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Murky waters: From carp to excess nutrients, Lake Okabena assessment continues

WORTHINGTON -- A pair of assessments under way on Worthington's Lake Okabena will be used to guide board members of the Okabena-Ocheda Watershed District as they identify ways to improve water quality in the local basin.

WORTHINGTON - A pair of assessments under way on Worthington’s Lake Okabena will be used to guide board members of the Okabena-Ocheda Watershed District as they identify ways to improve water quality in the local basin.

The district hired Wenck to complete both a carp population and Best Management Practices (BMP) assessment on Lake Okabena. The studies examine the prevalence of carp in the basin and the impact they have on water quality; as well as evaluate where nutrients are entering the lake. Both assessments are hoped to shed light on ways the watershed can reduce phosphorus loading and suspended solids in Lake Okabena and guide the district toward meeting Total Maximum Daily Load reductions.

A TMDL report previously completed identifies the need to reduce phosphorus loading in the lake by 57 percent. On Tuesday, Jeff Strom of Wenck said phosphorus reductions of 73 pounds per year from within the city of Worthington, 3,100 pounds per year from the rural area and 791 pounds per year from internal loading are needed to improve the lake’s health.

The watershed’s filtration project on the former Prairie View Golf Links property will help to reduce nutrients reaching Lake Okabena, but more work is needed. Wenck is evaluating the potential to implement practices at Olson campground, Sailboard Beach and the District 518 property along Crailsheim Road.

Nearly 50 acres in the watershed, between Second Avenue and Fifth Avenue and from the lake shore to just beyond 12th Street, drains toward Sailboard Beach. That area alone sends about 30 pounds of phosphorus and six tons of total suspended solids into Lake Okabena each year.

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Strom said options such as adding sediment traps, doing underground treatment of water or doing a series of smaller projects like permeable pavement and rain gardens can help to reduce the amount of nutrients reaching the lake from the downtown area.

The Olson Park campground and adjacent land comprise 208 acres of drainage area that reaches Sunset Bay. This area contributes 14 to 17 tons of total suspended solids to the water body each year, along with between 139 and 165 pounds of phosphorus.

Among the ideas to reduce nutrient loading from this area include adding a filter to the existing pond west of Crailsheim Road or creating a new water storage pond on the south side of the Olson Park entrance road.

“There could be a pretty good reduction with these options,” Strom said.

The third site in the study is the District 518 school development property and the potential for a project on the property that would help to treat the largest of the three drainage areas, at 1,385 acres. Water drainage from north of Interstate 90, between Crailsheim Road and Monroe Avenue delivers the highest amount of nutrients to Lake Okabena - 958 pounds of phosphorus per year and 149 tons of total suspended solids.

“It’s a large watershed; it will be costly to treat the phosphorus load,” Strom said, adding that the watershed district may want to consider a pond or a series of ponds to pre-treat water before it reaches the lake.

“You could do something similar to the golf course ponds - ponds with some sort of filter at the end,” Strom said.

As for the carp assessment, Strom and Tom Langer conducted an electrofishing study on the lake last fall. Using methodology developed by the University of Minnesota, they estimate 14,000 carp are in Lake Okabena, amounting to a density of 196 kilograms per hectare. A density of 100 kg/ha is considered to result in lake impairments.

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Among the ideas for carp control presented by Langer include doing radio tagging to learn where the carp go to spawn and where they congregate for winter. That information would then be used to target rough fish removal through winter-time seining efforts or open water seining by commercial fishermen, or through migration entrapments.

The Wenck representatives anticipate completing both studies within the next four to six weeks. Recommendations thus far include continued water monitoring in Lake Okabena, along with water monitoring in Sunset Bay, doing radio tracking of carp and considering roughfish removal methods.

In other business, the board:

  • Conducted a public input meeting regarding the replacement of the bridge over the Ocheyedan River, south of the Lake Bella spillway. Construction is planned this year, with two 8- by 12-foot precast concrete culverts to replace the existing bridge. The construction will infringe on the watershed district’s property, requiring a temporary easement be granted.
  • Approved the watershed district’s 2017 annual report and 2018 draft plan and budget. The 2018 budget is $586,500.
  • Discussed the need to change the boundaries of the Okabena-Ocheda and Kanaranzi-Little Rock watersheds to reflect the change in water flow after the diversion channel was closed at the Herlein-Boote Slough. Several landowners are being taxed by the OOWD when they should actually be taxed by the K-LR.

OOWD Administrator Dan Livdahl said Houston Engineering is working on LIDAR maps, which may be used to redraw the watershed lines. Once boundary determinations are made, one of the watershed boards will need to petition the state’s Board of Water and Soil Resources to grant the change.

Julie Buntjer became editor of The Globe in July 2021, after working as a beat reporter at the Worthington newspaper since December 2003. She has a bachelor's degree in agriculture journalism from South Dakota State University.
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