Upstream on the Sand Hill River in northwestern Minnesota, sturgeon and channel catfish are returning to their ancient spawning grounds for the first time in decades. The fish had been cut off from the gravel beds of the upper river — ideal habitat for laying and guarding eggs — by a series of concrete low-rise dams dating to the 1930s.

Over the past several years, those dams were either modified or entirely replaced with more natural rapids, allowing for fish to once again make it upstream.

As soon as the passages were reopened, the fish started returning, said Jamison Wendel, stream habitat supervisor for the Minnesota Department of Natural Resources (DNR).

"It's almost immediate," Wendel said.

Around the state, the same pattern has been playing out along dozens of rivers and tributaries.

On the Wild Rice River, which flows into the Red River, several species of fish were spotted 70 miles upstream for the first time in a generation less than a year after outdated low-rise dams had been replaced by more natural rapids, Wendel said.

Multiagency effort

Since the mid-1990s, several federal and state agencies including the DNR, Minnesota Pollution Control Agency (MPCA), U.S. Army Corps of Engineers and local watershed districts have been studying impacts on spawning grounds and biodiversity as they replace or repair dams and culverts that were built over the past 70 to 80 years.

It started with efforts to restore lake sturgeon populations in the Red River Basin, said Ted Sledge, assistant regional fisheries manager for the DNR.

"We knew we had to improve fish passage in combination with these stocking programs we were doing," Sledge said.

That meant opening up ways for the sturgeon to once again reach the "beach ridge" area of northwestern Minnesota, which includes the gravelly streams that were left behind thousands of years ago when the ancient Lake Agassiz retreated at the end of the last ice age.

Seven of the eight major dams and barriers that had been cutting off passage on the Red River have now been modified or removed, Sledge said. Studies are underway on the eighth — the Drayton dam.

The barriers don't just affect sturgeon. Bass, pike, catfish and even native river mussels are returning to areas where they haven't been seen in years, Wendel said.

"Mussels rely on these fish to transport them upstream where they can recolonize," he said. "A lot of fish, too, can go downstream when there are droughts or tough winters and just were not able to go back."

Dangerous dams

The opened waterways not only lead to better fishing opportunities, but in many cases, remove some of the most dangerous dams in the state, Wendel said.

The low-rise dams that were built in the early and mid-1900s cause a rolling circular current below the dam that can suck in both people and wildlife. As those dams are replaced, the best option in many cases is to install man-made rapids with a more gradual slope that mimics a natural river, Wendel said. In some cases, the rapids can be built in a way to still preserve the lakes and ponds above the dam for the cabins and homes that were built along the shore.

The MPCA has been studying the spawning barriers as one of many "stressors" on aquatic life, said MPCA spokesman Dan Olson.

Generally, the dams can cause water quality issues and deteriorated habitats with the buildup of silt as well as cutting the amount of biodiversity above the barriers, he said.

It's hard to say exactly how many dams agencies are preparing to remove or modify. In a 2010 study, DNR officials estimated that at least 100 low-rise dams should eventually be taken out or altered.

Several projects are ongoing throughout the state, including three dam removals along the Cottonwood River in southwest Minnesota, as well as pending removals in Hinckley and Hallock, Olson said.

"More and more of what's happening is that we're finding the benefits of removing these barriers outweigh the benefits of keeping them," Olson said.

Greg Stanley • 612-673-4882