South Twin Lake

South Twin Lake earns an A — water clarity, phosphorus, and chlorophyll-a all rate among the best monitored waters in Mahnomen County. 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.

A TSI near 43 places South Twin Lake in the mesotrophic band — moderate productivity, with seasonal swings between clearer spring water and richer late-summer conditions. The lake bottoms out at 29 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. At 1,127 acres, South Twin Lake is one of the larger lakes in Mahnomen County, with 5.1 miles of shoreline supporting multiple distinct use patterns. Within Mahnomen County's 16 graded waters, South Twin Lake sits at rank 3, near the top of the local distribution.

South Twin Lake is on the Minnesota infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. The fishery includes walleye alongside the lake's other 15 documented species — a notable draw for Minnesota anglers. Public access is available — the lake is on the Minnesota PCA public-access list. Ice-out has been recorded 10 times at South Twin Lake, with a median around Apr 17. 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.