9364 - Laserfiche WebLink

Home Browse Search

Analysis during the initial study was conducted to gain an understanding of the potential magnitude of contribution to peak flows that might be attainable through CROS efforts and to identify the types of water years where contributions by coordinated releases might be significant. Characteristic years were identified in the period of 1984 to 1994. It was found that during dry years, reservoirs would be operating strictly to meet existing demands and would have little, if any, discretionary water available. Further analyses were conducted for water years 1993 and 1994, which bracket the 'average' year of 1991 and represent the types of years in which discretionary releases may be able to be made and provide some benefit to peak flows. The analyses indicate that even the hypothetical removal of the total effects of existing transmountain diversions and reservoirs, which is not contemplated by the RIP, would not produce major increases in the peak spring flows. This is of particular concern as the operational modifications considered under this study are only a small fraction of 1 the total effects of present reservoir operations. Additionally, since discretionary releases represent a shifting of timing of releases from a reservoir and not an increase in overall volume released, there are seasonal tradeoffs to making the releases. The study team requested that the Program extend previous research into the flow effects upon habitat formation. Dr. John Pitlick of the University of Colorado found that certain velocities must be surpassed and sustained to cause channel formation in the gravel beds of the Reach. There are several non-fluvial factors that impede channel formation, particularly levees, riprap, and vegetative growth. If the flow increases due to coordinated reservoir operations are only a small percentage of the overall Colorado River flow during peak runoff, critical flow velocities for channel formation may not be obtained, or there may only be a marginal impact on channel formation. Dr. Pitlick is conducting further research into the flow effects upon silt movement, and it is possible that greater benefits may be attainable by targeting discretionary releases toward silt movement. The team analyzed historic climatological trends due to a concern that the 15-Mile Reach flow recommendations may have included data from a wetter period of record than is currently being experienced in the Colorado River Basin. The results indicate that up to half of the decreases in some mean monthly flows may be due to climatological changes during the past 90 years. Some of the flow recommendations are greater than 'virgin' flows, with all effects of development removed, for the recent drier period of 1954 to 1989. The study team simulated potential discretionary releases that might have been made in past years had the reservoirs been operated to enhance peaks. This analysis was being conducted with 1 very broad, simplifying assumptions. Initial indications were that there is some ability to enhance peak flows through coordinated discretionary releases though it is possible they should be aimed at cleaning out silts to improve habitat in the Reach. The U.S. Fish & Wildlife Service has also identified a range of peak flows outside of which it may be more beneficial to target releases at augmenting low flows during non-peak times ("Relationships Between Flow and Rare Fish Habitat in the '15 Mile Reach' of the Colorado River", U.S. Fish & Wildlife Service 1995). The conclusion of the feasibility study was that reservoirs should seek to bypass inflows during the peak, but only if bypasses would result in peaks greater than 12,900 cfs without exceeding the National Weather Service flood level of 26,600 cfs. This range is based on the the need for sediment flushing from the interstitial voids in the river channel for better survival of fish eggs and invertebrates (food source for the fish). 4