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Wetlands and river floodplains can reduce some flood effects by temporarily storing water, and they can improve downstream water quality when floodwater spreads across connected land. Their benefits are conditional: storage is finite, nutrient processing varies by site, and levees, dams, channelization, and altered sediment supplies can limit river-floodplain exchange. The best quantified examples cited here are from Iowa and Wisconsin in the neighboring Upper Mississippi basin—not measurements of Great Plains river projects.
How wetland storage slows floods
Wetlands and low floodplain depressions can hold rainfall, snowmelt, and water that spills out of a river. That temporary storage can slow runoff and delay some water from reaching a channel at the same time as other flows. The effect depends on how much storage is available before and during a flood, how readily water can reach the wetland, and the size and duration of the event. The U.S. Environmental Protection Agency describes wetlands and floodplains as part of the connected river system that can help mitigate flooding (EPA, “Mitigate Flooding”).
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A USGS account of the 1993 Great Midwest flood reports that a 1994 Scientific Assessment and Strategy Team analysis modeled upland wetlands as reducing flooding by 9–23% for a one-year event, compared with 5–10% for a 100-year event. These are results from that historical analysis, not a general performance rate for wetlands or a measurement for all Great Plains rivers. In the 1993 event, available wetland storage was exceeded, and areas that usually did not contribute runoff did so. The example shows why wetlands can moderate some floods without preventing major ones (USGS, “Effects of the Great Midwest Flood of 1993 on Wetlands”; USGS, “Wetland Functions, Values, and Assessment”).
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When floodwater spreads over a floodplain, its velocity can fall. Suspended sediment may settle out, and nutrients attached to those particles can settle with it. Wetland plants, soils, and microorganisms also influence nutrient cycling. The EPA’s review describes riparian wetlands and floodplains as physically, chemically, and biologically connected to rivers and identifies their role in downstream water quality (EPA, “Connectivity of Streams and Wetlands To Downstream Waters: A Review and Synthesis of the Scientific Evidence”).
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Nitrate can be removed through denitrification
In saturated, oxygen-poor wetland soils, denitrifying microbes can convert nitrate into nitrogen gas, removing some nitrogen from the water system. This is different from simply storing nutrient-bearing sediment: deposited material may later be disturbed or nutrients may be released, whereas denitrification can remove nitrate from the aquatic system. Its rate depends on local conditions, so a value measured at one site should not be applied to another.
Phosphorus may be retained or released
Phosphorus can settle with sediment or be retained in soils, but retention is not necessarily permanent. Soil saturation and phosphorus concentrations affect whether a wetland acts as a phosphorus sink or can release phosphorus. The USGS Maquoketa River floodplain work examined both nutrient retention and conditions associated with phosphorus retention or release (USGS, “Maquoketa River Floodplain-River Connectivity 2014–2016 Data”).
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What neighboring-basin measurements show—and do not show
USGS studies in the Upper Mississippi basin provide measured examples of the mechanisms, but they are not Great Plains river-wetland estimates. The figures below refer to specific Iowa and Wisconsin study sites and should not be treated as universal rates.
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| Study site and source | Reported result | How to interpret it |
|---|---|---|
| Maquoketa River floodplain, Iowa; USGS study covering 2013–2018, published 2022 (USGS study) | For the only inundation event in the study period long enough to leave quantifiable sediment, the deposited sediment amount was equivalent to 0.91% of nitrate load and 3.8% of phosphorus load. Potential denitrification ranged from 250 to 668 kilograms of nitrogen per day. | The sediment-deposition equivalents are for that event, not whole-system nutrient-removal rates. Denitrification figures are potential rates at this Iowa site, not measured Great Plains rates. |
| Halfway Creek marsh complex, Upper Mississippi basin; USGS study published 2013 (USGS study) | Average retention over the study was approximately 30 Mg sediment, 26 kg total nitrogen, and 20 kg total phosphorus per hectare per year. | These are averages for the studied marsh complex and period, not a forecast for another wetland or a Great Plains river. |
The Maquoketa work also covered a 93-hectare Iowa floodplain parcel, quantifying flood-transported sediment, carbon, nitrogen, and phosphorus alongside denitrification and factors associated with phosphorus behavior. Its results illustrate that both connectivity and site conditions shape what a floodplain retains or processes (USGS Scientific Investigations Report 2022–5030).
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Why effects depend on flood size, connection, and sediment
- Available capacity: Storage can delay water only while space remains. Large or prolonged floods can fill wetlands and floodplain depressions.
- Hydrologic connection: Water must be able to reach the floodplain for overbank settling and other river-wetland processes to occur. Levees and disconnected channels can reduce those opportunities.
- Inundation duration: Floodwater needs sufficient contact with floodplain surfaces for measurable sediment deposition and for some nutrient processes to occur.
- Soil and water conditions: These influence denitrification and whether phosphorus is retained or released.
- Sediment supply: Wetlands can trap excess sediment, but heavy agricultural sediment inputs can fill prairie wetlands, shortening their useful life and impairing function (USGS, “Sedimentation of prairie wetlands”).
- River engineering: Dams, levees, and channel modifications alter flows, floodplain access, and sediment delivery, changing both the potential benefits and the practical options for restoration.
What Missouri River management means for Great Plains restoration
The Missouri River is a relevant Great Plains example, but it has been heavily altered. USGS describes the Lower Missouri as trained into a fast, deep, single-thread channel with wing dikes and revetments, while levees disconnect portions of the river from its floodplain. Reservoir regulation has changed the river’s hydrograph, sediment loads, temperature regime, and nutrient budgets (USGS, “River-Corridor Habitat Dynamics, Lower Missouri River”).
Those changes constrain restoration. USGS analysis of the Lower Missouri explains that in incising river segments, floodplain reconnection may require flows beyond operational limits; in aggrading reaches, reconnection can put lower-lying farmland at risk of inundation. The feasibility and consequences therefore depend on the particular reach, sediment regime, and flow management—not simply on whether a wetland can be restored in isolation (USGS/GSA, “Sediment regime constraints on river restoration – An example from the lower Missouri river”; USGS Circular 1375, “Missouri River Basin studies: sediment, nutrients, and the river system”).
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The available evidence supports the mechanisms—temporary water storage, sediment settling, and nutrient processing—and documents the Missouri basin’s altered river-floodplain connection. It does not establish a single measured flood-peak reduction and water-quality improvement applicable to Great Plains wetland restoration projects. Project outcomes must be evaluated for the site and river reach in question.
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- Each map is oriented so the river flows upward on the page
- No more turning your map upside down or twisting your head to read it
- Printed on water-resistant paper specially made for all-weather use
- The maps measure 8-1/2 x 14 inches much larger than most river guide books
- Spiral binding and stiff covers that make them lay flat and easy to use
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