A year ago an unprecedented storm dumped up to 10 inches of rain on the Duluth area, literally rearranging North Shore streams and spewing tons of sediment — visible even from satellite — into Lake Superior and the St. Louis River estuary.
At the time, some feared the worst flood in a century would harm fisheries in the lake, streams and the St. Louis River, which dumps into the lake near Duluth.
Yet, the effects, so far, appear minimal. Instead, researchers marvel at the resiliency of the system and say it appears fish mostly weathered the floodwaters, the infusion of sediment and even the rearranging of stream beds.
"I'd say the Lake Superior fishery was little affected,'' said Don Schreiner, Department of Natural Resources area fisheries supervisor for Lake Superior. "Fishing this spring has been gangbusters. Lots of lake trout and coho and chinook salmon are being caught.''
The famed North Shore's trout streams also survived, although they have been permanently changed.
"They were rearranged in a big way,'' Schreiner said. "We did see mortality to young-of-the-year fish last year, especially steelhead.'' But he doesn't expect long-term impacts.
"In a nutshell, fish populations fared better than what most people were expecting,'' Schreiner said.
Schreiner said he also doesn't believe the sediment deposited by the storm has impacted lake trout spawning areas along the shore.
"I think wave action has cleaned off any spawning habitat that may have had sediment,'' he said. And the sediment only spread as far north as Two Harbors.
The bigger issue was the floodwaters washed sand and gravel out of streams and into the lake, and, combined with low water levels in Superior, formed large sandbars at the mouths of some streams, disconnecting the streams from lake.
"Fish couldn't physically get over the sand and rubble bars at the Lester, French and Knife rivers,'' Schreiner said. That would have prevented fish from spawning in those rivers.
"I thought we might have to get backhoes out [to create outlets], and I didn't want to set that precedent,'' he said.
But melting snow and rain blew through the bars last spring, reconnecting streams with the lake and providing access for spawning fish.
"We lucked out,'' Schreiner said.
St. Louis River changed
The impact to the St. Louis River estuary, a popular fishery that was blasted with floodwaters carrying trees and sediment, is more uncertain, but there are positive signs, too. Fifteen hundred tons of rock placed below the Fond du Lac dam in 2009 to provide sturgeon-spawning habitat remarkably withstood the brunt of the flooding, and officials say sturgeon and walleyes used the area to spawn this spring.
In fact, biologists recently found sturgeon are spawning successfully in the river — only the second time that recently hatched sturgeon have been found there.
A DNR spring fishery survey showed good numbers of walleyes.
"We didn't see a major issue there,'' said Deserae Hendrickson, DNR area fisheries supervisor. "One thing we definitely noticed was an increased amount of woody habitat in the water, which is a positive thing.'' The trees, partly buried in sediment, provide fish habitat.
The event itself killed some fish, including some that were stranded in backwaters after the floodwaters subsided. "We did rescue some sturgeon and other fish,'' Hendrickson said, "but we couldn't get them all. But I can't say we saw a lot of dead fish.''
Fish reproduction likely wasn't strong last year because of the flood, but Hendrickson said it will take a year or two to assess the impact.
Joel Hoffman, a research biologist for the U.S. Environmental Protection Agency in Duluth, was working on an invasive species research project in the estuary, surveying fish larvae, when the storm struck.
"We wondered if the fish larvae would be flushed out, but interestingly our catch rates after the flood didn't change much,'' he said.
Later last summer, however, Hoffman discovered the disappearance of a large amount aquatic vegetation — key habitat for young fish. "We literally couldn't find it,'' he said. "I'm not sure it was scouring [from floodwaters] or was a light-related affect from the turbid water.''
Hoffman doesn't know if the vegetation will reappear this year or if the flooding cause long-term damage. "It's too early to tell; it's just starting to come up now,'' he said.
Still studying impact
Elizabeth Minor, an associate professor of chemistry at the University of Minnesota Duluth's Large Lakes Observatory, is looking at the possible changes in the chemistry of the lake, including organics, nutrient levels, suspended solids and light penetration.
"The lake is very big and appears to be pretty resilient,'' she said. But the jury is still out on the long-term impact.
Something else bothers her.
Of the 12 "mega-rain" events in Minnesota history, five have occurred in the past 11 years. "If you look at climate change models, we're predicted to have more intense events,'' she said. "The question is, if the lake keeps getting hit this way, what happens?''
Doug Smith • doug.smith@startribune.com
