The Mississippi River in Aitkin County has long been noted as being muddy. This was confirmed in recent studies that found high levels of sediment in the river. This resulted in 147 miles of the river being listed as impaired for Total Suspended Solids (TSS), and triggering a Total Maximum Daily Load (TMDL) study. The impaired stretch extends from the Swan River in northern Aitkin County to the Crow Wing River south of Brainerd. A Total Maximum Daily Load (TMDL) study establishes the amount of a given pollutant that a water body can accept and still meet water quality standards. This was completed to determine the reduction in pollutants needed to again meet water quality standards. Where is the sediment coming from and how can it be reduced?
The main source of sediment in this stretch of river comes from non-point sources – especially bed and bank erosion. The soil deposits left behind from Glacial Lakes Aitkin / Upham are finely grained and easily erodible. They are comprised of fine, layers of sand, silt, and clay with very little coarse sand and gravel. Beds of silt and clay are found in areas where the lake was deepest. Sand and gravel can be found in areas where the lake was shallow and along beach ridges. As Glacial Lake Aitkin began to drain, peat deposits developed in the bog areas that remained. The flat topography associated with Glacial Lake Aitkin soils generally prevents erosion, however, these fine-grained soils are highly susceptible to erosion when disturbed, particularly along river and stream banks where the soil is on a slope or incline. Bends in the river also commonly experience erosion.
The effects of sediment on the aquatic life in the Mississippi River was a concern that was investigated through the TMDL study. The aquatic community reflects the cumulative impacts of pollutants, habitat alteration, and hydrologic modification on a waterbody over time. Degradation of surface waters can lead to changes in biological communities as pollutant intolerant species are replaced by species that tolerate more polluted waters. The presence of a diverse and reproducing aquatic community is a good indication that the aquatic life beneficial use is being supported by a stream.
All the Upper Mississippi River reaches that are impaired by TSS exhibited good fish ratings. The total number of fish species observed at each monitoring station ranged from 11 to 24. The good scores indicate that the current fish community is either not impacted by the existing elevated TSS, or has adapted to the existing sediment levels. These good scores also indicate that the fish community is not negatively impacted by pollutants (in this case high TSS levels), habitat alteration, and hydrologic modifications to the impaired portions of the Mississippi River.
So what can be done to reduce the sediment levels in the river? There are strategies that can be implemented to minimize sediment. The following were outlined in the TMDL report:
- Land Conservation through easements and acquisitions. Conservation easements typically restrict future subdivision and development of the property, favoring natural habitat values. Incentive payments may be available to property owners interested in this program.
- Excluding livestock from access to riverbanks. Exclusion fencing and alternate water sources can accomplish this. Technical and financial assistance may be available to assist landowners.
- Creating / protecting vegetation along the riverbanks. Deep rooted native vegetation can help to hold the soil in place. Trees and shrubs are especially important.
- Minimizing stormwater impacts. Reducing or slowing runoff from large rain events can reduce sediment entering the river. Rain gardens and vegetated swales are simple practices to implement.
Several partners were involved in this study including the Minnesota Pollution Control Agency, Minnesota Department of Natural Resources, U.S. Army Corps of Engineers, and Aitkin County Soil and Water Conservation District. The full TMDL report is available on the Minnesota Pollution Control Agency website at https://www.pca.state.mn.us/sites/default/files/wq-iw8-60b.pdf
Photo Credit: Kyle Fredrickson, Aitkin County SWCD