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HomeMy WebLinkAboutAddenda - RFP - P982 BOBCAT RIDGE TRAIL DESIGN AND CONSTRUCTON (15)Chapter 2. Physical Resources The physical resources analyzed at Bobcat Ridge Natural Area include topography, geology, soils, hydrology, and scenic vistas. Recommendations in subsequent chapters are based in part on the capabilities and limitations of the physical resources listed here. Topography The topography of Bobcat Ridge Natural Area is varied; mountains, cliffs, rolling terrain, drainages and flat valley areas are all found within the landscape of this natural area. Forested hills dominated by Ponderosa pine and Douglas fir and reaching to over 7,000 feet elevation dominate the western section and comprises a named ridge (Green Ridge). The valley section of the property consists of open, flat grasslands and agricultural areas. The elevation of the valley bottom ranges from 5,500 feet on the west to5,400 feet on the east side. Hogback ridges made up of red sandstone cliffs on the natural area’s eastern edge rise to an elevation of 5,600 feet and create two unique canyons that cut through the hogback and drain eastward. Buffman Canyon is located on the north end and provides the main access to the site. Brooks Canyon located to the south is ecologically vibrant canyon rich in plant and wildlife diversity. The varied topography of Bobcat Ridge is indeed the source of the site’s scenic and biological resources. Geology (Braddock, et al. 1970) The geology of Bobcat Ridge varies widely across both space and time. Different formations dating from the Precambrian era and up to the Quaternary period are found here. In general, the geologic formations are older on the western, mountainous areas of Green Ridge. Younger formations are found directly east along the hogback ridges. Green Ridge is entirely Precambrian era metamorphic Tonalite and metasedimentary rock. Tonalite is light gray and varies from medium-grained, equigranular to fine-grained, porphyritic (large crystals embedded in a finer-grained matrix). The mineralogy of the metasedimentary rock varies with metamorphic grade. Within Bobcat Ridge these rocks are primarily quartz-feldspar schist and gneiss, and mica schist and gneiss. The valley floor was formed in the Fountain Formation (Carboniferous period) overlain by Quaternary era alluvial fan deposits. The Fountain Formation is a red and minor gray, coarse-grained sandstone with lenses of siltstone and fine-grained sandstone. Alluvial fan deposits consist of material washed off of Green Ridge and deposited within the many west- east drainages that cross the area’s valley. As the valley floor rises to form the hogback ridge, the Fountain Formation can be seen on its western slopes as exposed red cliffs. Moving eastward, the top of the hogback is made up primarily of the Permian period Ingleside and Satanka Formations. The Ingleside Formation consists of red calcareous, fine - to medium-grained, well-sorted, cross-bedded sandstone while the Satanka Formation consists of red siltstone and fine-grained sandstone. These formations form the red cliffs visible on the higher elevations of the hogback and in Buffman and Brooks Canyon. Lyons Sandstone dating to the Permian period covers the eastern-most slopes of the hogback and is made up of red and pink fine- to very fine-grained, well-sorted, cross-bedded sandstone. Recommended Actions for Geologic Resources:  Do not allow rock climbing on fragile, easily erosive rock cliffs.  Assess site for potentially appropriate rock climbing area. Soils (Moreland 1980) The soils on Bobcat Ridge Natural Area vary depending on location and underlying geology. Valley soils are quite complex with varying slopes, soil textures, and series classification. Soils formed on Green Ridge are less complex, as are soils on the hogback since these areas are erosional rather than the depositional soils of the valley (Figure 2.1). Green Ridge Soils The upper elevations of Green Ridge are dominated by Wetmore-Boyle gravelly sandy loam mixed with rock outcrop with areas of Elbeth-Moen loams and Breece coarse sandy loam. The latter is found in areas such as Mahoney Park. These soil types are shallow, stone, occur on very steep slopes that exhibit rapid runoff, and are highly erosive. These soils may withstand limited grazing under a well planned grazing system that is directed toward maintaining adequate plant cover. There are severe limitations to recreational use and building site development mainly due to high angle slope, and shallow depth of bedrock. The Natural Resources Conservation Service recommends that these areas are primarily suited for native grasses and woodlands for wildlife habitat. The lower slopes of Green Ridge are dominated by stony or cobbly sandy loam Haploborolls mixed with rock outcrop. These soils have similar capabilities and limitations as those mentioned above. The Bobcat Gultch Fire in 2000 denuded the vegetation on the Green Ridge area. Subsequent rainfalls caused much erosion of the exposed soils and nearby drainages. Aerial seeding efforts of the burned area by the U.S. Forest Service have successfully created a ground cover of grasses. However, the area is still unstable and erosion potential is high. An assessment of areas with high erosion potential is needed and those areas should be stabilized to reduce soil loss. Valley Soils The soils found in the valley bottom at Bobcat Ridge display a very complex distribution. They are primarily loams of the Kirtley-Purner complex and Satanta series. Areas of Connerton-Barnum complex, Harlan and Purner fine sandy loams and other minor series are mixed throughout. Slopes range from nearly level to strongly sloping with slow to rapid runoff. Soils on the western section of the valley tend to have severe limitations that make them generally unsuited for cultivation due to the severe erosion hazard and are primarily suited to pasture and native grasses for wildlife habitat. Soils on the eastern section of the valley tend exhibit severe limitations related to erodability that restrict the choice of cultivated plants, requires special conservation practices, or both. Under irrigated conditions these soils support wheat, barley, corn, sugar beets or beans. In dryland conditions they are best suited to pasture or native grasses. Valley soils present few limitations with regards to recreational uses, however engineering designs for trails, structures, or other improvements may need to be made to account for high, shrink-swell potential, corrosiveness and general low strength of heavy-textured soils. Hogback ridge soils Soils on the hogback on the eastern edge of Bobcat Ridge are a mixture of rock outcrop, loamy soils of the Purner series and Haplustolls or Baller soils ranging from stony sandy loam to loam in texture. Slopes are generally steep with vertical cliffs present on portions of the rock outcrop. Runoff in this area tends to be rapid exhibit a severe erosion hazard. These soils are primarily suited to native grasses and wildlife habitat. Limited grazing under a well planned grazing system that emphasizes the protection and maintenance of plant cover may be tolerated. Hogback soils present severe limitations to recreational use and building site development primarily due to slope, depth to bedrock and presence of large surface stones. Recommended Actions for Soil:  Complete a grazing plan that is directed toward maintaining adequate plant cover to limit erosion on green ridge and hogback ridge soils.  Carefully engineer trails on valley soils to account for high shrink-swell potential, corrosiveness and general low strength of heavy-textured soils.  Complete soil assessment prior to construction of public improvements  Complete soil assessment prior to restoration work.  Assess areas with high erosion potential to prevent further erosion and improve water quality. Figure 2.1. Soil map of Bobcat Ridge. Hydrology/Water Resources Bobcat Ridge lies within the Buckhorn Creek watershed, which in turn is nested within the larger Big Thompson River watershed. The contribution of Bobcat Ridge to Buckhorn Creek in terms of surface runoff is minimal. However, its contribution to groundwater dynamics may be more significant as there are a number of springs located on the property. The hydrologic system on Bobcat Ridge includes several springs, two ephemeral streams and a small irrigation-fed pond. The Hansen Feeder Canal, although concrete lined, may play a part in the hydrologic system at Bobcat Ridge. The known springs on Bobcat Ridge can be found in and around the Mahoney Park area. One of these, on the northeastern side of Mahoney Park, feeds a small earthen-dammed detention pond used for watering livestock. Another, on the western side of the park, feeds wetland seeps among the rock outcrops. This same spring flows underground across Mahoney Park and resurfaces on the eastern side where it is used to supply a livestock watering tank. A third, located in the major drainage just south of the park, has been excavated out of the side of the hill to create a watering area for livestock. It is unclear whether this third spring would have produced surface water historically or just manifested itself as a seep within a wetland vegetation complex. Other springs may be found on Bobcat Ridge with further investigation. There are two ephemeral stream networks on Bobcat Ridge Natural Area. The northernmost collects several drainages including those containing the known springs in and around Mahoney Park. This stream course cuts the hogback ridge forming Buffman Canyon before continuing on to Buckhorn Creek. The southernmost stream network collects two major drainages and a handful of smaller ones before cutting the hogback to form Brooks Canyon and continuing on to join Buckhorn Creek. Surface water is present in this stream course in Brooks Canyon; however, it is unclear as to whether this water is the result of a naturally occurring spring, spillover from a irrigation pipe fed by a siphon off the Hansen Feeder Canal, or seepage coming directly from the canal. A small retention pond exists on the western edge of the valley just north of the Powerline Road. This pond is primarily fed with irrigation water pumped over from the Hansen Feeder Canal, leased to the ranchers (lessee of the city). The pond would also capture surface runoff from three minor drainages during heavy precipitation events or during the spring melt. The pond lies within the Buffman Canyon stream network although its role in the overall hydrology is unclear since there is no surface flow coming out of it. The Hansen Feeder Canal is a concrete lined ditch that is part of the Horsetooth Reservoir supply network. The canal is owned and operated by the Northern Colorado Water Conservancy District. Water from this canal is used to supply the retention pond mentioned above as well as a stock tank located west of the head of Brooks Canyon. Although it is lined some leakage may occur and contribute to the hydrology of Bobcat Ridge. The location(s) and extent of this contribution are uncertain at this time. Recommended Actions for Hydrologic Resources:  Assess the hydrologic systems to better understand sources of wetlands and drainages.  Restore dammed drainages to ensure continued and natural hydrology of wetlands and drainages.  Rehabilitate eroded drainages that were damaged by heavy rains following the 2000 Bobcat Gulch Fire. Establish sufficient ground cover over the burned areas for greater infiltration and reduction of sediment laden runoff.  Design trails to keep people away from canal and post signs that canal area is closed to public. Scenic Resources This area serves as a scenic resource for visitors to the area. Bobcat Ridge Natural Area is clearly visible from the top of Horsetooth Mountain and many of the other surrounding ridges (mostly privately owned). Not only does Bobcat Ridge serve to preserve a continuous scenic landscape when viewed from afar, but the varied topography, geology and plant communities found on the property provide a scenic and aesthetic resource for visitors to the natural area (Figure 2.2). Currently, a private small aircraft landing strip at the crest of the hogback on neighboring property and the Denver International Airport flight path over the property can affect the peaceful, scenic resource at times. Wildfires, residential development and expanded quarrying operations are all potential future negative impacts to the scenic and aesthetic values of this natural area. Recommended Actions for Scenic Resources:  Limit vehicle travel on site in order to reduce noise and improve air quality.  Achieve state and local air quality standards during any management actions such as prescribed burning for long-term air quality and visibility of scenic resources.  Investigate burying the power line through the grassland/meadow area of the natural area.  Design trail to provide scenic vistas of the unbroken landscape within the natural area. Chapter References Braddock, W. A., R. H. Calvert, S. J. Gawarecki, and P. Nutalaya 1970. Geologic map of the Masonville quadrangle, Larimer County, Colorado. U. S. Geologic Survey, Washington D.C. Moreland, Donald C. 1980. Soil survey of Larimer County area, Colorado. U. S. Dept of Agriculture, Washington D.C. Figure 2.2. Looking northeast toward Horsetooth Mountain (left) and east toward Milner Mountain (right).