Wonder Lake Dam is an earth dam located in McHenry County, Illinois, built in 1929. It carries a High hazard potential classification.
Dam profile
NID height · 27 ft27 ft tall and 1,050 ft long, taller than 59% of U.S. dams with a recorded height. Height is drawn to scale against a two-story house, about 25 ft; the profile is a generic embankment section and the water level is illustrative.
About Wonder Lake Dam
The primary purpose of Wonder Lake Dam is recreation. The dam creates a reservoir used for recreational activities such as fishing, boating, and swimming. The dam is owned by Master Property Owners Association, Inc. (private). It impounds the Nippersink Creek near Wonder Lake.
The dam stands 27 feet tall and stretches 1,050 feet across, creates a reservoir covering 728 acres, has a maximum storage capacity of 10,194 acre-feet, and collects water from a drainage area of 97 square miles. Completed in 1929, the structure is well-established, with more than 75 years of service.
This dam carries a high hazard potential classification, which means that a failure or uncontrolled release of water would likely cause loss of human life. This classification reflects the consequences of failure, not the likelihood — it indicates that people live or work in the area downstream. An emergency action plan is in place for this dam. The most recent inspection on record was 10/03/2025.
Living Downstream
If you live or work downstream of this dam, consider taking these steps: learn your local evacuation routes, check whether your county emergency management office has a dam failure inundation map, and sign up for local emergency alerts. The Illinois dam safety program can provide more specific information about emergency planning for this structure.
Search for dams near your address to see all dams in your area with hazard classifications and safety data.
Location
| State | Illinois |
| County | McHenry |
| Nearest City | Wonder Lake |
| River/Stream | Nippersink Creek |
| Coordinates | 42.40879, -88.34245 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 27 ft |
| Dam Height | 27 ft |
| Dam Length | 1,050 ft |
| Year Completed | 1929 (97 years old) |
Storage & Hydraulics
| NID Storage | 10,194 acre-ft |
| Max Storage | 10,194 acre-ft |
| Normal Storage | 4,877 acre-ft |
| Surface Area | 728 acres |
| Drainage Area | 97 sq mi |
| Max Discharge | 8,471 cfs |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Recreation |
| Owner | Master Property Owners Association, Inc. |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Not assessed |
| Last Inspection | 10/03/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | IL00585 |
| Federal ID | IL00585 |
| More Info | State Dam Safety Program |