Cobblestone Lake

On the scoring rubric Cobblestone Lake grades a D: clarity at 3.9 ft and 93 µg/L of phosphorus keep it in the lower bracket for Dakota County. Phosphorus is the parameter holding the grade back — runoff from the surrounding watershed appears to keep nutrient levels elevated through the summer.

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. Cobblestone Lake reaches 18 ft at its deepest point — typical for Minnesota mid-sized lakes, with seasonal stratification but limited cold-water refuge. Cobblestone Lake is small — 35 acres alongside 1.3 miles of shoreline — which makes it sensitive to even modest changes in watershed runoff or recreational pressure. Within Dakota County's 48 graded lakes, Cobblestone Lake ranks 31 — below the local median, though not at the bottom.

Eurasian watermilfoil is established at Cobblestone Lake, which can dampen recreational access and pressure the native plant community. The fishery includes walleye alongside the lake's other 10 documented species — a notable draw for Minnesota anglers. No formal public access is documented at Cobblestone Lake — most use is by shoreline residents and their guests. The grade is supported by multiple sampling years from Minnesota PCA volunteers and partner agencies, giving the letter a reasonably stable foundation.