by Claire DeLeo, Boulder County Parks and Open Space
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| Photo 1: Severe stream bank erosion and incised channel in Rock Creek before restoration. |
Introduction
The Carolyn Holmberg Preserve at Rock Creek Farm was one of the first large land purchases by Boulder County Parks and Open Space. At 1,150 acres, this open space was the first sizable piece of preserved public land in the southeast part of Boulder County and serves as a buffer between the growing cities of Broomfield, Lafayette, and Louisville. In addition to having a trail system and an active agricultural tenant, the property also has a long cultural history, and was the site of a bustling stagecoach stop in the 1860’s before the advent of railroads. Rock Creek Farm is also home to nesting raptors, prairie dog towns, and other wildlife.
A river runs through Rock Creek Farm, or more accurately, a stream, for which the property was named. Rock Creek was historically an intermittent stream, but now flows year round with runoff from developments upstream, including Interlocken and Flatirons Crossing Mall, and discharges from the Town of Superior water treatment plant. Rock Creek has experienced increases in peak discharges, total runoff volume, and frequency of discharges due to urbanization within the watershed. Increased flows in Rock Creek have lead to accelerated bank erosion and channel incision. The soils of these riparian areas are highly erosive clays. The bank erosion had reached up to 20 feet in some places.
Rock Creek Farm and the adjacent open spaces around it are becoming an isolated corridor for wildlife habitat and movement in southeast Boulder County. Once rural farms are being converted into housing and business developments. In 2000, construction began on the Northwest Parkway, which provides a faster route to the Denver International Airport as well as to locales in between. The new highway borders Rock Creek Farm on its northern boundary, creating a barrier to wildlife movement. The Northwest Parkway donated money to Boulder County Parks and Open Space (BCPOS) in compensation for cottonwood tree removal from the highway construction, and the money was invested into restoring Rock Creek.
In 2003, three consultants, ERO Resources, Moser Engineering, and Holdeman Landscape Architecture, completed a master restoration plan of Rock Creek for BCPOS. Two years later, the Urban Drainage and Flood Control District partnered with BCPOS to make the stream restoration a reality. Construction was completed in June 2005 by L&M Enterprises. The goals of the project were to halt channel incision, increase habitat for wildlife, protect historic resources, and create a more aesthetically pleasing riparian area.
Construction
The challenge of the restoration project was to stabilize the stream while attempting to make these changes look natural in a highly altered system. Six sculpted concrete drop structures were built along a 2-mile stretch of stream to prevent further channel incision and to raise the channel in some places. Sculpted concrete was used instead of the popular grouted boulder since boulders are not typically a part of grassland watersheds of Boulder County. Sculpted concrete was colored and shaped to mimic the sandstone outcrops in this area.
A bioengineering technique utilizing soil wraps protected the newly graded streambanks from eroding. This method was chosen instead of rock to stabilize the toe of the slopes because rock would have looked out of place. The soil wraps were a balance between providing the stabilization of rock and the softness of vegetation. First, the vertical banks were graded back to 3:1 slopes or gentler. The soil wraps were then constructed with a multi-layer erosion control blanket composed of biodegradable and permanent materials to hold the soils in place and provide benches for native riparian plants. The blanket was filled with one foot of topsoil for plantings and were anchored into the slope. The benefit of using a multi-layered blanket was to prevent soils from washing away during high flow events while native sedges and grasses were establishing, as well as providing long-term bank stabilization. The soil wraps initially looked like 1 foot high steps at a 3:1 slope, but over time, seasonal flooding and the growth of native plants has softened their appearance.
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| Photo 2: Rock Creek after restoration project showing gentle bank slopes and riparian plantings. |
Native Plants
Once the stream channel had been reshaped and the drop structures were poured, the final crucial step was to install carefully chosen native plants. Imported topsoil was used instead of salvaged streambank topsoil because the old banks were completely infested with reed canarygrass (Phalaroides arundinacea). Reed canarygrass is an aggressive noxious weed that is considered a native by some authorities (Stubbendieck et al. 1997) since it has been planted in this country for so long. The native Emory’s sedge (Carex emoryi) was transplanted densely at one foot spacings where the reed canarygrass might invade again. Emory’s sedge is the dominant sedge found in flowing water in Boulder County and difficult to purchase commercially in large quantities. Volunteer seed collections in previous years provided the seed for contract growing Emory’s sedge through Rocky Mountain Native Plants. Other sedges and grasses were also planted for diversity, including Nebraska sedge (Carex nebrascensis), woolly sedge (Carex lanuginosa), black creeper sedge (Carex praegracilis), three-square bulrush, (Scirpus pungens), and prairie cordgrass (Spartina pectinata). Coyote willow (Salix exigua) tubelings were planted on the higher soil wraps and watered through the hot dry summer.
Riparian shrubs and trees were also planted to improve wildlife habitat along the Rock Creek corridor. The native shrub species included leadplant (Amorpha fruticosa), native hawthorn (Crataegeus erythropoda), wild plum (Prunus americana), wood’s rose (Rosa woodsii), and snowberry (Symphoricarpus occidentalis). Russian olive trees (Elaeagnus angustifolia) were removed prior to the project, although non-native crack willows (Salix fragilis) were allowed to remain because they provide some similar structure and function of native willow trees. About 90 native plains cottonwood trees (Populus deltoides) and peachleaf willow trees (Salix amgydyloides) were planted.
Almost 9,000 native plants were planted, not including the native seeded areas. Wildlands Restoration Volunteers and the Boulder County Youth Corps got involved with the fun of riparian restoration by planting all the wetland plants and grasses, and some of the native trees. On other parts of Rock Creek where the banks were not regraded, it is hoped that raising the channel elevation will help saturate previously high and dry streambanks to provide habitat for riparian vegetation once again.
Stubbendieck, J, S.L. Hatch, and C.H. Butterfield. 1997. North American Range Plants, 5th Edition. University of Nebraska Press, Lincoln, Nebraska.