8208 - Laserfiche WebLink

Home Browse Search

Thus the flow that relies least on backwaters for maximizing total area of preferred habitat, is the flow most likely to result in the highest amount of preferred habitat that is actually usable. At 1,630 cfs, backwaters made up the smallest proportion (24%) of total weighted preferred habitat (at 1530 cfs it was 26%; at 1,240 cfs, 38%; at 2,870 cfs, 41%). Thus, our recommendation for summer is to maintain flows of 1,630 cfs during years of above average (50%) precipitation and for flows to not fall below 1,240 cfs during years of low precipita- tion when the recommendation of 1,630 cfs would be difficult to meet. During drought years (the lowest 20%), fish could probably be maintained at 810 cfs. For winter, flows of 1,630 cfs are recommended for most years (80%), and 1,240 cfs should be a minimum flow during years of drought. These recommended flows are for the top of the 15-mile reach, measured at the USGS gauging station, and assume that current irrigation return flows continue to provide additional water at various downstream points within the reach. SPRING FLOW NEEDS Background Recommendations for flows in the 15-mile reach during spring were previously provided by Osmundson and Kaeding (1991). They documented the extent to which spring flows have been reduced since the 1940's, largely as a result of upstream water storage and transbasin diversions. Effects of reduced flow on populations of Colorado squawfish and razorback sucker were described. The authors developed their conclusions using a combination of empirical data, observations, and current biological and geomorphological theory. Recommendations for spring flows in the 15-mile reach were designed to fulfill two purposes: 1) improve habitat conditions within the 15-mile reach ; and 2) together with flows from the Gunnison River, improve conditions downstream of the reach. Benefits described by Osmundson and Kaeding (1991) that result from higher spring flows included: 1) maintenance of channel complex- ity and habitat heterogeneity; 2) improvement of Colorado squawfish reproductive success; 3) inundation of bottomlands for razorback sucker spawning and nursery habitat; and 4) control of certain prolific, non-native fish species. Relationships between flow and reproductive success of Colorado squawfish and relationships between flow and control of non-native fish species were based on locally-collected, empirical data. The need to inundate low-lying lands adjacent to the river during the razorback sucker spawning season was based on biological data regarding attributes of razorback sucker life history. Im- portance of channel complexity was based on squawfish habitat selection data that was collected locally. The effect of flow on channel maintenance in the 15- and 18-mile reaches was based on observations of vegetation encroachment and deposition of fine sediments there over a six-year period coupled with causal explanations in the literature by geomorphologists studying similar trends in other river systems. Osmundson and Kaeding (1991) recognized the potential for habitat loss due to sedimentation of low-velocity sites and channel simplification but had no data to document its occurrence;. in 49