7848 - Laserfiche WebLink

Home Browse Search

each of these three periods (Appendix; Table H.1.) and a flow-duration curve was developed for each month for the post-Aspinall period at the Whitewater gage (Appendix; Figures H.1-H.6.). The quantity of water delivered from upstream Federal reservoirs during nonrunoff months has increased and thus has increased water availability to downstream irrigation users. Dams and reservoirs on mainstem and tributary streams now allow water managers to allocate and redistribute water more evenly throughout the year for flood control and to accommodate downstream irrigation users. Since construction of the Aspinall Unit dams and reservoirs, flows can be delivered to downstream reaches during extreme low flows to sustain endangered fishes. Since 1917, 300 cfs or less has occurred in the 2.3-mile reach less during 1967-1994 than the previous two water-development periods. During July, August, and September, months that have the lowest flow in the 2.3-mile reach, the number of days flows fell below 300 cfs was less following construction of Aspi nal l than during the previous water period (1939-1964; Appendix; Table H.1.). Based on current water availability, flows less than 300 cfs still occur in 3.5 out of every 10 years in July and in four out of every 10 years in August in the 2.3- mile reach. In September during the 1967-1994 period, flows fell below 300 cfs in two out of every 10 years compared to 7 of 10 years during two previous water development periods (Appendix; Table H.1.). During the 29-year period from 1967- 1995, the number of days that the mean daily discharge was from 0 to 300 cfs in the 2.3-mile reach for July, August, September, and October was 604 days, or 33.6% of the time. During the same period, there were 226 days (about 12.8% of the time) that discharge was 0 cfs (Appendix; Table H.2.). The greatest number of days in which flows were 300 cfs of less occurred in 1977. Flows were less than 300 cfs for almost the entire four-month period (121 days) and 0 cfs occurred for 90 days. Similarly, the frequency that flows fell below 300 cfs during the non- runoff months, October through March, occurred much less often following construction of the Aspinall Units. For example, during October, flows only fell below 300 cfs 18% of the time from 1967-1994, compared to 52% of the time from 1939-1964. A five-fold decrease was observed in December, 13% versus 69% comparing the same two periods. Surprisingly though, since 1965 flows less than 300 cfs still occur monthly in the 2.3-mile reach (Appendix; Table H.1.). Flow in the Gunnison River is currently managed to store water during runoff months and augment natural flows during pre- and post-runoff months. For the runoff months of May and June, the frequency of flows below 300 cfs was S- and 8%, respectively, during 1965-1994. For May, this was about 16 times greater than the 1939-1964 water development period and about 50 times greater than the 1917-1938 period (Appendix; Table H.1.). SYNTHESIS River Management Low- and Moderate-Water Years This study was to recommend a minimum flow for the 2.3-mile reach of the 28