William O'Brien State Park
William O'Brien State Park is a state park in Washington County. It was established in 1947. 8 graded lakes are within 15 miles of the park, including Square Lake, Unnamed Lake, Big Marine Lake.
William O'Brien State Park is a Minnesota State Park covering an unrecorded acreage acres. State parks are the most heavily-developed of the public-land designations — campground, picnic, boat-launch, interpretive-center. 8 graded lakes are within easy reach of the park, with an average grade of B. Square Lake (A) is the cleanest of the set.
For visitors planning around water quality, the nearby-lakes list below is the practical tool: a park is only as good as the lake you intend to use it from, and the LakeGrade for that lake gives you the water-clarity, nutrient, and algal-pressure context you need.
Official Resources
Nearby Graded Lakes (within 15 mi)
Lakes closest to William O'Brien State Park, weighted by size. Click a lake for its full water quality grade, fish species, and depth.
| Lake | County | Grade | Surface Area | Distance |
|---|---|---|---|---|
| Square Lake | Washington | A | - | 4.7 mi |
| Unnamed Lake | Washington | D | - | 4.9 mi |
| Big Marine Lake | Washington | A | - | 4.9 mi |
| Bald Eagle Lake | Ramsey | C | 1.0K acres | 14.3 mi |
| Big Carnelian Lake | Washington | A | - | 6.2 mi |
| Big Carnelian Lake | Washington | B | - | 6.4 mi |
| Bone Lake | Washington | D | - | 6.6 mi |
| Halfbreed Lake | Washington | D | - | 6.9 mi |
Other Parks in the Area
- Interstate Park— State Park, 12.9 mi
- Interstate State Park— State Park, 13 mi
- Wild River State Park— State Park, 20 mi
- Afton State Park— State Park, 24.7 mi
Source: EPA National Aquatic Resource Surveys, 2026.