Decorah Lake

Decorah Lake earns a D — measurements through 2021 show persistent nutrient pressure that limits clarity through the summer. High phosphorus is the leading drag on the score — likely from upland agriculture, lawn runoff, or in-lake sediment release.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. The lake bottoms out at 15 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. The lake's 314 acres and partial shoreline records put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. Within the 2 graded lakes of Juneau County, Decorah Lake sits at rank 2, near the bottom of the county list.

Eurasian watermilfoil is established at Decorah Lake, which can dampen recreational access and pressure the native plant community. Walleye are documented at Decorah Lake, one of 4 fish species on record for the lake. A documented public access point at Decorah Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.