Mirror Lake
Sauk County, WisconsinEutrophic
Mirror Lake earns a D — measurements through 2025 show persistent nutrient pressure that limits clarity through the summer. Clarity, phosphorus, and chlorophyll-a all rate similarly, so there is no obvious single lever to pull on watershed management.
Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. The lake's maximum depth is not yet documented in state morphometric records — context for its physical structure remains limited. At 137 acres, Mirror Lake fits the Wisconsin median for monitored lakes, with partial shoreline records. Within Sauk County's 11 graded lakes, Mirror Lake ranks 8 — below the local median, though not at the bottom.
No invasive species are currently listed at Mirror Lake — the lake remains off the Wisconsin infested-waters roster. The state fisheries records do not list documented species for Mirror Lake, which usually reflects a lack of formal fisheries survey work rather than an empty lake. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2025-08-02. Grade methodology: LakeGrade methodology.
Watch Mirror Lake water quality
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
Murky, only visible to about 4.6 ft. Phosphorus level: 65.4 µg/L. Trophic State Index: 60.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.6 ft | D |
| Phosphorus | 65.4 µg/L | D |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 60 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 137 acres |
| Public Access | Yes |
Water Quality Trend: → Stable
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.163 m/yr | 5 |
| Phosphorus | ↓ Declining | +5.7 µg/L/yr | 6 |
Recreation & Access
Location
County Ranking
Ranked #8 of 11 lakes in Sauk County
Cleaner Lakes Within 30 Miles
Mirror Lake holds Grade D. 2 nearby lakes hold higher grades.
- AJordan LakeAdams County · mi · Two grade letters higher (D → A)
- BFawn LakeAdams County · mi · Two grade letters higher (D → B)
Nearby Lakes in Sauk County
More Wisconsin Lake Rankings
State Parks Near Mirror Lake
Other Grade D Lakes in Wisconsin
Eutrophic Lakes in Wisconsin
Reservoir Info (USACE NID)
Mirror Lake is a man-made reservoir impounded by the Delton (completed 1931), built primarily for recreation on the DELL CREEK; gravity-type dam, 19 ft tall and 150 ft long.
- Surface area
- 137 ac
- Normal storage
- 870 ac-ft
- Max storage
- 1,280 ac-ft
- Drainage area
- 71 sq mi
- Hazard class
- Low
- Owner
- Sauk County
Source: USACE National Inventory of Dams, NID ID WI00190 · Operator website
EPA Impairment Status
Mirror Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit WI10002739 · Official waterbody report
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2025-08-02
Monitoring stations: 2
Where to stay near Mirror Lake
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