MANKATO — The increase in field drainage systems across southern Minnesota has caused a dramatic increase in the flow of water in the Minnesota River and other tributaries, leading to bank erosion and a widening of the channel that pollutes water and is causing a rapid fill-in of Lake Pepin on the Mississippi River.
That’s the conclusion of a three-year study published last week by a team of researchers at the Science Museum of Minnesota and several major universities.
“Any type of drainage that reduces the time water is on the landscape will increase river flow,” said Shawn Schottler, senior scientist at the St. Croix Watershed Research Station of the Science Museum of Minnesota.
Schottler said they were surprised to find it wasn’t simply the fact that drainage moves more rain water to rivers quickly, but that the advanced drainage systems have significantly reduced the amount of evaporation.
Historically, 70 to 80 percent of all rain that falls on southern Minnesota evaporates into the atmosphere, either through plants or from sitting water.
“Drainage takes water that would have evaporated and sends it to the river,” Schottler said.
There are critics of the study.
Leon Schoenrock, a farmer in Waseca County and former president of the Waseca County Soybean Association, read the study and said he isn’t convinced.
“I think even in their own study they say they don’t necessarily know all the answers yet,” he said.
“There’s a perception out there that farm drainage is causing this problem. But all the low wetlands were drained in the ’40s and ’50s and that may have caused a problem in the rivers then. But studies show that when you drain high ground, it acts like a sponge and reduces water coming off.”
Schoenrock said he believes the increased rainfall and more big rain events are more of a problem. “Those big events cause flooding.”
He said most tile systems are designed to take one-half inch of water off the landscape per day. “The tile lines meter the water, slow it down compared to it running off the landscape.”
The researchers say one thing they still want to determine is what will happen when virtually all farmland has modern drainage systems installed. “The Blue Earth River basin is already close to that,” Schottler said.
He said that when tiling is complete, rivers will eventually widen to a point that will become their new normal width.
“At some point the river flows won’t increase anymore and they won’t widen the rivers more, and there will be a reduction in sediment. We need to find out when that will happen and what the sediment loads will be at that time — will they be the same as before drainage or still higher than before drainage started.”
The study looked at three things that could add to river flow: drainage, changes to the landscape, and changes in the amount of precipitation over time.
Schottler said some increase in precipitation doesn’t nearly account for the added water going into rivers.
He said that while the increased flow from drainage is clear, they do want to more closely study the effect of more soybeans being planted in the state. The amount of farm land in corn has remained virtually unchanged since 1940, but soybeans — which were almost nonexistent before 1940 — have grown dramatically to the point they are now about half of the crop acres.
“Soybeans replaced alfalfa, oats, rye, pasture — all those crops that are green and growing in April are now are gone and have been replaced by soybeans.”
Schottler, one of a team of scientists who has been working on the drainage studies in recent years, said that despite some skeptics, he’s been pleasantly surprised that the basic thesis that drainage sends more water more quickly to the rivers, is being fairly well accepted.
“I talk to a lot of farm groups and they don’t really challenge that. They understand that if they have land that’s wet and soggy for three weeks and then you put in drainage and it’s dry in a couple of days — they know that water’s going to the rivers. That’s why you do drainage, to get the water out.”
The discussions and studies over sediment in the river is more than academic. For years, the federal government has been requiring states to do detailed monitoring of sediment and other pollutants in all stretches of rivers. When rivers don’t meet standards, they are placed on an “impaired waters” list. At some point, federal regulators may require states to bring those rivers within standards — something that could take dramatic action to achieve.
There is also growing momentum by Twin Cities policymakers and those living along the Mississippi to push for stricter regulations to slow the sediment coming out of the Minnesota River — sediment that is filling in Lake Pepin.
With over half the state’s population — and state lawmakers — living in those areas, there could be legislative action forcing more restrictions on those living in the Minnesota River Basin.
So far, few good options exist for slowing the drainage flow. There are new mechanical drainage systems that close and open tile to slow the flow of water or that have reservoirs to store water and slow the flow to rivers. But they are expensive, can consume valuable crop land and need to be maintained.
For more detailed information on the study, go to: http://onlinelibrary.wiley.com/doi/10.1002/hyp.9738/abstract.