8208 - Laserfiche WebLink

Home Browse Search

Because pools are highly preferred during winter, weighted area also goes up. However, because we do not know if backwaters, eddies and pools will still be preferred at flows less than 1,654 cfs during winter, we cannot recommend a winter flow of 557 cfs based on weighted area of pools, backwaters and eddies. Thus we recommend 1,630 cfs as the best discharge level for maximizing the area of preferred habitats during winter. Mean winter flows (November-March) during the historic period (1902-1942) range from 1,322 (January) to 1,789 cfs (November) and averaged 1,531 cfs for all months (see Osmundson and Kaeding 1991). Recent (1954-1989), mean, winter flows range from 1,765 (January) to 2,161 cfs (November) and average 1,920 cfs. Thus, our recommended winter flow of 1,630 cfs would be 100 cfs higher than average historic winter flows and 290 cfs less than recent winter averages. Habitat Depth Minimum Flows for Providing Suitable Depth Habitat quality needs to be considered along with maximizing habitat area. Two measurable variables that affect quality are habitat depth and habitat diversity. Depth, considered in this section, is an important component of cover. As discussed earlier, depth and turbidity are probably the primary sources of cover for fishes in this portion of the Colorado River, though during winter, ice provides a third cover component (Osmundson and Kaeding 1989). Backwaters, eddies and pools are the preferred habitat types of adult squawfish during summer and winter. During winter, pools replace eddies as the most preferred type. Depth at fish locations within these habitat types varied somewhat between summer and winter (Fig. 20; Table 2). Though depths at fish locations in eddies did not differ significantly between seasons, depths in backwaters and pools selected by the fish did differ between summer and winter. Fish used deeper backwater sites in summer than in winter and used deeper pools in winter than in summer. Our objective was to use the stage/depth information obtained from the 12 transects to see if depth of preferred habitats was suitable or not during the flow levels recommended from the mapping output. We felt that for each habitat type, the average depth used by the fish, or rather the range of variance about the mean (plus or minus one standard error), provided a reasonable estimate of optimum or at least suitable depth. Preferred depth, a better indicator of optimum depth, could not be determined because measuring preference requires a comparison between use and availability. We had too little depth data to determine availability. Using the depth-use data, backwaters, for example, would have a range of suitable depths during summer of 3.4-4.5 ft; during winter, 2.6-3.1 ft (see Table 2). Presumably, water more shallow than that generally used by the fish provides insufficient cover. Water deeper than average should provide additional cover and therefore is not considered detrimental. Low use of deep water likely reflects its low availability. Ideally, using the above example, we would like to know at what flow level the average and maximum depth of most backwaters in the 15-mile reach is at least 3.4 ft during summer and 2.6 ft during winter. This was the rationale behind monitoring stage at our transect sites. Stage, along with the bed cross-section data, allowed us to measure maximum and average depth at various flows (Table 3). However, there were not enough transects to allow us to make more general conclusions about relationships between stage and depth of habitats in general. For example, to 37