On a June morning, a 16-inch pipe spews rust-colored slurry onto the beach of Minnesota Point. Water drains from the mixture, and a bulldozer noses the remaining deposits along the edge of the beach.
The slurry, dredged from several sites in the shipping canal in the Duluth-Superior harbor, is mostly sand and water. But in some cases, it also contains silt and clay.
Minnesota Department of Natural Resources officials are concerned that silt and clay deposited on the beach could interfere with the spawning of lake trout and herring in Lake Superior.
Responding to those concerns, the U.S. Army Corps of Engineers has halted plans to dredge two other, higher-silt areas of the harbor.
The high-silt areas in question, just inside the harbor and near the Superior Entry, were re-sampled by the Corps of Engineers on Monday, said Steve Brossart, engineer for the Corps in Duluth.
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"We've done more testing, and the silt content is higher than we feel comfortable with," Brossart said Tuesday afternoon. "We're not here to destroy the habitat."
Minnesota Department of Natural Resources officials said they were pleased with the Corps of Engineers' action.
"I think that's a good decision," said Deserae Hendrickson, DNR area fisheries supervisor at French River. "I think resource-wise it's better to find an alternative and not to dispose of it on the shoreline."
Using hydraulic equipment has allowed contractors to remove harbor sediments and pipe them to the Minnesota Point beach. They will continue the process in less-silty sites. If high-silt areas need to be dredged, the work will be done mechanically and materials will be barged to an upland deposit site.
DNR fisheries biologists had been concerned that wave action from fall storms could redistribute silt and clay from the beach and spread it over lake trout and lake herring spawning areas.
The silt could suffocate the eggs by not allowing oxygen to pass through to developing fish, Hendrickson said.
Some Lake Superior anglers also have been concerned about sediments suspended in the lake recently.
"I can't recall in 23 years seeing this much clay in the lake, and it's really dense," said Lake Superior charter fishing captain Dexter Nelson.
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The murky water makes it difficult for fish to see anglers' lures. Anglers have had to go as far as Lakeview Castle on the North Shore to find clear water, Nelson said.
The sediment may be due in part to dredge already deposited on Minnesota Point and in part to rains last week that raised water levels in the Nemadji River, which has a high clay content.
Dredging to keep shipping channels open is done about every five years in the harbor, Brossart said. Dredge deposits often are used to fortify about 3,500 to 4,000 feet of beach along Minnesota Point near Superior Entry. That portion of the beach is vulnerable to erosion, Brossart said.
This summer, the Corps of Engineers had planned to dredge six sites in the harbor and Superior Entry.
But Minnesota Department of Natural Resources fisheries officials expressed concerns last month about one site just inside Superior Entry because its silt and clay content was high. Original sampling put the silt and clay content at about 34 percent, and resampling on Monday showed it is about 42 percent, Brossart said.
The site contains a higher percentage of silt and clay because it's near the mouth of the Nemadji River.
"Allowing disposal from [sampling] Site 3 into the lake is not a good idea," wrote Don Schreiner, DNR Lake Superior Area fisheries supervisor, in a May 28 internal memo. "We think if clay and silt is more than 20 to 25 percent, it should not be directly disposed of on the beach."
Another area next to Sampling Site 3 also has higher levels of silt and will not be dredged hydraulically, Brossart said.
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"We'll have to reevaluate and get with the shipping industry and verify what the critical concerns are for them," Brossart said. "If we do have to [dredge] them, we'll do them mechanically. The materials would be brought to an upland site and placed on it in a confined disposal facility."
