Ecological Engineering 26 (2006) 55–69
Restoration of wetlands in the Mississippi–Ohio–Missouri
(MOM) River Basin: Experience and needed research
William J. Mitsch a,∗ , John W. Day Jr. b
a
Olentangy River Wetland Research Park, School of Natural Resources, The Ohio State University,
352 W. Dodridge Street, Columbus, OH 43202, USA
b Department of Oceanography and Coastal Sciences, School of the Coast and Environment,
Louisiana State University, Baton Rouge, LA 70803, USA
Received 9 June 2004; received in revised form 27 December 2004; accepted 26 September 2005
Abstract
An ecological and hydrologic restoration of the Mississippi–Ohio–Missouri (MOM) Basin in the United States is proposed
as the solution to the reccurring hypoxic conditions in the Gulf of Mexico. Nitrate–nitrogen is the cause of this eutrophication
in the Gulf and its source is mainly due to increased fertilizer use in the American Midwest. In that same Midwest, the land has
also been artificially drained and 80–90% of the original wetlands have been lost. Our proposed restoration involves the strategic
creation and restoration of 2.2 million ha of wetlands in the MOM basin where in-field wetlands intercept agricultural runoff and
diversion wetlands are overflowed by flooding river water. Case studies that total 50 wetland-years of data from Illinois, Ohio,
and Louisiana are summarized as the basis for the restoration area estimate. Benefits of this restoration, in addition to solving
the Gulf hypoxia, include water quality improvement, reduction of public health threats, habitat creation, and flood mitigation
that will accrue to the locations in the MOM basin where the restoration occurs. Before the restoration commences, there is a
need for formal and rigorous large-scale research in the basin to reduce uncertainties.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Gulf of Mexico; Hypoxia; Olentangy River Wetland Research Park; Ohio State University; Louisiana State University; River
restoration; Wetland restoration; Nitrate-nitrogen
1. Introduction
The low oxygen conditions (hypoxia) in the Gulf of
Mexico are but one sign that America’s 3 million km2
watershed that feeds the Gulf—The Mississippi–
∗ Corresponding author. Tel.: +1 614 292 9774;
fax: +1 614 292 9773.
E-mail address: [email protected] (W.J. Mitsch).
0925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ecoleng.2005.09.005
Ohio–Missouri (MOM) River Basin—needs to be
restored. Humans have dramatically increased nitrogen
fertilizer use in the basin since the 1950s and significant amounts of this excessive nitrogen are transported
as nitrate–nitrogen via drainage tile, ditches, streams,
and rivers to the Gulf, leading to eutrophication and
episodic and persistent hypoxia (dissolved oxygen
<2 mg/L) in the Gulf; the hypoxic zone routinely
covers 20,000 km2 or more (National Research
56
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
Council, 2000; Alexander et al., 2000; Rabalais et al.,
2002; Turner and Rabalais, 2003; Scavia et al., 2003).
The connection between coastal eutrophication in
the Gulf of Mexico and the nitrate–nitrogen released
from the 3 million km2 Mississippi River Basin that
feeds the Gulf is well established (Goolsby et al.,
1999; Rabalais et al., 2002; McIssac et al., 2002). The
hypoxia in the inner continental shelf of the Gulf of
Mexico off Louisiana and Texas is also related to large
loss of wetlands in the Basin (Mitsch et al., 2001),
more rapid drainage and efficient drainage, and the
separation of the Mississippi River from its floodplain
and deltaic plain. Whereas the higher order streams
and rivers once spread out over floodplains and delta
during floods, flow is now mostly shunted directly to
the sea (Baumann et al., 1984; Day et al., 2000a, 2003;
Mitsch et al., 2001, 2005b).
A comprehensive series of studies sponsored by
NOAA, USEPA, USDA, and other Federal agencies
described the extent of the Gulf of Mexico problem
(Rabalais et al., 1999), the sources of nutrients in
the Mississippi River Basin (Goolsby et al., 1999),
model predictions of the connect between the extent
of the hypoxia and nutrient reductions (Brezonik et
al., 1999), economic analysis of both the effect in
the Gulf and the solutions to the problem (Diaz and
Solow, 1999; Doering et al., 1999) and, most important for this paper topic, the potential solutions (Mitsch
et al., 1999). These report findings were summarized
by CENR (2000) and an “action plan for reducing, mitigating, and controlling hypoxia in the northern Gulf
of Mexico” (Mississippi River Task Force, 2001) was
formally transmitted to the U.S. Congress as a result of
these studies.
The subject of this paper is the ecological restoration
of MOM to solve the Gulf nutrient problems but also to
improve the environment throughout the basin. Three
general approaches for reducing agriculturally derived
nitrogen that would otherwise reach the Gulf of Mexico
were summarized in a Federal report and subsequent
article by Mitsch et al. (1999, 2001): (1) change farming
practices to minimize nitrate loss by reducing the use of
nitrogen fertilizer and through a suite of management
practices; (2) intercept laterally moving groundwater
and surface water with nitrogen-sink ecosystems, particularly created and restored wetlands and (3) provide
a system 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 use of the second and third
options as part of a major hydrologic restoration of the
Mississippi–Ohio–Missouri River Basin and points out
the potential benefits to both the Gulf of Mexico and to
those living in the basin itself. But this restoration must
be done with lessons learned from previous large-scale
restorations in the Everglades and Louisiana delta. We
outline a research program of “big science” (see Mitsch
and Day, 2004 for definition) that is needed to reduce
uncertainties in the restoration approaches before it is
attempted on a large-scale.
2. Source of the hypoxia problem
The Mississippi–Ohio–Missouri (MOM) Basin
(Fig. 1) is 3.2 million km2 or about 40% of the lower
48 United States. The Ohio, Missouri, and Upper Mississippi Rivers combined, which account for 80% of
the Mississippi River Basin, contribute 90% of the
nitrate–nitrogen flux to the Gulf of Mexico (Table 1).
The Upper Mississippi and Ohio Rivers account for 43
and 34%, respectively of this flux, from comparablesized basins. The Missouri River, although more than
twice the area of the other two sub-basins, contributes
only 13% of the nitrates to the Gulf. The shaded area
in Midwestern USA shown in Fig. 1 is the region of
highest nitrate–nitrogen flux, generally greater than
1000 kg N km−2 yr−1 , in the basin. This area stretches
from southern Minnesota southward through Iowa then
eastward through Illinois, Indiana, and Ohio. Much of
this area is the “corn belt” of Midwestern USA and historically one of the most productive agricultural areas
in the world. That agricultural prosperity has led to
the development of many large urban complexes in the
region—Chicago, Minneapolis-St. Paul, Indianapolis,
Cincinnati, and Columbus—where a large portion of
the human population of the upper basin resides.
The increase in the hypoxia in the Gulf of Mexico
in the past 20 years appears to reflect the increased
nitrate–nitrogen flux to the Gulf and the increased use
of nitrogen fertilizer in the Mississippi River Basin
since the late 1950s. Nitrogen fertilizer use in the
Mississippi–Ohio–Missouri Basin (as estimated by the
sale of fertilizer in 20 basin states from 1951–1996)
increased seven-fold from 1960 to 1990s, from 1 to
7 million Tons yr−1 (Goolsby et al., 1999). Most of that
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
57
Fig. 1. Mississippi–Ohio–Missouri (MOM) River Basin in the United States, showing location and general extent of Gulf of Mexico hypoxia in
Louisiana coastline and location of high nitrogen loadings in the basin (>1000 kg N km−2 yr−1 ) (nitrogen loading source location from Goolsby
et al., 1999).
increase occurred in 1960–1980. 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. Goolsby et al.’s (1999) data
do not show any significant rise in any of these sources
in the period of the 1950–1990s and suggest that atmospheric and wastewater inputs are low compared to
fertilizer inputs to the basin. Release of nitrogen by soil
mineralization in the basin is poorly understood and
was assumed by Goolsby et al. (1999) to be constant.
Nitrate–nitrogen concentrations in the streams of
the MOM Basin show a clear seasonal pattern. From
August through November, nitrate–nitrogen concentrations are generally low in most major streams in
the basin as in-stream degradation of nitrates occurs,
during low-flow periods with warm stream and river
temperatures of late summer and fall (Fig. 2). High concentrations of nitrate–nitrogen occur in January–June
during the wet season of high runoff and river flow
and especially in May through early June immediately after fertilizer is applied. Concentrations as high
Table 1
Fluxes and watershed yields of nitrate–nitrogen in the Upper Mississippi–Ohio–Missouri River Basin (from Goolsby et al., 1999)
Basin
Area (km2 )
Flow (cm yr−1 )
Nitrate flux
(Tons)
Nitrate yield
(kg km−2 yr−1 )
Percentage
of total
Ohio
Missouri
Upper Mississippi
526000
1357700
489500
50.2
6.5
22.7
323500
125900
411100
620
90
840
34.0
13.2
43.2
Total “Upper” Mississippi River Basin
2373200
860500
362
90.4
58
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
Fig. 2. Patterns of weekly nitrate–nitrogen concentrations measured
in the Olentangy River, Ohio, 1996–2001.
as 20 mg N/L occur in small streams draining agricultural areas in the Upper Mississippi River Basin
(Randall et al., 1997; Mitsch et al., 1999). Concentrations of nitrate–nitrogen of 5–10 mg N/L are
the norm in third and fourth order streams, such as
the Olentangy River in Ohio during the wet season
and drop to 2 mg N/L or less during the dry season of August–November (Fig. 2). Concentrations of
nitrate–nitrogen in the Mississippi River in Louisiana
just before if flows into the Gulf of Mexico average
<2 mg N/L for most of the year (Fig. 3) but are sufficient to cause the Gulf hypoxia in this coastal system,
particularly since the highest concentrations are seen
April–July.
Fig. 3. Average monthly concentrations of nitrate–nitrogen in the
Mississippi River based on 7 years of monthly sampling at Caernarvon, Louisiana.
The effect of increased use of fertilizer on flux
of nitrate–nitrogen to the Gulf is exacerbated by the
increased drainage of the landscape—including the
drainage of wetlands—that occurred in the past century in the MOM basin (Fig. 4). About 30 million ha of
land has been drained in the Mississippi River Basin
in the 20th century (Mitsch et al., 2001). The portion
of the basin where drainage tiles have been used for
more than a century (Fig. 4), correspond closely to
the “red zone” high nitrate areas (Fig. 2). Seven states
in the upper half of the MOM basin (Indiana, Illinois, Iowa, Minnesota, Missouri, Ohio, and Wisconsin)
accounted for about 19 million ha of that drainage. In
those same states, 14 million ha of wetlands were lost
over the past 200 years (Dahl, 1990). Thus, not only
has the landscape 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 MOM Basin are the
reasons for the hypoxia in the Gulf of Mexico.
3. The ecological solution
Our national “problem-solving committee” (Mitsch
et al., 1999, 2001) that was part of the hypoxia study
described above recommended that to help solve
the persistent problem of the hypoxia, the flux of
nitrate–nitrogen to the Gulf of Mexico must be significantly reduced. Since the source and rapid transport of
nitrogen to the Gulf is due to a combined effect of two
human actions—the use of nitrogen fertilizer in excess
of what is required by crops, and artificial drainage
of much of the Upper Mississippi/Ohio River basins
that accelerates the flow of fertilizer-rich waters to
adjacent streams and rivers—our ecological solution
suggests landscape changes that both remove the
nitrate–nitrogen from runoff and rivers alike but also
slow the speed by which the runoff reaches streams
and rivers and streams and rivers subsequently reach
the coast.
The agricultural community is already responding
by identifying and implementing some management
practices, such as changing cropping systems, reducing
fertilizer application, controlled drainage, and manag-
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
59
Fig. 4. Mississippi–Ohio–Missouri (MOM) River Basin in the United States, showing location of land drainage (from Mitsch and Gosselink,
2000). Each dot represents 8000 ha of drained land.
ing manure spreading and timing of nitrogen application. These practices, by themselves, are estimated to
be able to reduce nitrogen discharge to streams by up
to 20% (Mitsch et al., 2001). If other solutions are not
found, mandated reductions in fertilizer use or water
quality standards may be the only options left to solve
the problem—these regulations could result in reduced
farm production.
We recommended that in addition to the above agronomic practices already being explored 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 main river channels and in the Mississippi
River delta during flood periods.
Nitrate–nitrogen retention has been effective in both
types of wetlands (Table 2). Case studies of each type
of wetland are described here.
3.1. 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. Hammer, 1992;
Knight, 1992; Arheimer and Wittgren, 1995; Kadlec
and Knight, 1996; Comin et al., 1997; Peterson, 1998;
Kovacic et al., 2000; Fink and Mitsch, 2004). This
wetland function is especially effective if the wetlands
are located in the headwaters of small watersheds and
downstream from farm runoff (Fig. 5).
3.1.1. Illinois agricultural runoff wetlands
Four surface-flow wetlands were constructed in
1994 on the Embarras River floodplain in east-central
Illinois, USA, to demonstrate if wetlands could inter-
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
60
Table 2
Nitrate–nitrogen retention in farm and river diversion wetlands in the Mississippi River Basin
Wetland, location and type
Farm wetlands
Agricultural wetlands, OH (one basin over 2 years)
Agricultural wetlands, IL (three basins over 3 years and one
basin for an additional year)
River diversion wetlands
Created river diversion wetlands, IL (three basins over 3 years)
High-flow conditions
Low-flow conditions
Created diversion wetlands, OH (two basins over 8 years)
Oxbow diversion wetland, OH (one basin over 4 years)
Caernarvon diversion, LA (one basin in 5 year)
Zone 1
Zone 2
Zone 3
Entire wetland, 1991–1994
cept tile drainage from farm fields, temporarily hold
the water, remove pollutants, and then pass the water
to the adjacent stream (Kovacic et al., 2000; Larson
et al., 2000; Hoagland et al., 2001). The four wetlands, 0.3–0.8 ha in size, were created by construction
of a berm 15 m from the river. The wetlands were
designed to intercept flow from field drainage tiles
from corn and soybean fields. Outlet weir structures
were installed on the downstream end of the wetlands
to measure outflow. Overall, with 10 wetland-years
of data, these wetlands decreased nitrate–nitrogen
export from 32 to 66%, with an average retention
of 23 g N m−2 yr−1 .
3.1.2. Ohio agricultural runoff wetland
An agricultural runoff wetland was constructed in
the spring of 1998 in Logan County, Ohio, USA,
Fig. 5. Conceptual diagram of farm runoff wetland.
Wetland
size (ha)
Nitrate–nitrogen
Reference
%
g N m−2 yr−1
1.2
0.3–0.8
40
44 ± 4
39
23 ± 5
Fink and Mitsch (2004)
Kovacic et al. (2000) and
Larson et al. (2000)
1.8–2
2.3
1
43–94
75–95
33 ± 2
12–43
2–8
38 ± 6
3
42 ± 13
20 ± 3
Phipps and Crumpton (1994)
Unpublished data
Mitsch et al. (1998, 2005a,b)
and Spieles and Mitsch (2000)
This study
This study
1000
3000
5000
39
59
72
78–95
90
46
31
5–9
Lane et al. (1999)
several kilometers upstream of a popular recreational
lake in northwestern Ohio called Indian Lake (Fink
and Mitsch, 2004). The multi-celled 1.2 ha “Indian
Lake Demonstration Wetland” receives drainage from
a 17 ha watershed, 14.2 ha of which was used for intensive row–crop agriculture and 2.8 ha of which was
forested. Thus, the wetland had a watershed:wetland
ratio of 14:1. Surface inflow in 2000 was 6.64 m yr−1
and groundwater inflow to the wetland was 4.2 m yr−1 .
Surface water levels over a 2-year period of study
varied over 40 cm in depth and muskrat activity in
one of the cells actually led to a 30 cm water level
decrease in the second year of study. Major storm
events led to dramatic but short increases in water level
of over 20 cm. These storm events, primarily in the
late winter and early spring, led to rapid flow-through
and poorer water quality enhancement. Surface inflow
had 2-year-average concentrations of 0.8 mg N/L for
nitrate + nitrite while groundwater had much higher
concentrations (2.0 mg N L−1 ). Interestingly, concentrations of nitrate–nitrite discharging from the wetlands did not increase significantly during precipitation
events even though inflow concentrations were generally higher then. Overall, the wetlands retained 40% of
nitrate–nitrite and retained a mass of 39 g N m−2 yr−1
(Table 2). The overall design of this wetland, with
multiple cells and a watershed:wetland ratio of 14:1,
appeared to be appropriate to buffer storm pulses of
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
Fig. 6. Conceptual diagram of river diversion wetland: (a) plan view,
(b) aerial view.
surface runoff coupled with more consistent yet higher
nitrate + nitrite concentrations in groundwater seeps
(Fink and Mitsch, 2004).
61
sites in the country to investigate the idea of river
diversions through small constructed floodplain wetlands (Sanville and Mitsch, 1994; Wang and Mitsch,
2000). Initially four constructed wetland basins, ranging in size from 1.8 to 3.8 ha were constructed in
Lake County in northeastern Illinois on the floodplain of the Des Plaines River. Excavation of the
four wetland basins began in 1986 in the alluvium
of the Des Plaines River floodplain. Water was delivered to each wetland basin through a pipe network
fed by a submersible pump at the river. Pumping
began in 1989 and most of the useful data at the site
were obtained in water years 1990 and 1991. During
that period, a whole-ecosystem experiment was established whereby two of the wetlands had a high rate
of flow (18–20 m yr−1 ) while the other two wetlands
had a much lower rate of flow (5–8 m yr−1 ) (Hey et
al., 1994). Nitrate–nitrogen budgets were developed
for three of the four wetland basins in 1991 (Phipps
and Crumpton, 1994; Table 2). All of the wetlands
were sinks for nitrate–nitrogen, removing 78–84%
of the inflowing nitrate by mass in that study. Over
a 3-year period, retention by concentration ranged
from 46 to 95%, with retention only slightly related
to the flow rates. Retention rates ranged from 2 to
43 g N m−2 yr−1 .
3.2. River diversion wetlands
A river diversion wetland is a wetland on the adjacent floodplain or behind artificial levees that receives
water by pumping or flood flow from the main channel of a river (Fig. 6). 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 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.
3.2.1. Illinois experimental riverine wetlands
Experimental wetlands at the Des Plaines River
Wetlands in northeastern Illinois were among the first
3.2.2. Ohio experimental riverine wetlands
A pair of 1 ha experimental diversion wetlands
basins were created in 1994 at the Olentangy River Wetland Research Park on Ohio State University’s campus
and used in a whole-ecosystem wetland experiment
from 1994 to 2002 (Mitsch and Wilson, 1996; Mitsch et
al., 1998, 2005a; Mitsch and Jørgensen, 2004). Continuously pumped inflows have averaged 20–30 m yr−1 to
each basin with day-to-day flow patterns corresponding
to Olentangy River flow. The water passes through the
wetlands in about 3–4 days and discharges to a common
swale that in turn flows back to the Olentangy River.
For 16 wetland-years of measurements (2 wetlands × 8
years), the Ohio wetlands retained an average of 34%
of nitrate–nitrogen by concentration and 33% by mass
(Fig. 7; Table 2). In years of high nitrate concentration
(1996, 1997, 2000, and 2001), nitrates decreased from
approximately 4 to 3 mg N L−1 in the 1 ha wetlands.
In low-nitrate concentration years (1994, 1995, 1998,
and 1999), nitrates decreased from approximately 2 to
1 mg N L−1 .
62
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
Fig. 7. Nitrate–nitrogen decrease through two experimental wetlands at Olentangy River Wetland Research Park, Columbus.
Decrease is overall percent reduction in nitrate–nitrogen concentration through each wetland basin from weekly samples for each
year.
3.2.3. Ohio created oxbow
A 3 ha river diversion wetland was constructed in
the summer of 1996 at the Olentangy River Wetland
Research Park. The wetland was created on the river
floodplain as mitigation for the loss of 1.1 ha of wetlands lost about 30 km south of the site in the same
major river basin. This freshwater oxbow wetland was
designed to receive water from the Olentangy River
during flood pulses similar to the flow pattern shown
in Fig. 6b. When the river level is higher than the wetland water level, water passively flows into the wetland
through a check valve at the inflow. When the river
pulse decreases to where its water level is below that of
the wetland, the check valve closes, keeping the water
from spilling back to the river. Thus all of the floodwater is “pushed through” the wetland and eventually
returns to the Olentangy River through a control weir at
the outflow about 300 m from the inflow. Fig. 8 shows
a typical hydroperiod for the wetland compared to the
adjacent river hydroperiod. Water level trends of the
Fig. 8. Water stage in Olentangy River in central Ohio and water
level in created floodplain oxbow that receives diversions from the
river. Data are for 2001.
oxbow follow a pattern of high water levels in winter and early in the growing season, followed by drier
conditions in late summer. Frequent winter and spring
flooding followed by summer drawdowns is typical of
Midwest USA wetlands. For example, 11 flood events
occurred between January and July in 2002 into the
wetland. These flood events provide nutrients and introduce seeds and small organisms from across the watershed. The spring/early summer flooding also provides
a period of standing water in the wetland considerably
longer than the number of hydrologic critical days that
were given by National Research Council (1995) for
wetland delineations; thus, the design of this wetland
was successful in establishing hydrologic conditions
conducive to development of a functioning wetland.
Nitrate–nitrogen and hydrology measurements at
the inflow and outflow of this created oxbow wetland illustrate a consistent pattern of nitrate retention
by concentration and mass (Table 3). Nitrate–nitrogen
concentrations were statistically lower in the outflow
compared to the inflow in three of the 4 years for which
data are available, with an average nitrate–nitrogen
concentration decrease of 42%. When hydrology data
are added to the evaluation, the wetland was shown
to retain about 46% of the inflowing nitrate–nitrogen
or 20 g N m−2 yr−1 . Overall, this type of wetland is a
prototype for how diversion wetlands can be created
or restored along floodplains throughout the MOM
Basin. The wetland is effective in retaining floodwater, in removing nitrate–nitrogen from that water, and
in establishing a habitat particularly suitable for migratory waterfowl during spring migrations and for herons
(e.g. Great Blue Heron, Ardea herodias) in the late summer dry season. In some years, the standing water in the
wetland is only a fraction of the 3 ha basin, thus causing
a concentration of fish and a feast for the herons.
3.2.4. Louisiana river diversions
In Louisiana, river diversions are designed along the
main stem of the Mississippi River to restore deteriorating wetlands in the Mississippi Delta by delivering
sediments meant to compensate for sinking land (Day
et al., 1997). One such diversion at Caernarvon on
the east bank of the river south of New Orleans is a
five-box culvert with vertical lift gates with a maximum flow of 280 m3 s−1 . River diversion began in
August 1991; average minimum and maximum flows
are 14 and 114 m3 s−1 , respectively, with summer flow
46 ± 13%
45 ± 5
8.5 ± 0.8
9.4 ± 0.9
42 ± 13%
53%
3.63 ± 0.81
6.69 ± 1.68
Average
0.0001
6.8
11.1
9.25
10.4
70%
12%
33%
0.0000
nd
0.0000
1.57 + 0.27 (34)
4.05 + 0.69 (69)
3.43 + 0.23 (45)
No data
5.48 + 0.54 (4)
5.30 + 0.39 (61)
4.60 + 0.30 (43)
5.15 + 0.26 (20)
No data
11.72 + 1.35 (34)
2001
2000
1999
1998
1997
Outflow
Inflow
25 ± 8
9.9
35.1
31.2
36.0
51.1
47.6
6.3
8.67
9.1
9.8
73%
31%
34%
Outflow
(g N m−2 yr−1 )
Inflow
(g N m−2 yr−1 )
Inflow
(m yr−1 )
Ouflow
(m yr−1 )
Nitrate–nitrogen flux
Hydrology
Percentage
decrease
t-Test p-value
NO3 –N concentration (mg N/L)
Year
Table 3
Nitrate–nitrogen retention in 2.5 ha natural-flow oxbow wetland created at Olentangy River Wetland Research Park, central Ohio
Mass %
retention
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
63
rates generally near the minimum and winter flow rates
50–80% of the maximum (Lane et al., 1999, 2004). The
diversion delivers river water to a the 260 km2 Caernarvon freshwater/brackish wetland that eventually discharges into the brackish/saline Breton Sound estuary
which is part of coastal Gulf of Mexico. The Caernarvon wetland retained 39–72% of nitrate by concentration, depending on the sampling location (Table 2)
and 31–90 g N m−2 yr−1 . Earlier studies at Caernarvon
(1991–1994) showed a long-term reduction of nitrate
of 76–95% nitrate reduction by concentration (Lane
et al., 1999). In a related study in Louisiana, Lane et
al. (2002) reported that nitrate concentrations in the
Atchafalya River outflow were reduced by 40–50% as
the water flowed into the Gulf of Mexico.
4. Restoring MOM
We recently re-estimated that creation or restoration
of 2.2 million ha of wetlands will be required to remove
40% of the total nitrogen discharging to the Gulf of
Mexico (Mitsch et al., 2005b). This estimate is based
on the relationship between loading and retention rates
for wetlands determined from several wetland-years of
data from several independent wetland basins in the
MOM Basin. If bottomland hardwood riparian forests
are included as “wetlands” in this management strategy,
the area required may be more; our analysis has shown
that bottomland hardwood forests generally retain less
nitrogen per unit area than wetlands do (Mitsch et al.,
2001). This ecologically engineered nitrogen reduction
of the MOM basin, combined with a 20% nitrate reduction we estimated that could be done by appropriate
agronomic practices (Mitsch et al., 2001) would assure
a sufficient reduction in nitrates entering the Gulf of
Mexico to ensure a decrease in the size of the Gulf of
Mexico hypoxia.
The restoration in agricultural areas might involve
a combination of wetlands intercepting surface drain
tiles and bottomland hardwood forests along streams
and rivers intercepting subsurface drainage. Along
river systems, natural and constructed backwaters can
be reconnected to the river system in a manner similar
to the created oxbow on the Olentangy River in Ohio,
but on a larger scale and at many locations. An illustration of a potential river diversion site along the lower
Mississippi River is shown in Fig. 9.
64
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
Fig. 9. Aerial photo showing pattern of floodplain geomorphology along the lower Mississippi River. These types of wetlands could be restored
to river diversion flooding with a minimal effort.
Our recommendation of setting aside less than 1%
of the Mississippi River Basin is an ecological solution
to the Gulf of Mexico hypoxia and provides a reasonable complement to a major reduction in fertilizer use
that could reduce agricultural output and cause an eco-
nomic impact in the Basin. Doering et al. (1999) agreed
with this assessment when they found that restoring
2 million ha of wetlands in the Mississippi River Basin
would have minimal impact on agricultural production
in the basin. Furthermore, they argued that if nitrogen
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
fertilizer use is restricted within the Mississippi River
Basin, then the crop production and hence nitrogen
pollution would simply be transferred to somewhere
else and some other coastal system might be affected.
Our solution to the nitrate problem prevents that transfer of pollution to another watershed and maintains
agricultural production in the MOM Basin.
This ecological restoration would return the MOM
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 hierarchical series of pulses that include tides, storms, and
floods as they interact with biological, chemical, and
geological processes (Day et al., 1995, 1997, 2000b).
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.
4.1. Local benefits of MOM restoration
One of the features that became apparent soon
after discussion began on solving the Gulf of Mexico
hypoxia with wetlands in the MOM Basin, was the
fact that such an ecological restoration would have
dramatic benefits to the Midwestern USA as well.
These benefits are summarized below.
4.1.1. Public health and local water quality
Warnings occur from time to time in the newspapers
and television in late spring in several Midwestern
cities about the threat to public health posed by
high concentrations of nitrate–nitrogen in-streams
and rivers. The chemical is known to cause methemoglobinemia (“blue baby” disease that can cause
fatalities to babies <6 months old). Water supply
companies are required to notify their customers
when nitrate–nitrogen concentrations are greater than
65
10 mg N/L, a frequent occurrence in the “red zone”
area shown in Fig. 1 during the first major rain events
after fertilizers are applied to crops. Currently several
communities in Midwestern USA use expensive water
treatment processes to reduce the concentrations of
nitrate–nitrogen in drinking water and thus avoid these
warnings to the public. Other communities spend
resources removing nitrate–nitrogen from wastewater
to meet water quality standards. Creating wetlands to
prevent nitrate–nitrogen from being discharged into
streams or to remove nitrates that are already in the
rivers and streams could reduce the number of nitrate
water quality alerts in the Midwest and could save
towns and cities a great deal of chemical treatment
costs.
4.1.2. Habitat restoration
Most states in Midwestern USA have lost 80–90%
of their wetlands. Many farmers cultivate their farms
to the stream’s edge, leaving little room for forested
riparian buffers that would protect the stream as well as
provide some water quality improvement. 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 10 million acres (4 million ha) of wetlands
by the year 2010, largely through reconverting crop
and pastureland and modifying or removing existing water control structures”. The restoration and
creation of 2.2 million ha of wetlands in the MOM
basin would immediately bring the nation more than
halfway towards that national goal presented by the
NRC. Restored and created wetlands greatly enhance
wildlife benefits to the region. Hickman (1994) found
that the number of breeding bird species increased
from 37 to 54 at the Des Plaines River Wetland
Demonstration project after wetland creation. He also
found a four-fold increase in the number of waterfowl species after the wetlands were created. There
are now over 150 species of bird species identified at
the Olentangy River Wetland Research Park, many of
which came to this urban area after the wetlands were
constructed.
4.1.3. Flood control
An additional advantage of creating and restoring
wetlands in the Midwestern United States is the mitigation of floods. Hey and Phillipi (1995) estimated
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W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
that restoration of about 5 million ha 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 Upper
Mississippi River Basin in 1993. This potential benefit
of wetlands in the Mississippi River Basin needs more
research from flood control specialists but the concept
of wetlands contributing to flood control has been frequently documented (Mitsch and Gosselink, 2000).
4.1.4. Protection of agricultural production
The productivity of the American heartland would
be preserved if a small percentage of the landscape
were converted to wetlands. Using only a small percent
of the landscape to allow the rest of the landscape to
remain productive farmland might be the best tradeoff
that the agricultural community could possibly have.
If the ecological solution that we propose here is not
enacted and the Gulf hypoxia continues to grow worse,
fertilizer limits and nitrate water quality standards and
discharge permits may eventually become policy.
5. Needed research
Funds have already been provided, through such
mechanisms as the USDA’s Farm Bill and Wetland
Reserve Program, to farmers and farming interests
to implement some conservation methods including
wetland creation and restoration. There are also several
initiatives to provide funds to farmers to recreate and
restore wetlands and riparian buffers in the Midwest
through the Conservation Reserve Enhancement Program (CREP). There are also many projects for river
diversion, flood management and wildlife protection
in the basin. However, few of these projects are being
monitored for their effectiveness and the research
base is inadequate to model and estimate the effects
of large-scale restoration of wetlands, riparian zones,
and river diversions with a high degree of certainty
to improve water quality in the Basin. Pilot- and
full-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 thoroughly investigate the uncertainties of these two general approaches (farm wetlands
and river diversion wetlands) for helping to solve a
major national pollution problem in the Basin and Gulf
of Mexico. Our findings will be applicable to help solve
inland and coastal pollution problems throughout the
USA. 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 MOM basin and Mississippi delta are needed to test alternative water quality
improvement strategies that use wetlands as permanent
sinks for nutrients. These projects will allow development of design parameters, confidence intervals, and
validated simulation ecosystem models to predict the
behavior of these wetlands. A scale of the research is
suggested here.
5.1. Farm wetlands
Ten to twenty full-scale, existing and new agricultural wetland demonstration projects should be located
throughout the Midwest and instrumented to study
agricultural runoff into wetlands in a variety of soil
conditions. Most of these sites should be in the Ohio
and Upper Mississippi “red zone” region shown in
Fig. 1. Studies of the use of wetlands for reducing
nitrogen and other pollutants 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 loadinguptake relationships. Similar studies could be carried out using riparian buffer forests for water quality
improvement. This will also provide flood control, sustainable forestry and wildlife habitat in these riverine
systems.
5.2. River diversions
River diversions are being carried out in the Mississippi River delta for coastal restoration. Such studies
should investigate the potential for managing these
diversion systems for pollutant reduction and coastal
fisheries enhancement. In addition, pilot and full-scale
studies are needed of diversions into riparian systems
along river channels throughout the MOM basin
similar to the created oxbow project in Ohio described
above. River diversions also need to be studied to
determine that they do not result in any unacceptable
water quality problems, such as eutrophication or
harmful algal blooms.
W.J. Mitsch, J.W. Day Jr. / Ecological Engineering 26 (2006) 55–69
5.3. Adaptive management research
These studies should use the same methodologies
and approaches at all sites to determine if our recommendations of using agricultural wetlands, riparian
ecosystems, and river diversions can be effective in
controlling nutrients to the Gulf before embarking on
a full-scale restoration in the Basin. Also, having multiple sites throughout the basin in different hydrologic,
soil, and nutrient conditions will provide more certainty regarding ecological engineering design criteria
for these systems.
6. Conclusions
• Ecologically engineered solutions should be the
focus for solving problems that are watershed-scale.
These solutions need to be sustainable and involve
enhancing ecological processes.
• The restoration of the MOM Basin and the Gulf
of Mexico will require a combination of agronomic
and ecological techniques and strong collaboration
among all stakeholders.
• Restoration of 2.2 million ha of wetlands is needed
in the MOM basin to reduce the nitrogen load to the
Gulf of Mexico sufficiently to ensure a reduction in
the size of the hypoxia.
• Agronomic techniques, by themselves, are neither
politically feasible, nor ecologically sufficient in
scale to cause a significant reduction in the size of
the hypoxic zone in the Gulf of Mexico.
• The benefits of wetland restoration in the MOM
Basin, of and by themselves, are sufficient reasons
for this large-scale restoration.
• Our eco-solution deals with three main causes of
the problem: wetland/habitat loss; major drainage
networks and hydrologic disruption; and excessive
fertilizer use.
• Our eco-solution rehabilitates the natural functioning of riverine, wetland, and deltaic ecosystems.
• Some large-scale riparian restoration projects are
beginning in the MOM basin and are supported by
U.S. Department of Agriculture. These projects need
comprehensive monitoring to determine their effectiveness and applicability to other regions of the
basin.
67
• A major, interdisciplinary research program, as a
lead-in to the actual restoration of MOM, needs to
take place with sufficient funding, study sites, and
time to reduce remaining uncertainties about the efficacy of wetlands rebuilt in the MOM Basin to solve
the coastal hypoxia problems in the Gulf of Mexico.
Acknowledgements
We appreciate support from the CREST Program at
Louisiana State University. Support for this paper was
also provided by the Louisiana Department of Natural Resources, the U.S. Army Corps of Engineers, the
U.S. Geological Survey, the U.S. Department of Agriculture’s Nutrient Science and NRI Programs, the U.S.
EPA Watersheds and Gulf Coast 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. Li Zhang,
Rob Lane, and Chris Anderson assisted with some
of the details in the paper. Olentangy River Wetland
Research Park Publication 06-001.
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