Algonquin Dam is a concrete dam located in McHenry County, Illinois, built in 1947. It carries a Significant hazard potential classification.
Dam profile
NID height · 9 ft9 ft tall and 300 ft long. Height is drawn to scale against a two-story house, about 25 ft; the profile is a generic concrete gravity section and the water level is illustrative.
About Algonquin Dam
The primary purpose of Algonquin Dam is flood risk reduction. The dam is designed to control downstream flooding by temporarily storing excess water during storms and releasing it at a controlled rate. The dam is owned by Illinois Department of Natural Resources (state). It impounds the Fox River near Algonquin.
The dam stands 9 feet tall and stretches 300 feet across, and has a maximum storage capacity of 24,200 acre-feet. Completed in 1947, the structure is well-established, with more than 75 years of service.
This dam has a significant hazard potential classification, meaning that a failure could cause significant economic damage — flooding roads, bridges, or property downstream — but is not expected to result in loss of life. An emergency action plan is in place for this dam. The most recent inspection on record was 10/28/2025.
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Location
| State | Illinois |
| County | McHenry |
| Nearest City | Algonquin |
| River/Stream | Fox River |
| Coordinates | 42.16574, -88.29009 |
Physical Characteristics
| Dam Type | Concrete |
| NID Height | 9 ft |
| Dam Height | 9 ft |
| Dam Length | 300 ft |
| Year Completed | 1947 (79 years old) |
Storage & Hydraulics
| NID Storage | 24,200 acre-ft |
| Max Storage | 24,200 acre-ft |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Flood Risk Reduction |
| Owner | Illinois Department of Natural Resources |
| Owner Type | State |
| Hazard Classification | Significant |
| Condition Assessment | Not assessed |
| Last Inspection | 10/28/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | IL55063 |
| Federal ID | IL55063 |
| More Info | State Dam Safety Program |