Reservoirs in Wisconsin
3 of 16 graded Wisconsin lakes (19%) are man-made reservoirs in the USACE National Inventory of Dams. 3 carry a High or Significant hazard rating.
Why these Wisconsin reservoirs were built
All 3 Wisconsin reservoirs (by surface area)
| Reservoir | County | Grade | Surface area | Year built | Purpose | Hazard |
|---|---|---|---|---|---|---|
| Ftrlk0880a Lake | Charles Mix | B | 118,400 ac | 1954 | Flood Risk Reduction | High |
| Bbdlk0987a Lake | Lyman | B | 60,000 ac | 1963 | Flood Risk Reduction | High |
| Canyon Lake | Pennington | A | 29 ac | 1976 | Irrigation | High |
Frequently Asked Questions
How many reservoirs are in Wisconsin?
3 of 16 graded Wisconsin lakes (19%) are man-made reservoirs in the USACE National Inventory of Dams. 3 are rated High or Significant hazard, meaning failure would put downstream lives or property at substantial risk. The oldest matched dam dates to 1954.
What's the largest reservoir in Wisconsin?
The largest matched reservoir in Wisconsin is Ftrlk0880a Lake in Charles Mix County at 118,400 acres of normal surface area, impounded by the Fort Randall Dam (completed 1954).
What's the difference between a reservoir and a natural lake?
A reservoir is a body of water impounded by a man-made dam — typically built for flood control, hydroelectric power, water supply, irrigation, or recreation. A natural lake formed without human intervention, usually from glacial scouring, volcanic activity, or river meandering. Reservoirs typically have shoreline that fluctuates with seasonal water level management, while natural lakes have more stable shorelines. The USACE National Inventory of Dams classifies every regulated dam in the United States, which is how we identify which LakeGrade lakes are actually reservoirs.
What does the hazard class mean?
USACE assigns each dam a hazard potential classification based on what would happen downstream if the dam failed — NOT how likely the dam is to fail. "Low" hazard means dam failure would cause no probable loss of life and only low economic loss. "Significant" means probable loss of life is unlikely but appreciable economic damage would occur. "High" means probable loss of life and significant economic damage. The classification is about consequence, not condition. Many High-hazard dams are perfectly safe; they're rated High because populated areas have grown downstream over time.
Where does this reservoir data come from?
Every reservoir record on this page is matched to the USACE National Inventory of Dams (NID), the federal database of all dams ≥25 ft tall or impounding ≥50 acre-feet, plus any dam that poses a Significant or High hazard regardless of size. The NID is published by the U.S. Army Corps of Engineers and updated annually. Surface area, storage capacity, dam height, and hazard classification come directly from NID records.
Data source
Reservoir data from the USACE National Inventory of Dams (NID), the federal database of all regulated dams. Dam attributes (height, year completed, storage capacity, hazard class) are matched to LakeGrade lakes by proximity and dam-name similarity. Some lakes may be reservoirs that aren't matched if the dam record uses a name substantially different from our lake name.