could be the most economically valuable aspect of creating wetlands in the basin. For
example, Hey and Phillipi (1995) estimated that restoration of about 5 million hectares of
wetlands and backwaters in the Upper Mississippi River Basin could have provided
substantial flood control value, even for the 100-year flood conditions experienced in the
1993.
The restoration approaches described above will help preserve the agricultural
economy in the Mississippi basin. One of the primary threats to farmers is the growing
demand that the runoff of fertilizers be reduced. The construction and restoration of
wetlands will reduce fertilizer runoff and thus solve this problem. This might require 2 to
3 percent of the landscape in the Midwestern farm belt. Thus, using 2-3% of farmland
for nutrient reduction would allow the remaining 97-98% of farmland to remain in
production with the problem of excess nutrients solved. Otherwise, there may be legally
mandated reductions in fertilizer use to meet water quality goals.
Research Needs
Although there has been considerable study on the use of wetlands for water quality
improvement, the research base is inadequate to model and apply with a high degree of
certainty the effects of large-scale restoration of wetlands, riparian zones, and river
diversions to improve water quality in the Basin. Pilot-scale projects, modeling, and
technology transfer are needed before a restoration project on the scale of 2 million ha is
undertaken. We propose a comprehensive, coordinated applied research program to
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thoroughly investigate the uncertainties of these two general approaches for helping to
solve a major national pollution problem in the Basin and Gulf of Mexico. The findings
of such a program will be applicable to help solve inland and coastal pollution problems
in other areas of the U.S. and other countries.
Big-science research projects (see Mitsch and Day, 2004) using a combination of
multiple full-scale demonstration wetlands on farms, wetland research parks, and river
diversions in the Midwest and the Mississippi basin and delta are needed to test
alternative water quality improvement strategies that use wetlands as permanent sinks for
nutrients. These projects will allow us to develop design parameters, confidence intervals,
and validated simulation ecosystem models to predict the behavior of these wetlands.
Ten to twenty full-scale, existing and new agricultural wetland demonstration
projects should be located throughout the Midwest to study agricultural runoff into
wetlands in a variety of soil conditions. These studies should include timing of
floodwater input, methods for retention of floodwaters, fate of nitrogen, and investigation
of factors such as temperature, soils, microbiology, etc. on loading-uptake relationships.
Similar studies could be carried out for riparian buffer forests in terms of water quality,
flood control, sustainable forestry and wildlife habitat. Similar studies should be carried
out for river diversions along river channels and in the Mississippi delta.
Conclusions
• The focus on solving problems that are watershed-scale should be for ecologically
engineered solutions, i.e. those that are sustainable and involve enhancing
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ecological processes.
•
The restoration of the Mississippi Basin and the Gulf of Mexico will require a
combination of agronomic and ecological techniques and strong collaboration
among all stakeholders.
•
Restoration of 2.5 million ha of wetlands is needed to reduce the nitrogen load to
the Gulf sufficiently to see an effect on the hypoxia.
•
Simply using agronomic techniques by themselves in the Mississippi River Basin
is neither politically feasible, physically possible, nor ecologically sufficient in
scale to cause a significant reduction in the size the hypoxic zone in the Gulf of
Mexico.
•
The benefits of wetland restoration in the Basin, of and by themselves, are
sufficient reasons for this large-scale restoration.
•
This eco-solution deals with 3 main causes of the problem: wetland/habitat loss;
major drainage networks and hydrologic disruption; and excessive fertilizer use.
•
This eco-solution also mimics the natural functioning of riverine, wetland, and
deltaic ecosystems.
•
A major “big-science” research program needs to take place with sufficient
funding, study sites, and time to reduce remaining uncertainties about the
restoration program.
Acknowledgements. Support for this paper was provided by the Louisiana
Department of Natural Resources, the U.S. Army Corps of Engineers, the U.S.
12
Geological Survey, the U.S. Dept of Agriculture’s Nutrient Science and NRI programs,
U.S. EPA Watersheds and Gulf of Mexico programs, and NOAA Coastal Ocean Program.
We also appreciate the support of the Ohio Agricultural Research and Development
Center and the Olentangy River Wetland Research Park at Ohio State University and the
School for Coast and Environment at Louisiana State University.
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Figure Legends
Figure 1. The Mississippi-Ohio-Missouri (MOM) Basin showing the location and general
extent of the Gulf of Mexico hypoxia zone, the location of high nitrogen loading in
the upper Midwest, and the location of enhanced drainage (compiled from Scavia et
al. 2003, Goolsby et al. 1999, and Mitsch et al. 2001)
Figure 2. The relationship between nitrogen loading to wetlands and the retention of
nitrate (from Mitsch et al. 2001).
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