Mirror Lake
Mirror Lake in Sauk County, Wisconsin has a water quality grade of D (Nutrient-rich). Water clarity: 4.6 ft visibility. Phosphorus: 65.4 µg/L. Surface area: 137 acres.
- Clarity
- 4.6 ft
- Area
- 137 acres
- Type
- Eutrophic
- Trend
- → Stable
- Public access
- Yes
Grade D measures nutrients and algae, not whether this is a good place for a cabin trip. Cabins and rentals near Mirror Lake →
Next lake in Sauk County
Swipe left or right on this bar to open the next or previous lake.
Mirror Lake is a 137-acre productive, nutrient-rich eutrophic lake in Sauk County, Wisconsin. Water quality is graded D based on 1 sampling years through 2025. Water clarity is improving over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings. Compared to the 6 other monitored lakes in Sauk County, Mirror Lake ranks #6 for water quality.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2025-08-02. Grade methodology: LakeQuality methodology.
Watch Mirror Lake water quality
Subscribe for LakeQuality updates by email. No spam, unsubscribe anytime.
Swimming Safety
Grade onlyBacteria C
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Based on E. coli testing, Mirror Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 5.4% of 112 samples across 8 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Wisconsin’s current beach and health-department advisories before swimming.
Water Quality Grade: D (Nutrient-rich)
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 | Not graded |
| Phosphorus | 65.4 µg/L | Poor |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 60 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Mirror Lake
Compare lakeside cabins, hotels, and vacation rentals near Mirror Lake in Sauk County, Wisconsin — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Mirror Lake are listed below.
Top-rated places to stay near Mirror Lake
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Wisconsin’s top lakes →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 137 acres |
| Public Access | Yes |
No bathymetric survey has been published for Mirror Lake, so its depth is not documented here.
Recreation & Access
- Mirror Lake State ParkPublic
- DeltonPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Mirror Lake
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 |
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
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Mirror Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Christmas Mountain Sanitary District WwtfDellona · 3.5 mi away · NPDES WI0036064Recent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #6 of 7 lakes in Sauk County
Nearby Lakes With Higher Grades
Mirror Lake holds Grade D. 2 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Sauk County
Frequently asked questions about Mirror Lake
Where is Mirror Lake?
- Mirror Lake is a lake in Sauk County, Wisconsin. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mirror Lake?
- Mirror Lake in Sauk County, Wisconsin has a water quality grade of D (Nutrient-rich). Water clarity: 4.6 ft visibility. Phosphorus: 65.4 µg/L. Surface area: 137 acres.
Is Mirror Lake safe to swim in?
- Based on E. coli testing, Mirror Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 5.4% of 112 samples across 8 seasons. That works out to a bacteria swim-safety grade of C — a historical summary of past testing, not a current reading. Mirror Lake has a water quality grade of D (Nutrient-rich). Bacteria levels can spike quickly after heavy rain, so always check Wisconsin's current beach and health-department advisories before you swim.
Are there boat ramps on Mirror Lake?
- Mirror Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Mirror Lake State Park, Delton. Public access is listed as yes.
Explore Mirror Lake across LakeQuality
20.7 mi away · cable-pulled, no boat wake
Fish species and top-graded lakes across Wisconsin
Fish species and best lakes in Sauk County
Species activity outlook by water temperature and season
Top lakes and where to stay in the Madison Area Lakes area
Where to stay at Wisconsin's top lakes, each with its grade
Lakes in the Wisconsin River Watershed, ranked by water quality
High nutrients, frequent algae, reduced clarity
Wisconsin's cleanest and most-polluted lakes, ranked
More Wisconsin Lake Rankings
State Parks Near Mirror Lake
Other Grade D Lakes in Wisconsin
Eutrophic Lakes in Wisconsin
Where this data comes from
43 measurements from 2 monitoring stations, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Wisconsin Department of Natural Resources
WIDNR_WQX43 samples · 2020–2025
Analytical methods
- Phosphorus by Colorimetry
Monitoring station identifiers (2)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Named boat-access points from the USGS National Boat Ramps inventory