Northern Wilds Magazine
Points North

Points North: New Study Links Tile Drainage to River Degradation

By Shawn Perich

On the intensively farmed landscape of southern and western Minnesota, about the only natural habitat you’ll find is along waterways and river corridors. Even there, the heavy hand of humanity is evident. Rivers that once ran clear and teemed with native fish and mussels are now muddy, phosphate-laden flows infested with nonnative carp.

The deterioration of our river systems is well documented, from the writings of the first explorers to the satellite imagery of the present. We know, broadly speaking, our waterways have diminished as we’ve developed land within the watersheds; converting native prairies, marshes and oak savannahs to agricultural purposes. We also know the decades of land conversion coincide with an era of increased precipitation. There lies the rub. Big Ag and conservationists disagree whether more drainage or more rain wrecked the rivers.

New, peer-reviewed research from the Science Museum of Minnesota and several universities, headed by researcher Shawn P. Schlotter, adds significant context to this issue. Researchers compared changes in flow in 21 southern Minnesota rivers from 1940-2009. In about half of the rivers, flows increased significantly during this period, with some rivers nearly doubling in flow. The remaining rivers showed less or no change in flows. The flow increases were strongly correlated with changes in land use.

To more clearly understand the effects of climate and land use, researchers compared changes in precipitation, crop conversion and the extent of drained wetlands within the watershed, using a detailed analysis called a water budget. They found artificial drainage and wetland loss was the major driver of increased flow, exceeding the effects of land conversion and precipitation. The greatest increases were in watersheds with extensive networks of drain tiles and ditches.
As flows increase, so does the erosion along stream banks and channels. The study found stream channels widened significantly in rivers where extensive artificial drainage occurred. The erosion leads to greatly increased siltation downstream. In Lake Pepin on the Mississippi River, sediment cores show siltation has increased tenfold since the onset of modern agriculture. The contribution from non field sources—such as eroding stream banks—has especially increased in recent decades.

The agricultural conversion of Minnesota’s landscape occurred in stages, beginning with a patchwork of row crops, pasture and fallow ground and progressing to the industrial-scale row crop production of today. The biggest hydrological change was the conversion to growing corn and soybeans nearly to the exclusion of other crops. These two plants grow best in soil with low moisture content, which leads farmers to install drain tiles beneath their fields to get rid of excess water.

In the past, most of the water that fell as precipitation was returned to the atmosphere via evaporation and transpiration, meaning it passed through water bodies and vegetation. Drain tiling sidesteps this natural process by simply draining the water into the nearest ditch, which eventually empties into a river. Water that once was used by vegetation is now simply whisked away, adding to the flow of the receiving river. Those increased flows are carrying silt, polluted run-off and flood waters downstream. The Science Museum study is important because it quantifies artificial drainage, especially tiling, as the source of these longstanding problems. In that respect, the study could become a game-changer for improving the state’s drainage practices and policies.

“The industrial agriculture folks consistently complain they are not the problem and point to increased precipitation as the reason for increased flows,” says Whitney Clark, executive director of the Friends of the Mississippi River. “This study gives us good science to be very clear about what the problem is. Then we can devise smart policy to address it.”

It won’t be an easy task. Agricultural drainage was not addressed in the 40-year-old Clean Water Act. You don’t need a permit to install drain tile, nor is there regulatory process to guide tile installation. The present system of agricultural subsidies leaves farmers with little opportunity or incentive to try other practices. Right now, with corn garnering record prices, many farmers are investing their profits into drain tiles, creating a drainage boom.

Steve Morse, executive director of the Minnesota Environmental Partnership, says that in order to address tiling and drainage issues, conservationists must wrestle with an 800-pound gorilla—powerful agricultural interests. The new study undercuts Big Ag’s arguments that increasing stream flows can be blamed on greater rainfall or “solved” by leaving more crop residuals on the land.

“We’ve been focusing on the decoy rather than the ducks,”says Morse. “The real problem is the intensification of corn and soybean production and drain tiling.”

Minnesotans place a priority on improving water quality and, with the passage of the Legacy Amendment five years ago, they’ve created a source of money to do so. They are also aware of the extent of our pollution problems—40% of the state’s waterways are listed as impaired by the Minnesota Pollution Control Agency. Via the regulatory framework of the Clean Water Act, we’ve addressed most primary pollution sources, including point source pollution from factories, urban runoff and municipal wastewater treatment. The only major pollution source that remains to be addressed is agricultural runoff—and we are making little progress in doing so.

“It may come as an unpleasant surprise to Minnesota voters that we are taxing ourselves to address this problem, but we are making little progress in this area,” says Clark.

For Morse, the study results point in the direction the state needs to go in order to restore water quality in southern Minnesota rivers. And it prompts him to ask a bigger question; one a certain 800-pound gorilla surely doesn’t want to hear.

Asks Morse, “Can we support so much intensified row crop agriculture and the drain tiling that comes with it and protect our water quality?”

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