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MINNEAPOLIS — Renville County must conduct an environmental review to determine whether a proposed drainage ditch improvement could harm one of the last free-flowing creeks in the heavily agricultural western county, the Minnesota Court of Appeals ruled on Monday.

The appeals court reversed a decision by the county board, which concluded that the study wasn’t necessary. The Minnesota Center for Environmental Advocacy, which challenged the county’s decision, said the court’s ruling sets a precedent that means any drainage project across the state that affects public water will require an environmental review.

The group said the decision “extends protections to hundreds of miles of rivers and streams that meet the legal definition of ‘public waters,’ but aren’t explicitly listed” on the official inventory kept by the state Department of Natural Resources.

The dispute involves proposed improvements to a century-old ditch that empties into the upper reaches of Limbo Creek. Backers say the project would lead to better drainage of farmland in the area. The Renville County Board declined in 2020 to order a review known as an environmental assessment worksheet because the creek’s upper reaches didn’t appear on the official list.

But the Court of Appeals ruled that upper Limbo Creek nevertheless meets the state’s definition of public waters, so the environmental review is mandatory.

Limbo Creek has a watershed comprising over 14 square miles in Renville County. It flows directly into the Minnesota River, which has long been a major contributor of silt and farm chemicals to the Mississippi River. So, according to the environmental group, an increase in stream flows as a result of the ditch project could potentially cause increased erosion and contribute to pollution as far downstream as the Gulf of Mexico.

“While Limbo Creek is just one of the tributaries that feed the Minnesota River, the cumulative impact of hundreds of ditches funneling the water from hundreds of thousands of acres of tiled farmland has transformed the entire Minnesota River Valley,” the center said in a statement.

The county had argued that the creek’s upper reaches were just a ditch created by farmers in 1918. But the DNR sided with the environmentalists and informed the county in 2020 that the upper creek should have been classified as a public water all along — and that it was in the process of adding it to the official list. State law defines public waters as “natural and altered watercourses with a total drainage area greater than two square miles.” The appeals court said the evidence in the record shows that the upper creek is a natural watercourse, and that the county “unreasonably disregarded” evidence that it met the definition of a public water.

State law requires an environmental assessment worksheet for projects that will “change or diminish the course, current, or cross-section of one acre or more of any public water,” the appeals court noted. The worksheet is meant to be a brief document that sets out the basic facts necessary to determine whether a broader review known as an environmental impact statement is required.