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CLAYTON, Iowa—The Sny Magill Unit of Effigy Mounds National Monument is a hidden wonder.

A dozen miles downstream from the park’s visitor center along the Mississippi River, the path starts with a turn you might miss if you’re not looking closely. Follow that path under a railroad bridge to a boat landing, then go by foot through the woods until the floodplain opens out flat in front of you, revealing more than 100 sacred mounds built by Native Americans thousands of years ago.

These ceremonial and burial mounds are one of the densest collections still existing in North America. It’s clear the people who built them had a special connection to the river valley cradled between the bluffs of the Driftless region, and wanted to add their own features to it, said park superintendent Susan Snow.

Today, though, that river has significantly eroded the bank they built on, eating away at some of the mounds at the water’s edge.

It’s a product both of climate change, which is causing wetter conditions across the upper Midwest, and engineered alterations to the river’s flow. There’s now an urgent need to protect the mounds from further damage, Snow said. A multimillion-dollar bank stabilization project proposed by the U.S. Army Corps of Engineers could accomplish that.

Since mounds should not be rebuilt by modern hands, once they’re gone, they’re gone, said Sunshine Thomas Bear, tribal historic preservation officer for the Winnebago Nation of Nebraska, who are descended from the mound builders.

“All we can do is try to save what we can,” she said.

Fast-flowing Mississippi River causing mound erosion

Nineteen tribal nations are affiliated with the mounds that make up the Sny Magill Unit, including the Ho-Chunk Nation, which has a strong presence in Wisconsin.

“The area itself is part of our homeland,” Bear said. “Our connection to these lands goes back thousands of years.”

Bear said the area around Effigy Mounds National Monument used to have more ancient Indigenous mounds, but many were destroyed in the last 150 years by developers as towns were built. And many other mounds were destroyed by amateur archeologists in the last century who desecrated the burial mounds and stole artifacts and human remains.

Most of the approximately 106 mounds that are part of the Sny Magill Unit are conical — or round — which are likely burial mounds, said Sheila Oberreuter, the park’s museum technician. Others are effigy mounds taking the shapes of birds and bears. It’s likely that ancient peoples returned to the area for hundreds, if not thousands, of years for mound building during the Woodland period, Oberreuter said, which occurred between 2,500 and 900 years ago.

Because it is low-lying, the land on which the mounds were built floods seasonally when the Mississippi floods. Sometimes, the mounds themselves are completely underwater, Oberreuter said — something that would seem unbelievable while walking among them, if not for visible high-water marks on nearby trees.

The serene backwater adjacent to the mounds is connected to the Mississippi River’s main channel by Johnson Slough. In recent decades, more water has rushed through the slough and hit the river bank, which Snow estimated has eroded the bank by five to 10 feet since the 1940s.

That’s happening in part because of the construction of the lock and dam system on the upper Mississippi River during the 1930s, which transformed the way the river ran to make shipping easier. By converting the free-flowing river into a series of pools, the lock and dam system causes consistent high water levels in some areas. On top of that, heavier rainfall and more severe, longer-lasting flooding events driven by climate change caused more water to move through the upper Mississippi in the last few decades.

Notes from park staff as early as the 1980s mention mound erosion, Snow said, with the first project proposed to stop it in 1994. Wooden support beams were placed along the bank, but were washed out. Reinforcing those beams didn’t work either. In 2022, large logs made of coconut fiber were placed along the parts of the bank experiencing the worst erosion. The following spring, the river saw near-record flooding, and many of those logs were swept from the bank immediately.

Army Corps project would stabilize bank with 2,000-foot rock berm

As park staff considered a more permanent solution, they were approached by the U.S. Army Corps of Engineers, which has managed the Mississippi River for decades and recently unlocked a new pool of money that funds ecosystem improvements along the river in addition to improvements to navigation for shipping.

The Navigation and Ecosystem Sustainability Program, or NESP, as it’s commonly called, also supports the protection of cultural resources along the river, said Jill Bathke, lead planner of the program. The Sny Magill project would be the first to access it for that protection.

After consulting with tribal officials, the Army Corps put forth a proposed fix: a 2,000-foot long berm the height of the floodplain, made of large rocks. They’ll place sand scraped out of the main channel behind the rock wall as an added barrier between the water and the mounds. The berm would be designed with current and future climate conditions in mind, Bathke said, a long-term solution to stop the erosion.

Bear and other members of her tribe are serving as consultants on the project, as is William Quackenbush, the tribal historic preservation officer for the Ho-Chunk Nation in Wisconsin, and his tribe. They also lead teams of volunteers to help care for the mounds, which includes removing invasive European plants and replacing them with native plants that reduce soil erosion.

Some are skeptical of this manmade solution to a manmade problem. There are some tribal partners who’ve expressed that the river should be allowed to keep flowing as it wants to, Oberreuter said. Snow also acknowledged that people have been hesitant about making such a change to the natural bank.

But, she pointed out, “The bank is (already) no longer what it was.”

Construction of the rock berm should begin in 2026. As they build, they’ll have to take care not to harm a population of federally protected freshwater mussels that live buried in the sand at the river bottom. The U.S. Fish and Wildlife Service, which manages the land around the Sny Magill Unit and Johnson Slough as part of the Upper Mississippi River National Wildlife and Fish Refuge, will help with that.

When the berm is complete, Snow said, there’ll be a trail atop it that visitors can walk. That may help protect the mounds better than the current way to see them, which is to walk among them, she said.

The Sny Magill Unit has been part of Effigy Mounds National Monument since 1962, Snow said, but it’s not advertised like the rest of the park. That’s in part because there are no staff stationed there to properly guide people through the mounds. But if people visit respectfully, she believes it’s one of the best places to take in the mounds because it’s on a flat, walkable surface, unlike the rest of the park, which is on a blufftop.

For Bear, that education is key to the mounds’ survival. She believes many of those who visit leave with a better understanding of the mounds, and why they need to be protected.

This story is a product of the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri in partnership with Report for America, with major funding from the Walton Family Foundation. Republish stories like this one for free.