L Thunder Lake

L Thunder Lake carries a solid B grade — strong on most parameters, with measurements that hold up well across the summer season. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

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's maximum depth is not yet documented in state morphometric records — context for its physical structure remains limited. The lake's surface area is not consistently recorded across state datasets — physical context remains partial. Within Cass County's 133 graded lakes, L Thunder Lake ranks 99 — below the local median, though not at the bottom.

L Thunder Lake has no invasive species recorded in Minnesota state databases as of 2024, though prevention practices still apply at all access points. The state fisheries records do not list documented species for L Thunder Lake, which usually reflects a lack of formal fisheries survey work rather than an empty lake. Public access is available — the lake is on the Minnesota PCA public-access list. The lake has a partial ice record — 6 observed ice-outs, centered near Apr 18. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.