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Restoration of critical sediment source areas in the Lower Poplar River
August 2011 (volume 6 - issue 7)
Contributed by John L. Nieber and Brad Hansen (Department of Bioproducts and Biosystems Engineering, University of Minnesota)
The Poplar River located along the north shore of Lake Superior at Lutsen is one of Minnesota’s prized north shore streams. The lower part of the river (shown in Figure 1) consists of steep channel slopes, large cobble and boulder channels, and slumping bluffs consisting of glacial till. As a result of observations of high turbidity levels in the lower Poplar River, water quality monitoring was initiated in 2002 at two stations, one at the upper end of the lower Poplar River watershed and the other near the mouth of the Poplar River where it enters Lake Superior.
A plot of turbidity in the lower Poplar River at the lower monitoring station is presented in Figure 2. Analysis of this monitoring data led to the designation of the lower portion of the Poplar River on the MPCA’s impaired waters list in 2004 for violation of the aquatic life turbidity limit of 10 NTUs. The plot in Figure 2 shows that the turbidity limit is often exceeded at the lower monitoring station.
Figure 2: Monitored turbidity in the Poplar River at the lower gauge for the period 2002 – 2006. Figure from Gregory Johnson, Minnesota Pollution Control Agency.
A visual comparison of the differences in turbidity in the river near the upper (a) and lower (b) monitoring stations during a storm in 2002 is shown in Figure 3. The monitoring shows that not much sediment enters the lower Poplar River upstream of the area, while there is significant turbidity measured at the lower station. This result indicates that the source for the turbidity impairment must be from within the lower Poplar River watershed.
Figure 3: Visual comparison of turbidity levels in the lower Poplar River during a storm in 2002. a). Near the upper gauge, b). near the lower gauge.
Possible sources for the sediment in the lower Poplar River watershed include the channel bottom and channel banks, the slumping bluffs, ravines and gullies, and/or the land surface. In July 2009 the University of Minnesota, Department of Bioproducts and Biosystems Engineering initiated a study (Principal Investigator Dr. John Nieber) funded by the MPCA (MPCA project managers Karen Evens and Greg Johnson) to conduct detailed field reconnaissance of the lower Poplar River watershed. The reconnaissance involved the measurement and observation of different sediment sources and associated factors, including the following; 1) soil hydraulic conductivity, 2) soil erodibility, 3) observations of vegetative cover, 4) road and trail infrastructure including location of culvert crossings, 5) river channel characteristics, 6) identification and characterization of ravines entering the Poplar River channel, and 7) surface area, slope, toe elevation, and erodibility of slumping bluffs. Detailed topographic maps with contour intervals of 2 feet and 10 feet were already available (from Salo Engineeering, Inc.) for the study area.
Implementing the topographic surveys into ArcGIS facilitated the identification of flow paths for surface runoff on the landscape within the lower Poplar River watershed. The presence of roads, drainage ditches, and other infrastructures such as parking lots, buildings, and ski runs interrupt the natural landscape and change the course of natural flow paths. In some cases these changes have led to concentration of surface runoff, which can then cause the development of gullies and ravine features on the landscape. Several of these features were very obvious from the field reconnaissance work and are estimated to be significant sources of sediment transported to the Poplar River.
Work on mitigating the effects of the concentrated runoff flow pathways has been undertaken by the Poplar River Management Board (PRMB) in collaboration with the Cook County Soil and Water Conservation District. To implement sediment reduction measures, this collaboration has raised over $1.1 million in grant funding in addition to the more than $500,000 in private funds raised by the PRMB. Work on mitigation is directed at removing the concentrated flows from the affected flow pathways and thereby facilitating stabilization of those highly erosive features. An example approach for mitigation is to pass the concentrated flow through a buried pipe (tightline) rather than having it pass through an eroding ravine, in addition to reshaping and revegetating eroded concentrated flow path surfaces. Other approaches involve the realignment of the flows to dissipate the energy of the concentrated flows and revegetating the eroded concentrated flow path surfaces. Engineering consultants for the PRMB have estimated that once the mitigation work is completed the sediment load to the lower Poplar River will be reduced by 48%. This work is planned to be completed by 2013.
The ravines and gullies associated with concentrated flow paths are only part of the overall sediment that is loaded to the lower Poplar River from the lower Poplar River watershed. Other sources include sheet erosion from the various land use areas of the lower Poplar River watershed, the channel itself, and the slumping bluffs. The land uses within the lower Poplar River watershed include residential/recreational housing, unpaved roads/trails, forest, a golf course, and ski slopes.
Analyses providing estimates of sediment yield from the various land uses in the lower Poplar River watershed and the bluffs have been completed using the WEPP model. Assessment of sediment contribution from the channel has also been completed using geomorphic methods. We intend to report on those results in a subsequent article of UPDATES.
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- Does the extra snow added on the ski slopes by the snowmaking operation significantly increase the erosion from the ski slopes?