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Evaluation of Flushing Flows in the Fraser River and Its Tributaries

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Evaluation of Flushing Flows in the Fraser River and Its Tributaries EVALUATION OF FLUSHING FLOWS IN THE FRASER RIVER AND ITS TRIBUTARIES Prepared for Trout Unlimited Prepared by Brian P. Bledsoe, Ph.D., P.E. Professor Johannes C. Beeby, M.S. Research Associate Kyle W. Hardie, M.S., E.I.T Research Assistant September 19, 2013 Colorado State University Department of Civil and Environmental Engineering Daryl B. Simons Building at the Engineering Research Center Fort Collins, Colorado 80523 EVALUATION OF FLUSHING FLOWS IN THE FRASER RIVER AND ITS TRIBUTARIES Prepared for Trout Unlimited Prepared by Brian P. Bledsoe, Ph.D., P.E. Professor Johannes C. Beeby, M.S. Research Associate Kyle W. Hardie, M.S., E.I.T Research Assistant September 1, 2013 Colorado State University Department of Civil and Environmental Engineering Daryl B. Simons Building at the Engineering Research Center Fort Collins, Colorado 80523 EXECUTIVE SUMMARY The Fraser River has a storied history as a trout fishery, and trout have been identified as a key indicator of its ecological health. Sustaining trout and their food base in the Fraser River requires the long-term maintenance of physical habitat quality and ecosystem characteristics on which they depend. In addition to adequate streamflows, spawning fish and aquatic invertebrates depend on open interstices in the river bed. Field reconnaissance performed in the Fraser River watershed over the last decade has indicated that fine sediment storage is occurring in several locations in the main stem and tributaries. Evidence of sediment storage includes bars, other deposits of sand and very fine gravel, and embedded substrates that lack interstitial space. Streamflow depletions in the Fraser River watershed have increased the risk of habitat degradation associated with sediment deposition and clogging of the river bed. The proposed Moffat Collection System Project could alter the capacity of the Fraser River and its tributaries to flush fine sediment and maintain physical habitat for fishes and aquatic insects. By further reducing streamflows, the proposed project would decrease the number of days on average that flows exceed the thresholds necessary to clean and rejuvenate the river bed. If there are longer multi-year periods between flushing events, more sediment could accumulate at time scales relevant to the reproductive cycles of trout and aquatic insects. Recent research conducted in comparable river segments in the upper Colorado River has documented habitat degradation and negative impacts to aquatic life in the absence of high flows that clean the river bed. The key to evaluating whether sediment accumulation is likely to be exacerbated by a proposed reduction in streamflow is to examine and quantify potential changes in the “flushing flows” that primarily influence cleaning and rejuvenation of the river bed. These flows may also be termed substrate maintenance flows. Flushing flows can achieve multiple objectives that include removing surface veneers of fine sediment, scouring algae, and opening-up interstitial space in the river bed that would otherwise remain continuously embedded with fine sediment. In developing management recommendations for flushing flows, it is advisable to avoid “single factor ecology” approaches that focus exclusively on one element of the streamflow regime for one type of organism. Given that trout are a highly-valued amenity and primary indicator of ecological health in the Fraser River, maintenance of their food base is also an important consideration. Trout depend on aquatic insects at all life stages from rearing to death, and production of aquatic insects depends on the quantity and quality of habitat available in the river substrate. Therefore, flushing flows are important for riffle habitats used by aquatic insects in addition to trout spawning and rearing habitats. The potential effects of additional high-flow alterations on aquatic habitat in the Fraser River cannot be thoroughly assessed until the flows that mobilize and clean coarse substrates are more explicitly quantified and understood. The overall goal of this study was to evaluate and quantify i | P a g e flushing flows that maintain sediment sizes on the bed and their mobility in the Fraser River and its major tributaries. Improved quantification of flushing flows can inform decision-making focused on the potential long-term environmental impacts of further flow extraction. The specific objectives of the study were to: Characterize spatial variability in channel morphology and grain-size distributions in the bed of the Fraser River, Ranch Creek, and other significant tributaries. Perform a sediment entrainment (shear-stress based) analysis at each bed material sampling and survey location to assess the likelihood of two types of flushing flows: 1) flows required to remove surface deposits of fines, and 2) flows required to mobilize appreciable amounts of coarse gravel and cobble at the surface of the river bed to remove surface veneers of fine sediment and perform the additional function of opening-up interstitial space deeper in the bed that would otherwise remain continuously embedded with fine sediment. Estimate critical discharges for flushing flows with explicit consideration of uncertainty and describe resulting estimates of critical discharges for substrate maintenance as probability distributions to inform decision-making. Combine the sediment entrainment analysis with existing hydrologic data to estimate the magnitude, frequency, and duration of dimensionless shear stress values for the baseline condition vs. the tentatively-selected plan for the Moffat Collection System Project at each survey location. Identify flushing flows that are likely to perform substrate maintenance across the range of variable conditions observed in the field, based on the best available information from this and other studies. We established sixteen study sites and used a weight-of-evidence approach to estimate critical discharges for flushing flows of varying effectiveness. Shear stress analyses based on highly-detailed characterizations of the channel substrate (including riffle habitats) were performed at each site. Key evidence used to estimate flushing flows included: pre- vs. post-2013 runoff evidence of flushing based on field substrate characterization; tracer rocks; post-2013 runoff evidence of sediment flushing, based on highly-detailed field characterizations of the river bed at each site; magnitudes and durations of 2013 peak flows as measured by the U. S. Geological Survey (USGS) (where available); sediment entrainment computations including relative confidence as quantified by uncertainty analysis; sediment entrainment information available in the Grand County Stream Management Plan (GCSMP); and convergence of evidence between this study and the GCSMP. ii | P a g e A frequency analysis of flushing flows was conducted for eleven sites (six locations on the Fraser River main stem, three on Ranch Creek, one on Vasquez Creek, and one on St. Louis Creek). We compared two Platte and Colorado Simulation Model (PACSM) scenarios of Base 285 existing flows (Current Conditions) vs. Alt 1a Moffat Tunnel Collection System Project (Projected Conditions). Overall, the results of the sediment entrainment analyses revealed variation along the river system in the magnitude of flushing flows required for coarse substrate mobilization. Despite this variability, there is considerable convergence between the estimates generated in this study and the GCSMP. Flushing flows identified in this study are, in some instances, higher than the GCSMP recommendations because sediment entrainment analyses and field observations indicate that some of the GCSMP values have a low probability of providing substantial surface flushing and/or coarse substrate mobilization. Based on multiple lines of evidence for each segment, the flushing flows summarized in Table ES.1 are, in our judgment, the most probable estimates of minimum streamflow required for coarse substrate mobilization and flushing surface veneers of fine sediment. Table ES.1. Estimates of minimum flows required for coarse substrate mobilization and flushing surface veneers of fine sediment. Minimum flows for superficial flushing of the river bed are only provided for Fraser River main stem sites with bed slope <1% to reflect higher uncertainty in steeper river segments. Flushing flow magnitudes are compared to values recommended in the GCSMP. Minimum Estimated Minimum Estimated GCSMP Flushing Flow for Coarse Flow for Flushing Surface Recommended Site Substrate Mobilization Veneer s of Fine Sediment Flushing Flow (cfs) (cfs) (cfs) Fraser above Diversion 45-100 – – Fraser below Diversion to 100 – 80 Vasquez Creek Fraser between Vasquez Creek 200 200 – and St. Louis Creek Fraser River between St. Louis – 280 200 Creek and Ranch Creek Fraser River between Ranch 370 280 – Creek and Crooked Creek Fraser River between Crooked 470-640 400 400 Creek and Granby Upper Ranch Creek 55 – 40 Lower Ranch Creek 150 – 150 Vasquez Creek 70 – 50 St. Louis Creek 100 – 70 iii | P a g e Flushing flows for surface removal of fine sediments are recommended for every year. The recommended minimum frequency for coarse substrate mobilization flows is every other year (2- year return period over long-term average). A tentative duration of 3 days is recommended for both types of flows. In practice, an event that mobilizes the coarse substrate on a strict interval of every other year is not likely to be feasible under the current management infrastructure given inter-annual
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