Lower Long Lake
Lower Long Lake is an earth dam located in Jefferson County, Colorado, built in 1939. It carries a High hazard potential classification and has a unsatisfactory condition assessment.
About Lower Long Lake
The primary purpose of Lower Long Lake is water supply. The dam stores water for municipal, industrial, or agricultural use, helping ensure a reliable supply for the surrounding area. The dam is owned by Denver Board Of Water Commissioners (public utility). It impounds the Ralston Creek-os near Arvada.
The dam stands 19 feet tall and stretches 1,350 feet across, creates a reservoir covering 30 acres, and has a maximum storage capacity of 409 acre-feet. Completed in 1939, 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. Its condition is rated unsatisfactory, indicating serious safety deficiencies exist that require immediate attention or remedial action. An emergency action plan is in place for this dam. The most recent inspection on record was 06/08/2022.
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 Colorado 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 | Colorado |
| County | Jefferson |
| Nearest City | Arvada |
| River/Stream | Ralston Creek-os |
| Coordinates | 39.81939, -105.22673 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 19 ft |
| Dam Height | 19 ft |
| Dam Length | 1,350 ft |
| Year Completed | 1939 (87 years old) |
Storage & Hydraulics
| NID Storage | 409 acre-ft |
| Max Storage | 409 acre-ft |
| Normal Storage | 292 acre-ft |
| Surface Area | 30 acres |
| Max Discharge | 330 cfs |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Water Supply |
| Owner | DENVER BOARD OF WATER COMMISSIONERS |
| Owner Type | Public Utility |
| Hazard Classification | High |
| Condition Assessment | Unsatisfactory |
| Last Inspection | 06/08/2022 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CO01784 |
| Federal ID | CO01784 |
| More Info | State Dam Safety Program |