Field Notes
Field Notes

The right wetlands can work wonders for clean water

Thursday, December 20, 2018
Posted by
Greg Seitz

Determining which ones hold the most runoff provides a chance to target restoration efforts.

A restored wetland in southern Minnesota

A wetland showing its value as wildlife habitat, in addition to protecting water. This site in southwestern Minnesota was restored with federal funds from the Conservation Reserve Enhancement Program. (Photo courtesy MN Board of Soil and Water Resources)

Not all wetlands are created equal, but they all soak up a lot of water. This is important where big rainfalls cause big problems, like eroding river banks and flooding.

Eroding banks are a major problem on some rivers in southern Minnesota. When the amount of water flowing downstream increases, it slowly widens the channel, washing away many tons of soil in the process.

This sediment muddies the water, buries streambeds, carries nutrients, and is even filling in Lake Pepin on the Mississippi River.

A gentler journey

Different types of wetlands found in the project area. (Source: BWSR Wetland Restoration Guide)

Different types of wetlands found in the project area. (Source: BWSR Wetland Restoration Guide)

Wetlands hold runoff and slowly release it, reducing river flows downstream. The water either soaks into the groundwater or evaporates, or continues a more gentle journey than unrestrained runoff. Often wetlands go through a cycle of filling with stormwater, slowly spilling downstream, and refilling again after the next rain.

A big question that remains unanswered is just how much water a wetland can hold, and what that means downstream.

That depends on several factors — a big one is the topography. The shape of the landscape often determines how much water, if any, flows out of the wetland and at what speed.  Water in southern Minnesota often flows from one wetland to another and another before reaching a river.

Understanding how wetlands work and which ones hold the most water is critical, because conservation and restoration can be costly. Keeping existing wetlands may also soon become more difficult, as some are set to lose protection under the federal Clean Water Act. Earlier this month, the Environmental Protection Agency removed short-lived and unconnected waters from federal regulation. 

Pushing the data

Minnesota's National Wetland Inventory Update makes it possible to see the many different types of wetland that can be found across a landscape.

Minnesota's Department of Natural Resources is completing a fascinating database of almost every wetland within Minnesota's borders, including their size, depth, plant types, groundwater connections, and more — but a key missing piece of information is a wetland’s ability to hold water.

Computer and mapping whiz Jason Ulrich of the St. Croix Watershed Research Station has been working on that problem with colleagues at the DNR.

“We’re pushing the data we have to give us more information about how wetlands function,” he says. “This way we can evaluate each one based on how they affect the whole watershed.”

By using high-resolution topographic maps created by laser-equipped satellites or aircraft, combining it with existing data, and running a lot of mathematical formulas, the team is able to show on a map where water is stored in any given landscape, how much is stored, and for how long.

Variable value

This map shows wetlands in the Maple River headwaters watershed, which ultimately drains into the Le Sueur River, to the Minnesota River, to the Mississippi. The dark blue areas are the wetlands that hold almost all the water that flows into them; red areas hold almost none.

The work has already shown that a wetland in southern Minnesota can capture anywhere from zero to one hundred percent of the rain that flows in from a big storm. That’s a broad spectrum. So to slow down runoff, and protect clean water, it’s important to figure out which wetlands will hold the most rainfall and snowmelt.

“We want to be able to say that if we increase wetland storage in a watershed by a certain amount, it will reduce flows in a given stream by a certain amount,” Ulrich says.

The project will also make it possible to find places where natural wetlands have been drained in the past, which are often excellent candidates for restoration.

By studying how water flows in and out of wetlands, the scientists seek to guide strategic conservation and restoration efforts, figuring out what is needed to capture runoff and protect lakes and rivers.

The work is expected to be completed in the summer of 2019.

Funding for this research was provided to the Department of Natural Resources by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative‐Citizen Commission on Minnesota Resources (LCCMR).

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