Dead trees line a channel in the Reno Bottoms in southern Wisconsin.

Dead trees line a channel in the Reno Bottoms in southern Wisconsin. Richard Mertens

As Flooding Increases on the Mississippi, Forests Are Drowning

Ever-worsening floods are killing trees at an increasing rate along the upper Mississippi River, and invasive grasses are taking over. The Army Corps of Engineers has launched a project to restore forest and boost tree diversity, and to improve habitat for fish and birds, too.

Mike Valley saw 10 years ago that the trees were dying. A fourth-generation commercial fisherman on the upper Mississippi River, Valley, 63, had fished for half a century among the backwater channels, sloughs, oxbow lakes, islands, marshes, and floodplain forests that lie between the high sandstone bluffs on both sides of the river at Prairie du Chien, Wisconsin, where he lives.

Come spring, Valley recalled, more and more trees failed to leaf out. In patches up and down the river, many floodplain species, mostly silver maple but also swamp white oak, cottonwood, green ash, and elm, were turning gaunt and lifeless. The die-off began to hurt his fishing. In high water he would fish for catfish on the flooded islands, but now there was too much downed wood to set his nets. And the danger of falling trees became a constant worry.

“It got progressively worse and worse,” he said. “It’s just devastating.”

In recent years scientists have been studying the causes of, and in some cases working to slow, a trend that today even the most casual observer can see: All along the upper Mississippi River, floodplain forests are dying. From St. Paul to St. Louis, trees are disappearing and in some places being replaced by thick stands of invasive reed canary grass. Since scientists began keeping track of the changes in the late 1980s, thousands of acres of floodplain forests have been lost. In recent years the rate of decline has only accelerated.

“The magnitude is mind-blowing,” says a forester. “You just don’t see hundreds of acres of dead trees.”

By the time scientists began monitoring the upper Mississippi floodplain forests, a combination of agriculture, urbanization, and river control efforts had destroyed nearly half the forest that had been present in 1891. They noticed worrisome trends, including increased flooding and lower tree density and diversity. Aerial photos consulted since 1989 revealed that the stretch from St. Paul to Rock Island, Illinois, a distance of 330 miles by car, lost about 6,200 acres of its bottomland forest. The stretch from Rock Island to St. Louis — 230 miles — lost 5,500 acres. Since then, the dying has only accelerated. Researchers are still analyzing more recent data, but photos show losses of 50 to 90 percent in some places.

“The magnitude of this is kind of mind-blowing,” said Andy Meier, a forester with the U.S. Army Corps of Engineers. “You just don’t see hundreds of acres of dead trees.”

Scientists say the main cause of overall tree mortality seems to be ever-worsening floods, linked with climate change, which according to a 2021 Frontiers in Water paper, is projected to increase spring precipitation and precipitation intensity across the Midwest through the end of this century. Already, trees that can easily tolerate a few weeks of spring flooding are inundated for months at a time.

Silver maples in the upper Mississippi River floodplain, Fort Snelling State Park, Minnesota.

Silver maples in the upper Mississippi River floodplain, Fort Snelling State Park, Minnesota. Dominique Braud / Dembinsky Photo Associates / Alamy Stock Photo

The problem is not just that trees are dying, but that in many places they aren’t growing back, said Nathan De Jager, an ecologist with the U.S. Geological Survey. For reasons that scientists still don’t fully understand, even areas with living trees sometimes have no sprouts and seedlings growing beneath. Another cause of forest decline is the increasing abundance of canary grass, which grows so thickly in marshes that nothing else can survive.

Meanwhile, changes in land use are making the problem worse. Across the Midwest, farmers are installing ever more tile drainage systems under their fields, making the land more productive for row crops, especially corn and soybeans. The systems also contribute to flooding, sending water quickly through pipes to nearby ditches and creeks.

Floodplain forests are known to be ecologically rich. They form long corridors that allow birds and other animals to move up and down the river. The Mississippi River Valley is also one of the great migratory flyways in North America, for waterfowl, raptors, and neotropical songbirds. But no one really knows how the changing forest structure might affect birds. More marshland and less forest may hurt some and help others. Tara Hohman, the conservation science manager for Audubon Minnesota, noted that red-headed woodpeckers have been thriving in the dying forests, while American redstarts, which prefer the shrubby understory of forests, are in decline. So are Carolina wrens.

The U.S. has lost between 57 and 95 percent of floodplain forests since European colonization.

Globally, floodplains are considered among the most threatened of all ecosystems. One estimate suggested that across the United States and Europe, as much as 90 percent of floodplain has been converted to farmland. Meanwhile, studies focusing on floodplain forests estimate that the U.S. has lost between 57 and 95 percent of those ecosystems since European settlers arrived.

Climate change is, of course, affecting floodplain forests around the world. Heavier rains and greater flooding across central and northern Europe are killing trees along the Rhine River, for example. But sometimes the problem is too little water — as in Italy, where oaks and black locusts are dying on the floodplains of the Ticino River, which is suffering from a long drought. Cottonwoods in the American West, where drought and declining snowfall in the mountains have starved rivers of water, are experiencing the same fate.

One of the largest expanses of intact floodplain forest in the U.S. can be found along the upper Mississippi in the Reno Bottoms, an 11-mile stretch upstream of Prairie du Chien. Here the Mississippi appears as it might have once looked, before the Corps began to build locks, dams, and levees: a wild patchwork of marshes, lakes, islands, backwater channels, and forests.

Andy Meier, a forester with the U.S. Army Corps of Engineers, in the Reno Bottoms.

Andy Meier, a forester with the U.S. Army Corps of Engineers, in the Reno Bottoms. Richard Mertens

The patches of dead trees — about a quarter of the forest, Meier estimates — are relatively new, however, and that’s why this area is a focus of restoration and rehabilitation. The efforts, by federal agencies and conservation groups, aim to improve habitat of all kinds, including backwaters where fish spawn and marshes visited by waterfowl.

Restoration work in the Reno Bottoms has been ongoing, at a small scale, for years. But recently the Corps and its partners have been putting the finishing touches on plans for a $37 million project that could start this year. Called the Reno Bottoms Habitat Rehabilitation and Enhancement Project, it calls for, among other things, dredging sediment that has filled overwintering habitat for fish, building new islands to replace those lost to erosion, and restoring, or improving, more than 500 acres of forest.

On a recent February morning, Meier and Jeff Butler, an ecologist with Audubon Minnesota’s Upper Mississippi River Program, met at the New Albin Landing, which sits within the project area on a backwater channel called the Minnesota Slough. The day was strangely warm for late winter, and the sun dazzled on open water. Geese gabbled and honked, while woodpeckers rapped against the dying trees.

“The reality is we aren’t going to restore or rehabilitate all the forest that’s been lost,” says a forester.

Scientists, like fishermen, have observed dying trees on the floodplain for many years. But a series of wet years and frequent high floods beginning in 2016 seemed to push the death toll much higher. Floodplain trees are tough. But Meier speculated that a couple years of heavy flooding, which deprives roots of oxygen, might have weakened them enough to finally kill them.

Not far from the New Albin landing, the two men crossed mats of dead rice cutgrass, a native wetland species, to a small clearing among the trees. Here, where the ground was slightly lower than the surrounding forest, the Corps plans to pile a foot or two of river sediment to raise the clearing’s elevation — enough, it hopes, to allow trees to grow. Some species, like cottonwoods, will colonize the site on their own. Others, like swamp white oaks, will need to be planted. The Corps also wants to plant young trees of different species in the surrounding forest, which still includes many living maples.

“We’re going to try to increase the diversity of the forest and the resiliency of the future forests,” Meier said. “We’re trying to get ahead of the game by getting other trees established.”

Volunteers plant young trees in a Mississippi floodplain in southeast Minnesota in 2019.

Volunteers plant young trees in a Mississippi floodplain in southeast Minnesota in 2019. Anthony Souffle / Star Tribune via Getty Images

Restoring floodplain trees is far from simple. Scientists are trying to understand which trees are most vulnerable to the new hydrological conditions; why, in many places, trees won’t grow back; and what kind of restoration strategies work best. So far, they have focused on stands that still contain healthy trees. Sites where all the trees have died may simply be too wet for restoration.

“The reality is we aren’t going to restore or rehabilitate all the forest that’s been lost,” Meier said. “The cost would be astronomical. And we don’t really know we could maintain it for the long run.”

Farther from the river, against the western bluffs, lie 300 acres of floodplain that show both the successes and shortcomings of restoration strategies. Most of the area was once rich farmland — fields that flooded regularly but still produced an occasional crop of hay. Abandoned in the 1970s, they were quickly invaded by canary grass. On one five-acre patch, workers cut the canary grass and stirred up the wet muck with their machines. Within a couple years, cottonwoods were sprouting up. But not many, and before long, the canary grass had smothered them.

A diverse forest resembles better the forests of the past. It’s also more likely to survive the future.

Still, the canary grass taught an important lesson. Cottonwoods reproduce freely in the floodplains, without human assistance. But they need bare soil for their seeds to germinate and to set their roots. In other experimental patches, workers first killed the canary grass with herbicides, then cleared the soil with tractor and disc. There, the cottonwoods are growing thickly, and the canary grass is in retreat.

The aim here, as elsewhere, is not only more trees, but more kinds of trees. In different places workers have planted cottonwoods, river birches, and swamp white oaks — a long-lived species and a favorite habitat for birds and insects. They have also planted shrubs like buttonwood and red osier dogwood. A diverse forest resembles better the forests of the past. But more importantly, Meier said, it’s more likely to survive the uncertainties of the future.

A hundred yards away, Butler and Meier stopped at a spot that seemed especially promising: a grove of trees where young swamp white oaks were thriving amid slightly older cottonwoods. Here, in 2015, Meier and 25 local schoolchildren spent a day planting 3,000 tiny bare-root oak seedlings. The oaks outcompeted the canary grass and outgrew all the children. They now stand 15 feet tall.

“This is what we’d love to see everywhere,” Meier said, clearly delighted. “We’ve gone from a reed canary grass monoculture to a young forest.” He paused and reflected. “When I first came here it felt like it was very difficult to do floodplain forest restoration,” he said. “But seeing this gives me hope.”