Third River Flowage Lake

Third River Flowage 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.

Mesotrophic conditions dominate at Third River Flowage Lake: enough nutrients for a healthy fishery, not so much that algal blooms become a chronic problem. The lake's 70 ft maximum depth is in the upper tier for Minnesota — deep enough for cold-water fish to find refuge in summer. Third River Flowage Lake covers 56,471 acres alongside 69.8 miles of shoreline, large enough that conditions can vary meaningfully between bays. Within Itasca County's 141 graded lakes, Third River Flowage Lake ranks 100 — below the local median, though not at the bottom.

Zebra mussels have been documented at Third River Flowage Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. Third River Flowage Lake's fishery is anchored by muskellunge, with 20 documented species in total — a destination water for muskie anglers. Public access is available — the lake is on the Minnesota PCA public-access list. The lake has a long ice-out record — 30 observations on file, with a median ice-out around Apr 27. 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.