St. Louis County culvert inventory paves way for improved safety, trout habitat

ST. LOUIS COUNTY — With more than 3,000 miles of roads spanning six watersheds from Lake Superior to the Canadian border, Carol Andrews figures St. Louis County must have hundreds of thousands of culverts.

Exactly how many — and the GIS coordinates, measurements and condition of each — is being tallied in a years-long inventory that will resume this spring. A $205,000 planning grant from the Minnesota Board of Water and Soil Resources is funding the fifth phase, which started last year and is expected to cover 570 miles by the time it wraps up this season.

Andrews, St. Louis County Public Works’ environmental project manager, said the inventory will help staff move from “firefighting mode” to planning mode.

“Nothing gets fixed the next day, but knowing about it and getting it on a five- to 10-year plan (is) better than sinkholes in the road,” Andrews said.

Public safety is St. Louis County’s №1 priority, followed by infrastructure protection. Fish habitat and passage are other considerations in prioritizing culvert replacements. The Fond du Lac Band of Lake Superior Chippewa shares those priorities, and adds wild rice habitat protection to the list.

Data shared with universities and agencies such as the Minnesota Pollution Control Agency and Department of Natural Resources will be used in larger-scale watershed models.

“There are so many uses for a culvert inventory,” said Kari Hedin, watershed specialist for Fond du Lac’s Office of Water Protection.

St. Louis County’s oldest metal culverts are about 50 years old, installed before rust-deterring aluminized metal became standard. Concrete construction is reserved for large culverts on more heavily traveled roads. Culverts range from about 1 foot to 8 feet wide.

“The biggest disaster is during very, very high flows you can wash out enough material in and around the culvert that the road fails and the culvert washes away. That’s catastrophic failure,” Andrews said.

The most common problem is connectivity.

“Over time, especially if (culverts) are under-sized, the water — as it’s swirling around waiting to get in the culvert, and then as it comes out of the culvert at high speed — will erode the stream,” Andrews said.

Even culverts installed flush with the stream level can become perched, resulting in a 6- to 12-inch drop from pipe to pool.

“It disrupts the ability of fish and other aquatic organisms to move up and down a stream,” Andrews said. “For organisms to try to jump up and get through that — and then the flow in the culvert itself is too shallow and too fast — even if the pipe’s not sitting up in the air, if it’s under-sized, the velocity … can impede organisms being able to swim against that flow.”

Some insects and other critters won’t cross exposed concrete or metal.

“Even if a fish could get through the culvert, if there’s no food upstream it’s not going to move up there,” Andrews said.

Now, replacement culverts match the width of the stream. They’re set 1 to 2 feet deeper to compensate for erosion. Substrate is designed to mimic the streambed.

About half of St. Louis County’s road miles and a quarter of its geographical area have been inventoried so far.

Starting west of Duluth, this spring’s work will include the Stoney Brook watershed in Stoney Brook Township, which borders Carlton County. The Fond du Lac Band and U.S. Geological Survey have been developing a watershed model for Stoney Brook.

“Road crossings and culvert size are important. We can’t have an accurate watershed model without those,” Hedin said.

The 101,000-acre Fond du Lac Reservation spans St. Louis and Carlton counties. Because the topography is flat, Hedin said a small change in culvert size can have a big effect on road crossings and wild rice habitat. The grain is susceptible to fluctuations in water level.

“In a landscape like the reservation where there isn’t a lot of development, road crossings are one of the biggest things that impact our water resources. We’ve got some really flat stretches of bog where these roads are the only thing (running) through them. Having the right-sized culvert can make the difference between having a tamarack bog that survives or one that dies,” Hedin said.

“For how small a culvert is, it can have these hugely out-sized impacts on the landscape.”

The Minnesota Board of Water and Soil Resources’ mission is to improve and protect Minnesota’s water and soil resources by working in partnership with local organizations and private landowners. Website:



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Minnesota Board of Water and Soil Resources

Our mission is to improve and protect Minnesota’s water and soil resources by working in partnership with local organizations and private landowners.