O'Brien Lake

On the LakeGrade scale O'Brien Lake grades a B, with clarity at 13.1 ft and phosphorus readings still being added placing it above the Minnesota median. 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 40 places O'Brien Lake in the mesotrophic band — moderate productivity, with seasonal swings between clearer spring water and richer late-summer conditions. O'Brien Lake is unusually deep for Minnesota at 63 ft, which buffers the lake against summer warming and helps the deep water stay oxygenated. The lake's 539 acres and 13.3 miles of shoreline put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. O'Brien Lake sits at rank 76 of 146 in Itasca County, in the lower half of the local distribution.

No invasive species are currently listed at O'Brien Lake — the lake remains off the Minnesota infested-waters roster. Walleye are documented at O'Brien Lake, one of 10 fish species on record for the lake. 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.