St. Croix Lake

St. Croix Lake earns a C — middle-of-the-pack water quality, with one or two parameters dragging the average down from a stronger grade. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

The lake sits in the mesotrophic zone — the band where most well-managed Minnesota lakes fall. The lake's 78 ft maximum depth is in the upper tier for Minnesota — deep enough for cold-water fish to find refuge in summer. With 8,393 acres of surface and 124.5 miles of shoreline, St. Croix Lake is a large water by Minnesota standards — recreational pressure is spread across a long shoreline. Within Washington County's 31 graded lakes, St. Croix Lake ranks 19 — below the local median, though not at the bottom.

St. Croix Lake carries an invasive watermilfoil record — boat-launch decontamination and shoreline care matter more here than at uninfested lakes. St. Croix Lake's fishery is anchored by muskellunge, with 36 documented species in total — a destination water for muskie 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.