Big Deep Lake

Big Deep Lake earns an A — water clarity, phosphorus, and chlorophyll-a all rate among the best monitored waters in Cass County. Clarity, phosphorus, and chlorophyll-a all rate similarly, so there is no obvious single lever to pull on watershed management.

A TSI value of 34 puts Big Deep Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. The lake's 107 ft maximum depth is in the upper tier for Minnesota — deep enough for cold-water fish to find refuge in summer. The lake's 536 acres and 5.0 miles of shoreline put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. Big Deep Lake ranks 44 of 133 in Cass County — solidly in the upper half of the local distribution.

Big Deep Lake has no invasive species recorded in Minnesota state databases as of 2024, though prevention practices still apply at all access points. The fishery includes walleye alongside the lake's other 13 documented species — a notable draw for Minnesota anglers. The lake lacks a documented public access point, so visitor use is limited. 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.