Thornapple Flowage Lake

On the LakeGrade rubric, Thornapple Flowage Lake pulls an A: clarity at 18.0 ft and phosphorus readings still being added put it in the top bracket for Wisconsin. Sub-grades cluster within a single letter of each other, which usually means the lake is in stable trophic balance rather than fighting one specific stressor.

The lake's low TSI puts it in oligotrophic territory — the cleanest of the four trophic classes, but also the most vulnerable to nutrient-driven shifts. The lake bottoms out at 19 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. At 266 acres, Thornapple Flowage Lake fits the Wisconsin median for monitored lakes, with partial shoreline records. Among the 13 graded lakes in Rusk County, Thornapple Flowage Lake sits at rank 6, above the county median.

Thornapple Flowage Lake has at least one documented invasive species (Purple Loosestrife), so boaters and anglers should observe clean-drain-dry steps when leaving the lake. The fishery includes walleye alongside the lake's other 8 documented species — a notable draw for Wisconsin anglers. A documented public access point at Thornapple Flowage Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.