Rocky Mountain National Park staff recently made the decision to repair the Lily Lake Dam and retain the lake and its features in a manner similar to what currently exists. The repair work will being on Tuesday, September 4, and may last through the end of November. During the repair work a roughly 500 feet of area of the Lily Lake Trail across the dam will be closed. Park visitors will be able to access the trail but will not be able to complete a loop hike. The parking area nearest the lake will be closed during the project. The parking area across Highway 7 will be open but has limited parking available.
The U.S. Bureau of Reclamation evaluated the Lily Lake Dam in 2010 and determined that it exhibited several high risk failure modes. Given the private development downstream, they rated the Lily Lake Dam as high potential hazard. While there are concerns with the dam it is regularly inspected and failure of the dam is not imminent.
To reduce the potential risk, Rocky Mountain National Park was faced with the choice of either repairing the dam or removing the dam and allowing Lily Lake to return to its natural configuration. The park sought public input on these two alternatives, and received about eighty comments.
During the public comment period, a legal judgment and stipulation that was rendered in 2000 was brought to the attention of park staff. As part of the judgment and stipulation, Rocky Mountain National Park is legally bound to permanently hold and retain the water rights and structures associated with Lily Lake. For this reason, the dam cannot be removed.
Before, during, and after the repairs are made, the water elevation and the volume of water in Lily Lake will not be artificially altered. The storage capacity of 75 acre feet of water will remain unchanged.
The repairs will involve the removal of surface vegetation on the downstream face of the dam, stripping and salvage of topsoil, regrading the downstream face of the dam and the area around the toe of the dam, placement of filter fabric (geotextile), filter gravel and another layer of filter fabric, and installation of Articulated Concrete Blocks (ACB).
Funding for the repairs is coming from the National Park Service Dam Safety Program. Once the repairs are completed and the vegetation on the face of the dam has been reestablished, Lily Lake and the dam will look much the same as they do now, but the risk of dam failure will have been significantly reduced.
Source: U.S. Department of the Interior, National Park Service
