Researcher says increase in sediment discovered in Minn. River

MONTEVIDEO -- Researchers are discovering an interesting story about sediment in the Minnesota River by scooping muck from Redwood Lake near Redwood Falls.

MONTEVIDEO -- Researchers are discovering an interesting story about sediment in the Minnesota River by scooping muck from Redwood Lake near Redwood Falls.

Farm drainage systems and urban development have greatly accelerated the sediment delivery rate of the Minnesota River, according to Dr. Carrie Jennings, a geologist with the Minnesota Geological Survey. She told an audience of nearly 100 people Thursday in Montevideo how sediment core samples taken from Redwood Lake are helping researchers understand why Lake Pepin is filling up with sediments, and where they're coming from.

The Minnesota River has historically provided a relatively greater share of sediment to Lake Pepin than either the St. Croix River or Upper Mississippi River, but nothing like it does today, according to Jennings.

The Minnesota River is responsible for about 25 percent of the flow of water to the lake, but it accounts for 85 to 90 percent of the sediment load, according to University of Minnesota research.

It's partly because the Minnesota River is well, naturally messy. The basin is large, and it has a complicated "stratigrahy" or mixture of disturbed, glacial till.


And ever since the river was carved as a deep gorge in a catastrophic action over 10,000 years ago, natural forces have been seeking "equilibrium,'' according to Jennings.

Tributaries that initially reached the river as waterfalls have been fanning out and their "knock points'' retreating. In geologic terms, the waterways "zipper'' open. This creates more river bank and sends what is called glacial legacy sediment into the river.

The process is repeated upstream by smaller, feeder streams and now, the ditches and straightened channels we have created.

In pre-settlement days, the flow of glacial legacy sediments to Lake Pepin was on a pace to fill the lake in 3,000 years, according to research cited by Jennings.

We have changed all of that, she said.

The conversion of prairie into open farm fields in the last century created a new source of sediment in the river. And, the drainage of wetlands eliminated the natural system that captures and slows the movement of sediments.

Now, the proliferations of drainage tile lines and urban development have greatly accelerated the rate at which both field and legacy sediments move through the system. The vast majority of the sediments are carried in April, May and early June when these systems flush snow melt and rainwater immediately into the river, said Jennings.

Redwood Lake helps tell the story. Created by a dam in 1902, it was originally 28 feet deep. Sediment now fills much of the 87 acre lake, leaving only two- to seven-feet of water.


Sediment cores show that from 1902 to the dust bowl years in the 1930's, most of the sediment filling the lake came from plowed farm fields. Conservation programs and farming changes adopted in response to the dust bowl helped. From the 1930's to 1964, the amount of sediment coming from farm fields was reduced to 44 percent of the load.

We're doing an even better job today. The sediment originating from farm fields now represents 35 percent of the total.

But the advent of modern drainage has accelerated the natural processes that allow tributaries and smaller streams to fan out and send sediments into the waterway. We have speeded up the natural system of seeking equilibrium.

"We are engineering a very different hydrologic system,'' said Jennings. "We are connecting everything up.''

The results are already obvious in sediment-filled Redwood Lake, and will be soon in Lake Pepin. Instead of an expected life span of 3,000 years, researchers have determined that the northern half of the lake is on pace to fill with sediments in 300 years.

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