More water in Mississippi River leading to loss of floodplain forests over time
Published 6:03 pm Tuesday, August 30, 2022
By Kirsti Marohn, Minnesota Public Radio News
The trees on Goose Island, near the Minnesota-Wisconsin border, are used to floods.
Almost every spring, Mississippi River floodwaters cover the bottomland forest, then recede later in the summer. Silver maples and rare swamp white oak trees thrive, along with willow, cottonwood and ash.
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But when Andy Meier looks up at the trees, he sees signs of trouble.
“You can see a lot of these silver maple trees, they’ve got dead branches in the tops of the trees,” said Meier, a forester with the U.S. Army Corps of Engineers, based in La Crescent. “Those are trees that are stressed from flooding.”
Meier’s office manages thousands of acres of floodplain forest in the Mississippi River valley, from the Twin Cities down to Iowa, including on Goose Island south of La Crosse.
The dense trees, lush vegetation and backwater sloughs of this region provide critical habitat for migrating songbirds, sandhill cranes and other wildlife.
“There’s really nowhere else in the Upper Midwest where you have a 2-mile-wide floodplain with islands that have forest on them, and backwaters that are bordered by forest,” Meier said.
Floodplain forests provide another key benefit: They slow down the water and capture sediment and nutrients, reducing pollution downstream.
But in recent decades, this important ecosystem has been showing signs of stress from continuous high water.
“We’ve been kind of seeing slow tree loss over the last 30 or 40 years,” Meier said.
The catastrophic floods of 2019 were an extreme event that killed numerous trees. But many already were stressed.
“That was really the result of really three or four years of heavy summer flooding,” he said. “If it would have been just one year, they probably would have survived.”
Losing floodplain forests is one consequence of what researchers say is a concerning trend in the Upper Mississippi — higher flows that occur more frequently and last longer.
“There’s just more water in the river more of the time,” said Molly Van Appledorn, an ecologist with the U.S. Geological Survey’s Upper Midwest Environmental Sciences Center in La Crosse. “It’s just a wetter place out there, and that is a pretty systemic pattern.”
Van Appledorn helped produce a report released last month on the ecological health of the river from the Twin Cities down to south of St. Louis.
The report was prepared by the Upper Mississippi River Restoration program, a partnership of federal and state agencies and other groups. It analyzes almost 30 years of data on water quality, aquatic vegetation and fish.
The report says the most widespread change during that period is the increase in the amount of water flowing through the river system.
“We know our climate is changing,” Van Appledorn said. “We’re getting more precipitation. Rain events are getting more intense.”
Changing land use practices also are playing a role, she said, including an increase in paved surfaces and farm drainage systems that move water off the landscape more quickly.
Human efforts to control flooding by building levees and other structures also can increase the volume of water flowing downstream, Van Appledorn said.
“We can tell that there’s been shifts in the hydrology that we think are in there being really stressful on the trees,” she said.
When trees’ trunks and roots are submerged for a long time, they lose oxygen, stop photosynthesizing and eventually die.
“We’re losing a lot of trees,” said Megan Moore, a district manager for the Minnesota Department of Natural Resources. “You’ll see just these exposed root systems and trees falling into the water almost anywhere you look, from the Twin Cities through the Minnesota border.”
Protecting floodplain forests
There are efforts underway to protect floodplain forests. Meier said forestry managers are trying to figure out ways to naturally regenerate tree species that are more flood-tolerant, including cottonwood and willow.
And they’re attempting to control invasive species like reed canary grass, that can take over a forest and prevent seedlings from growing.
“Our primary objective is to maintain forest cover,” he said. “Then, we want to try to diversify that forest and make it into a more diverse habitat. But the first thing is to try to keep trees on the landscape.”
On Goose Island, there are some hopeful signs of resiliency. Meier pointed out a patch where some new willows are sprouting from dead stems.
“These three dry summers are enough that those willows are actually sprouting back up again, and putting out new leaves,” he said.
But climate scientists predict that the Midwest will be wetter in the future, which could make saving these trees a challenging task.
Meier said the future of the forest depends a lot on what happens in the next 10 to 15 years. If they are drier years, the surviving trees likely will fill in and recreate the canopy that used to exist.
But if those years are wetter, the trees probably will die, and the area will convert to more of a marsh habitat, he said.
Meier said conservation officials need to weigh the pros and cons of such changes, and where they should focus their efforts.
“Are we losing quality habitat? Or are we just shifting to another quality habitat?” he asked. “We want to maintain forests wherever we can, but we sometimes need to think about that … Is it alright if this converts to something else, so that we can focus on another area that we’ve got a better chance of saving?”
The Upper Mississippi report does include some positive trends. The amount of suspended solids — tiny particles of sand, soil or other matter — declined, leading to an improvement in water clarity.
“Then you see a rebound in vegetation with that clearer water,” said Kirsten Wallace, executive director the Upper Mississippi River Basin Association. “That traps sediment, makes the water even more clear. It’s kind of a rebound effect.”
However, concentrations of nutrients such as nitrogen and phosphorus in the Upper Mississippi system remain high, exceeding federal benchmarks.
The report also says the Upper Mississippi continues to support diverse and abundant fish populations, although invasive carp are taking over in lower stretches of the river. They have not yet established a reproducing population in Minnesota.