Colby Lake

On the scoring rubric Colby Lake grades a D: clarity at 3.8 ft and 61 µg/L of phosphorus keep it in the lower bracket for Washington County. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

At a TSI of 62, Colby Lake reads as eutrophic — nutrient-rich enough that summer algal growth and reduced clarity are expected, not unusual. Colby Lake is a shallow lake at 11 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. Colby Lake is small — 71 acres alongside 2.3 miles of shoreline — which makes it sensitive to even modest changes in watershed runoff or recreational pressure. Colby Lake ranks 71 of 83 in Washington County — at the lower end of the locally monitored distribution.

Eurasian watermilfoil is established at Colby Lake, which can dampen recreational access and pressure the native plant community. 8 fish species are on record, with panfish making up the recreational core. The lake lacks a documented public access point, so visitor use is limited. Multiple sampling years from Minnesota PCA volunteers and partner agencies back the grade, so the letter should hold steady absent a major watershed shift.