REGIONAL— While fingerprints are used mostly to fight crime, a different kind of fingerprint is helping researchers better understand the sources of the chronic turbidity that has long impaired the Little Fork River.
The Minnesota Pollution Control Agency, in coordination with the North St. Louis, Itasca, and Koochiching counties’ soil and water conservation districts, is using “sediment fingerprinting” as a way to identify the specific locations of the sediment that is constantly entering the river from various parts of its 2,000 square mile watershed.
According to Mike Kennedy, with the MPCA, staff from the cooperating agencies have gathered more than 200 soil samples from across the watershed to establish a library of the varying soil particles found in the surrounding landscape. Using a variety of methods, from appearance under a microscope to chemical, magnetic, and even radioactive signals, the researchers can tell where on the landscape the river sediments may be coming from. Unfortunately, said Kennedy, the research hasn’t found a “silver bullet” solution, but rather that the sources of sediment are widespread and that any solution to the problem would be long-term and expensive.
Much of the Little Fork’s watershed is comprised of the former bed of glacial Lake Agassiz, said Kennedy, and it contains a thick layer of sediments that settled on the bottom of the ancient lake. While the lake is gone today, that sediment remains, consisting of dense, silty clay and fine sands, all of which are easily eroded over time, contributing to the river’s turbidity.
Turbid waters are normally a sign of a river system that’s in trouble from a biological standpoint, which is why the Little Fork is listed as impaired by the MPCA for sediment levels that are about four times the allowable limit. Yet, while the river is remarkably turbid, Kennedy notes that the biological health of the river, as measured by the abundance and diversity of fish and aquatic insects, remains robust. “The good news is that the biology of the river is exceptional, probably at the 95th percentile,” said Kennedy.
That would be expected given that much of the river’s watershed is remote, much like the rest of the region, where river water quality tends to be exceptional. The Little Fork watershed, unlike most in the region, does have pockets of more intensive land use, such as agriculture, which appears to be a significant contributor to the sedimentation problem in the river.
Agricultural properties are more likely to erode than forested lands, which otherwise dominate the Little Fork watershed. And Kennedy notes he’s seen evidence that there may be some more direct impact from agricultural operations in the area. This reporter has personally witnessed a livestock producer near Cook emptying a trailer load of manure-encrusted straw into the river from the side of a bridge. Kennedy said he’s found significant amounts of straw in the river at times, suggesting this kind of illegal dumping may be occurring relatively frequently.
Many sources of sediment
If there is one thing that the sediment fingerprinting has revealed, it is that the sediment isn’t coming from just one, or even a handful of locations. Kennedy notes that the Little Fork watershed is interlaced with hundreds of ravines, cut into the fine clay and sand layers by water as it moves through the landscape. While any one of those ravines may contribute a relatively small amount of sediment to the river, the cumulative effect can be seen every day. “It’s like death by a thousand cuts,” said Kennedy.
The river begins its journey at Lost Lake, just west of Tower, and spends its first several miles percolating through the vast Lost Lake peatland, emerging with a bit of bog stain, and some organic material, but little sediment. But Kennedy said that changes abruptly as the river nears Cook. “Right about Hwy 53, the soil changes and we start picking up that load,” he notes. From there until Hannine Falls, about ten river miles west of Cook, Kennedy said it appears that most of the sediment is coming in from surrounding upland sources, farm fields, logging operations, and even dust from gravel roads.
Further downstream, he said most of the sediment appears to be coming from within the river channel itself, which could make it extremely difficult to address in a meaningful way. It’s not clear whether the river has always experienced significant sedimentation or if the current conditions are a legacy of the original logging era. Prior MPCA analyses have pointed to the effects of the big log drives that used to be conducted on the river as big pine cut from the region was ferried north to the mill in International Falls. Those log drives effectively scoured the riverbanks, likely exposing new pockets of sediment that have continued to erode over time.
All of which makes a solution exceptionally challenging. While the MPCA is charged with finding solutions to the river’s sedimentation problem, it isn’t clear how it can be accomplished without an enormous investment of public resources. Kennedy notes that addressing sedimentation from a singe ravine could cost hundreds of thousands of dollars, and that would need to be repeated potentially hundreds of times to make much difference. And reducing sedimentation from the river channel itself would be even more challenging and expensive.
And given that the fishing in the river remains very good, Kennedy said it’s tough to generate much public enthusiasm for an expensive mitigation effort. Which suggests that even as work continues to understand the problem, a change in the river’s turbidity could still be a long way off.
Eventually, notes Kennedy, the Little Fork will cut its way through the sediment layer from the old lake bed to the bedrock below, and it could run clear again once it does. “But that’s not going to happen in any of our lifetimes,” said Kennedy.