St. Marys Lake Dam
St. Marys Lake Dam is a gravity dam located in Lake County, Illinois, built in 1921. It carries a High hazard potential classification.
About St. Marys Lake Dam
The primary purpose of St. Marys Lake Dam is recreation. The dam creates a reservoir used for recreational activities such as fishing, boating, and swimming. The dam is owned by Archdiocese Of Chicago (private). It impounds the Bull Creek near Libertyville.
The dam stands 12 feet tall and stretches 389 feet across, creates a reservoir covering 96 acres, has a maximum storage capacity of 966 acre-feet, and collects water from a drainage area of 3 square miles. Completed in 1921, the structure is over a century old.
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 09/12/2025.
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 | Lake |
| Nearest City | Libertyville |
| River/Stream | Bull Creek |
| Coordinates | 42.28314, -87.98053 |
Physical Characteristics
| Dam Type | Gravity |
| NID Height | 12 ft |
| Dam Height | 12 ft |
| Dam Length | 389 ft |
| Year Completed | 1921 (105 years old) |
Storage & Hydraulics
| NID Storage | 966 acre-ft |
| Max Storage | 966 acre-ft |
| Normal Storage | 778 acre-ft |
| Surface Area | 96 acres |
| Drainage Area | 3 sq mi |
| Max Discharge | 320 cfs |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Recreation |
| Owner | ARCHDIOCESE OF CHICAGO |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Not Available |
| Last Inspection | 09/12/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | IL00654 |
| Federal ID | IL00654 |
| More Info | State Dam Safety Program |