Fan lake & fall river restoration: site characterization, design elements, and recommendations

Cooper, David; Gage, Edward

In this report, we present the results of research conducted during 2005 in Horseshoe Park in the area impacted by Fan Lake, a water body formed following the 1982 failure of the Lawn Lake dam. The lake inundated an area of ~17 acres, drowning the riparian willow community dominant on the site. Rocky Mountain National Park is interested in restoring the communities and ecological functions lost or impaired by the formation of Fan Lake. However, to develop appropriate restoration goals and approaches, basic information is needed. Our work was intended to fill some of these information gaps by addressing the following questions: how does the site function hydrologically and are the current hydrologic regimes suitable for the restoration of riparian willow communities? Are there other constrains such as inadequate seed rain preventing natural recovery of the site? Where might willows be successfully planted across the landscape and are there differences in cutting survival among species or planting methods? What modifications to the site’s topography and hydrologic regimes need to be undertaken to allow for reestablishment of the riparian shrub communities historically present in the site? The main body of the report presents an overview of the baseline studies we undertook to answer these questions. We took several approaches including the installation of groundwater monitoring wells and staff gauges, installation and monitoring of a network seed rain traps providing information regarding the spatial and temporal patterns of seed rain across the site, and establishment of experimental cutting plots in a variety of hydrologic settings in order to evaluate the efficacy of planting pre-rooted and unrooted willows stakes from the dominant willows species in the area. Our results suggest that, although the lake was partially breached in 1995, it still floods extensively during the spring through the early summer, creating an environment unsuitable for willow establishment and favoring the dominance of herbaceous emergent marsh species. Data from cutting plots and well networks suggest that willow cover could be effectively increased in some areas under existing hydrologic regimes, but that absent redirecting the Fall and Roaring River channels into their historical paths, complete restoration would be difficult to impossible. In the appendices, we detail specific restoration options and recommendations for the Fan Lake site. These include design specifications for restoration of the Fall and Roaring River channels, grading plans and earthwork calculations, suggested planting protocols and species lists. In addition, we outline approaches for plant installation, erosion control and sediment stabilization, and long-term monitoring of restoration success.