Thomson Reservoir

Thomson Reservoir grades an F: water clarity, phosphorus, and chlorophyll-a all rate poorly, placing it among the most stressed lakes monitored in Minnesota. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. A maximum depth of 22 ft puts Thomson Reservoir in the middle of Minnesota's depth distribution. Thomson Reservoir covers 390 acres alongside 10.7 miles of shoreline — a mid-sized water that supports a working fishery without being so large that conditions diverge between basins. Thomson Reservoir ranks 32 of 33 in Carlton County — at the lower end of the locally monitored distribution.

No invasive species are currently listed at Thomson Reservoir — the lake remains off the Minnesota infested-waters roster. The fishery includes walleye alongside the lake's other 10 documented species — a notable draw for Minnesota anglers. The lake lacks a documented public access point, so visitor use is limited. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.