Pepin, which is fed by the Mississippi, is loaded with sediment — almost all of it coming from the Minnesota River. The Minnesota delivers just 25 percent of the water flow to Lake Pepin but carries nearly 90 percent of the sediment filling in the lake.

“There’s been a tenfold increase in sediment in Pepin and the major source is the Minnesota River. That’s well documented,” Jennings said.

Jennings said it’s not known exactly why the Le Sueur gives up so much soil, or if the sediment load coming from the river has increased significantly over time.

The team of scientists will conduct a variety of tests and ongoing research to decide exactly how much dirt is flowing from the river and where it’s coming from.

Long a dirty river

It is known the Le Sueur and Blue Earth have long poured dirt into the Minnesota River. Early explorers wrote of noticeable muddy water flowing out of the Blue Earth long before white settlement and farming in the region.

Modern agriculture and tiling of fields have put more sediment into the rivers. Jennings said some farmers in the Blue Earth River basin have taken part in using best-management practices, such as putting grass buffers around tile intakes and along drainage ditches. “It’s helped, but there may be better ways to do it.”

One thing researchers want to know is the changes in cut — width — of the Le Sueur as well as changes in its elevation.

When the Minnesota River valley was carved out about 9,000 years ago, the tributaries were higher than the Minnesota.

“All the tributaries, like the Blue Earth, would have been waterfalls, but those have been nicked now. We’re trying to estimate how rapidly ravines have deepened,” Jennings said.

They also want to measure another natural phenomena — the widening of the river at the mouth. Rivers — unless they hit hard rock — naturally widen at the mouth, with the expansion moving upstream.

“The Minnesota basin is relatively young,” Jennings said. “What it wants to do now is to continue moving up the tributaries to drain the whole watershed. The river will work up the landscape eventually and completely drain it.”

Discovering trends

Ganske said the detailed study of the Le Sueur watershed is possible because of funding through the state’s Water Legacy Act, passed in 2006.

“It’s part of a statewide effort where every major watershed in the state, about 80 of them, will have this biological assessment done,” Ganske said.