Tiny particles, costly problem: Too much sediment in Upper Mississippi River

The Upper Mississippi River is known for its pristine water quality near its headwaters in north-central Minnesota. 

But just downstream, too much sediment — small particles of sand, silt and clay — is a problem for the iconic river. 

A new study released this week reports that significant efforts to reduce sediment will be needed for the Mississippi’s upper reaches to meet state water quality standards.

The Minnesota Pollution Control Agency issued the draft report, which focuses on roughly 150 miles of the Mississippi, from the point where it’s joined by the Swan River near Grand Rapids, to its confluence with the Crow Wing River near Brainerd.

That stretch of the Mississippi, as it meanders through Aitkin and Crow Wing counties, is on the state’s list of impaired waters because of the amount of suspended solids, or sediment, in the water.

A major culprit behind the sediment problem: The region around the river’s upper reaches is made up of fine soils that erode easily, said Meghan Funke, a water resources engineer with EOR, a consulting firm that helped draft the report. 

But there are human-created causes, too. The Upper Mississippi has several dams, which store water in reservoirs and release it throughout the summer to control floods. That means the river tends to have more water flowing between its banks throughout the year, not just during the spring snow melt, Funke said.

“Because the water levels are higher, there’s just more chance for bank erosion,” she said. As banks erode, more of that soil ends up downstream.

Forests and other deep-rooted plants within the river’s buffers help slow runoff and reduce erosion, Funke said. But over time, people have cleared the forests and vegetation for farming and development, exposing the soil and leaving it less stable.

Historically, landowners also created ditches to drain peat land for farming, which carry water to the river more quickly, said Anna Bosch, watershed project manager with the MPCA.

“All of those ditches really contribute a lot of extra water into the river every time there's a rain event,” Bosch said. “That sort of concentrates the flows and increases the amount of sediment that then gets into the system.”

Sediment doesn’t pose a health threat for people, but it can increase water treatment costs for communities that get their drinking water from the river, Bosch said. Soil particles also can carry other pollutants, such as phosphorus and nitrogen, that can affect the river’s health.

And excess sediment can have an impact on the fish and bugs that live in the river, Bosch said. It can bury the riverbeds that fish use for spawning and reduce the amount of light that penetrates the water, depriving aquatic plants of sunlight.

In addition, soil particles absorb warmth from the sun, increasing the water temperature in streams and rivers.

A challenging fix

The draft report is a follow-up to a 2017 MPCA study, which identified excess sediment as a problem in this stretch of the Upper Mississippi.

The reviews are part of the state’s efforts to document the health of all 80 major watersheds in the state. The Upper Mississippi was one of the first large rivers in the state to be assessed.

The federal Clean Water Act requires the state to identify the maximum amount of a pollutant that a lake or river can take on and still meet water quality standards. 

The draft report says sediment in the Mississippi needs to be reduced by about 60 percent from the Swan River near Grand Rapids to the Pine River, and by 25 percent from the Pine River to the Crow Wing River near Brainerd.

That’s a lofty goal, especially since most of the sources of the sediment aren’t regulated by government. Wastewater and stormwater systems — which do require a government permit — contribute only a small fraction of the sediment in the river, Funke said.

It’s trickier to get at the major sources of sediment cited in the report, including increased water flow from dams and ditches, livestock trampling the riverbanks and people replacing forests with development or shallow-rooted lawns and crops.

Protecting a buffer of forest and vegetated land through conservation easements, the report says, is critical to the efforts to improve the river’s water quality. 

That’s already happening, to some degree. In one case, the Mississippi Headwaters Habitat Corridor Project, launched in 2016 by the Mississippi Headwaters Board, has preserved more than 38 miles of shoreline along the Upper Mississippi.

Nonprofit conservation groups are working together to protect the forestland in the region from being converted to farms or development, said Todd Holman, Mississippi headwaters program director for the nonprofit Nature Conservancy’s Brainerd office.

One of the challenges in trying to reduce erosion is the heavy rainfalls that are occurring more often than in the past, Holman said.

“It’s just very difficult, because we can’t control the frequency and the volume of those rain events,” he said. “But we can work to armor the shore by identifying the most important properties to keep forested, to keep in a natural state.”

The MPCA report suggests other strategies for reducing erosion, including working with landowners to plant buffers of trees and vegetation, to keep livestock away from the river banks. The total cost of all these efforts is estimated at more than $17 million.

The public has until Sept. 16 to comment on the findings of the draft report, which will then be submitted to the U.S. Environmental Protection Agency for approval.

Bosch said Minnesotans have a responsibility to take care of the Mississippi headwaters region for the rest of the country that uses it downstream. 

“Everything that we can do to protect it benefits ourselves and everyone in the rest of the basin,” she said.

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