Gunnison River: Executive Summary

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Biological and Field Survey Data The NPS has conducted scientific studies and analysis as reported in the May 9 th 2003 letter from the DI to Greg Walcher, Executive Director, Colorado Department of Natural Resources. These studies indicate the Black Canyon and Gunnison Gorge are matchless environments, which have created a diverse group of isolated biological communities that provide unique opportunities for scientific study. This segment of river also contains one of the finest trout fisheries in the world and has been designated by the Colorado Division of Wildlife as Gold Medal and Wild Trout Water (see "Scientific Basis for the Requested Appropriation to Protect the Natural Environment of the Black Canyon of the Gunnison National Park to a Reasonable Degree" in Appendix A — Attachment B). Instream Flow Recommendation The CWCB staff relied upon the expertise of the cooperating agencies to interpret output from the data collected and to develop the initial instream flow recommendation. This initial recommendation is designed to address the unique requirements of the river segment without regard to water availability. For this segment of river, the NPS collected data on Sediment, Aquatic Invertebrates, Fish and Streamside Vegetation (see Streamflows Necessary to Protect the Natural Environment — Appendix A — Attachment B). These NPS studies conclude: • Sediment - "To protect critical biological habitats and the spectacular gorge, sediment transport capacities and main stem channel proportions must be maintained. Peakflows are necessary to scour the riverbed and rework debris flows and other coarse - grained deposits within the main stem channel. Peaks between 2,500 cfs and 10,000 cfs effectively mobilize gravels and cobbles within the low flow channel, adjacent low flow banks, and overflow channels. However, flows in excess of 10, 000 cfs are needed in some years to mobilize materials on lateral gravel bars and debris flow deposits. "; • Aquatic Invertebrates — "To protect aquatic invertebrates populations and habitats in the main stem channel and adjacent alluvial deposits requires peak flows to redistribute sediment, and circulate water, oxygen, and nutrients through the channel alluvium. Peaks between 2,500 cfs and 10,000 cfs effectively mobilize gravels and cobbles within the low flow channel, adjacent low flow banks, and overflow channels; thus serving to scour attached algae, flush entrained organic matter, re- oxygenate channel alluvium, and wet aquatic habitats within the active channel. Flows in excess of 10, 000 cfs, which move gravels and cobbles on lateral gravel bars, are needed to rejuvenate and create new aquatic insect habitat. Appropriate ramping rates associated with the peak must allow ample time and opportunity for hyporheic and other aquatic insects to respond to changing flow conditions... "; • Fish — "To enhance native fish species, while at the same time assuring the continued presence of a self - sustaining exotic sport fishery, requires base flows; annual peak and shoulder flows with appropriate timing, magnitude, and duration: and occasional large peaks to create new biotic habitats... Appropriately timed annual peaks between 2,500 cfs and 10, 000 cfs remove fine sediment, accumulated organic debris, and algae from the active channel without disrupting species life cycles. Rare, large peaks greater than 10, 000 cfs redistribute inorganic substrate and create new biotic habitats. "; • Streamside Vegetation - "To prevent the permanent establishment of woody riparian vegetation and to limit the encroachment of herbaceous vegetation on depositional features located between the current active channel and the canyon walls requires peak -3-