The Blue Earth River is 50 percent wider along its entire length than it was seven decades ago, largely because a generation of artificial drainage on the surrounding landscape has doubled the flow of water coursing between its soft banks.

It’s one of many Minnesota rivers that has seen a significant increase in flow, and all of which are sending millions of tons of dirt and pollution downstream to the Mississippi River and its tributaries. Those turbid rivers, in turn, are scouring their banks, accelerating a natural geological process that could fill the top third of Lake Pepin with dirt within the century.

The research, published online last week, is just the latest to paint a picture of how agriculture and, more recently, rising commodity prices are changing the Midwestern landscape in significant ways. Since 2006 farmers have converted 350 square miles of Minnesota grassland into row crops, according to a new review of land and satellite data by scientists at South Dakota State University.

Altogether, the five western Corn Belt states lost a total of 2,000 square miles of pasture, grassland and prairies between 2006 and 2011 — much of it on environmentally sensitive or poor agricultural land, the review found. More than a third of it was land that, without drainage, would have been too wet to farm, said Christopher Wright, the author of the study published in Proceedings of the National Academy of Sciences.

“It raises alarms on how to protect what’s left,” he said.

Drainage in the Minnesota River valley has been a contentious issue for years. State regulators are at the tail end of a multiyear effort to identify the sources of pollution and sediment that are contaminating the lower end of the Mississippi River, and to develop a plan to fix it.

Conservationists, scientists and farmers debate whether the rise in river flows is from more rain, which has increased substantially in recent decades, or from the agricultural drainage that whisks water off the land.

“If you don’t know what the cause is, you can’t devise policy,” said Whitney Clark, executive director of Friends of the Mississippi River.

Surprise: Evaporation

The river study, published in the journal Hydrological Processes, is one of the most comprehensive analyses to date. The Minnesota researchers looked at decades worth of land and climate data in 21 watersheds. And it is the first to look at the role of evaporation, which is considerable.

“It was a surprise to us,” said Shawn Schottler, the lead author and an expert on water and soil at the Science Museum of Minnesota’s St. Croix Watershed Research Station.

Only 20 percent of the region’s natural water “budget” flows into streams and rivers, he said. Before farmers started digging ditches and laying drainage tubes, 80 percent of the water was used by plants or evaporated after ponding on the land or collecting in wetlands.

But drainage is designed to remove water as quickly as possible — before it has a chance to cycle through plants and into the air, he said.

Higher rainfall does play a part, he said, but the loss of evaporation plays a larger one: instead of returning to the atmosphere, now most of that water is sent to the rivers.

“We don’t appreciate evaporation,” Schottler said. It’s invisible, it processes vast amounts of water, “and it’s not measured.”

Results questioned

Satish Gupta, a professor and soil scientist at the University of Minnesota, disputed the results. He questioned the methodology, and said the increase in river flow is primarily due to more rain, not agricultural practices.

That part of Minnesota now receives on average four more inches of rain each year, he said. “and that water has to go somewhere. The land is flat. There is not enough capacity to hold four inches of rainfall year after year.”

Still, artificial drainage in Minnesota appears to be on the rise, said Wright of South Dakota State University. While other western Corn Belt states lost other types of grassland to agriculture, the change in Minnesota was clearly related to drainage, he said.

Some conservationists say there are ways to protect the rivers without hurting agricultural productivity. New designs in drainage systems hold water back by storing it in underground tanks, ponds and wetlands, greatly reducing the flow water during heavy rains.

“What we need are innovative solutions” that address both conservation and farmer productivity, said Rylee Main, project manager for the Lake Pepin Legacy Alliance.

Schottler said that at some point every square foot of land could be drained, and the flow of water and loss of soil along the river banks will reach equilibrium. That may already be happening in some places, he said — the flow in the Blue Earth, for example, hasn’t changed in 15 years.