Case Study 4 The Eleven-mile Canyon Demonstration Project Project Overview The Eleven-mile Canyon Demonstration Project is located on the South Platte River in Park County, Colorado (figure 1). This project area is located in a semiarid (less than 15 inches of precipitation per year) portion of the foothills zone of the Rocky Mountains (Pennak 1977). The bedrock underlying this canyon area is comprised primarily of biotite granite that produces highly erodible, unproductive soils. This condition also results in sparse ground cover, contributing to the area’s erosive nature. The landscape is bisected by relatively steep, narrow drainages with abundant erosional stream channels. Figure 1: Vicinity map of Eleven-mile Canyon. Study reaches are identified by reach numbers. (The inset map identifies the location of the canyon within the Pike National Forest.) In the late 1800s a railroad was built in the Eleven-mile Canyon, further narrowing the Eleven-mile Canyon floor. Large native trees were removed Eleven-mile Canyon Demonstration Project from the valley floor for use in the nearby metropolitan areas. In the early 1900s the city of Denver began building reservoirs and associated conduits for water storage in the South Platte River corridor, for use by the increasing population. The releases from these reservoirs have created a hydrograph considerably different than what would be expected under historic conditions (figure 2). The removal of trees, location and maintenance of the road corridor, and modified flow releases from upstream reservoirs has created a stream system considerably altered from pre-European settler conditions. 3—43 Developing Monitoring Plans— Chapter 3 Figure 2: Comparison of a hydrograph from a regulated section of the South Platte River downstream of Eleven-mile Canyon to a more natural flow regime in Tarryall Creek, a tributary to the South Platte River. In 1993, a complete stream-condition inventory determined limiting factors for trout production (Winters, Gallagher, McMartin, 1993). The results showed that the lack of inadequate habitat complexity was the primary limiting factor in portions of the canyon. Low-gradient “glides” were areas particularly associated with relatively homogenous habitat conditions. To remedy the poor habitat conditions, the Pike-San Isabel National Forests and the Colorado Division of Wildlife (CDOW) signed a multiyear cooperative agreement. We identified strategic boulder placement, anchoring large coniferous trees in sections of the stream channel, and reconstructing the stream channel as appropriate for this type of system (Rosgen C-type channel). Construction began in 1996. We placed structures primarily in glides with fairly uniform current velocities and little heterogeneity in habitat conditions. Eleven-mile Canyon Demonstration Project From our inventory efforts, we agreed that physical changes in the morphology of the river were necessary for providing better quality habitat availability for various life-stages of brown and rainbow trout. The desired changes included: 1) A decrease in the width-to-depth ratio where appropriate. 2) An increase in residual pool depth in identified pools. 3) An increase in habitat complexity in relatively sterile glide habitats. 4) No loss or improvement to aesthetic values for visitors. 5) An ultimate increase in abundance of adult rainbow and brown trout. 3—44 Case Study 4 To determine the effectiveness of the instream channel structures, we attempted to answer these four questions. 1. Do instream channel structures in a regulated mountain river increase aquatic habitat complexity and ultimately adult trout abundance (density and biomass) by increasing habitat complexity and residual depth, and decreasing the width-to-depth ratio of the project area? 2. Do structure orientation and elevation determine the effectiveness (scour success and self-maintenance) of instream channel structures? 3. Which types of structures are most efficient in increasing aquatic habitat complexity, residual pool depth, and fish abundance (density and biomass)? 4. Which structures and placement characteristics result in limited maintenance while providing the desired effects? Project Methods, Design, and Monitoring In 1993, initial preproject basinwide stream habitat surveys used a modified Hankin-Reeves method (Winters, et al, 1991, rev. 1997). We identified stream reaches according to channel type and valley configuration (Rosgen 1996). Within each reach, we recorded specific stream channel morphological and biological habitat conditions. Also, we took photographs for each habitat unit (e.g., pool, riffle, glide) and other unique or impacted areas, for later comparisons with postproject conditions. To further quantify the potential influence of modifying various habitat complexity to homogenous glides in the project area, we modeled changes in stream velocities, substrate, and depth using the physical habitat Eleven-mile Canyon Demonstration Project simulation model (PHABSIM) for different life-stages of brown and rainbow trout developed by Bovee (1982). The purpose of this exercise was to determine whether the PHABSIM model could accurately predict the changes in habitat for different life-stages of brown and rainbow trout. The CDOW developed habitat suitability curves for the South Platte River, and used them to identify potential changes in adult and juvenile life- history stages of both species. We identified a glide in the project area that had conditions of both a typical homogenous habitat in the project area and areas of complexity due to boulders and a large tree within the stream channel. These natural “structures” comprised a relatively small percentage of the larger habitat. 3—45 Developing Monitoring Plans— Chapter 3 We carefully placed seven transects within the glide, one at the downstream hydraulic control and six to represent the various conditions within the habitat unit. To apply the PHABSIM model, to represent the glide without structure, we weighted the transect at the hydraulic control and those upstream that weren’t influenced by structure and modeled it for various flows, species, and life-history stages. To elucidate the effect of adding structure to the river, we entered the remaining transects into the model and ran the procedure a second time. We hypothesized that the difference in weighted useable area (WUA) between the modeling effort using the different transects would be realized when various structures were built. Because of modeling limitations, we did not incorporate instream cover, such as overhead seclusion objects, into the model. The CDOW had been conducting population estimates of salmonids in the demonstration area for several years, using the 2-pass, Seber-LeCren removal methodology (Seber 1982). This method locates two stations in the project reach that do not overlap the structure placement sites. Two stations occur at the structure sites. The CDOW used truck-mounted multiple-array electrofishing gear with each fish weighed and measured for population density and biomass estimates. The CDOW made maps of specific structure placements. After identifying the specific habitats, the CDOW strategically placed cross- sectional transects. To determine what (if any) changes in stream channel morphology occurred as a result of the project implementation, the DCOW also established permanent benchmarks for reference, and measured both elevation and stream current velocities before project implementation. We used pre- and post-project monitoring design to determine changes in channel scouring and deposition associated with the structures as Eleven-mile Canyon Demonstration Project well as with fish population changes. We also determined reference habitat conditions outside of the project area were not warranted for this study. Comparisons between pre- and post-microhabitat conditions at the particular project site became the means by which we would measure the success of the structures. In addition, we regularly documented the effectiveness of each structure to remain functional over time. Fish sampling took place in the fall. We conducted habitat transects and stream mapping at the low-flow period following runoff, and conducted PHABSIM measurements at three different flow levels to encompass the range of flows in the project area (Bovee 1982). We measured facet slope of structure boulders—as well as maximum and residual pool depth downstream of each of the boulders that make up the vortex and deflector structures—once in 2003. The purpose of collecting 3—46 Case Study 4 this data was to determine whether any measurable benefit existed to angling the slope of the top of individual boulders slightly downstream, to maintain effective scour. We began this project with the following assumptions: l The monitoring techniques would be sensitive to changes realized by the project. l Funds and personnel would be available when needed. l The South Platte River would exhibit typical dynamic flow regimes during the monitoring efforts. l Changes in stream channel morphology and fish population dynamics could be related to the project. l Consistency in monitoring efforts was paramount. l Stochastic events such as landslides, unusual flooding, drought, or anthropogenic activities would not compromise the success of the project. Monitoring Results and Interpretation (a) Basinwide Inventories Table 1 presents a subset of the basinwide inventory results conducted prior to project implementation. We designed this type of inventory to identify