8208 - Laserfiche WebLink

Home Browse Search

when flows were approximately 2,300 cfs. Flows during 1990 and 1991 would need to have been greater than 8,000 cfs to create a comparable depth. We suspect that sedimentation of this backwater and the lack of adequate flushing flows during the low-flow years of 1988-1990 was responsible for reducing depth and thereby degrading the quality of this backwater. Backwater No.2, within site No.3, was sampled with transects Nos. 7, 8 and 9. Transect No.9 was later dropped from the analysis because it was laid out diagonally across the backwater instead of perpendicular to the long axis. Transect No.7 bisected the top end of the backwater and No.8 bisected it about midway up its length. Considerable bed change occurred at the midway site between 1990 and 1991. Scouring of bed sediments during the spring of 1991 left the backwater about three feet deeper. We graphed stage at various flows separately for the 1990 and 1991 cross- sectional profiles (Appendix Fig. VII). To achieve suitable summer depth at the deepest point, the midway transect in 1990 would require flows in excess of 4,426 cfs. For winter use (2.9-4.3 ft), flows in excess of 2,870 cfs were required at the midway transect. In 1991, flows of only 810 cfs were adequate to provide a suitable summer depth at the deepest point on the midway transect. For winter use, flows as low as 557 cfs were adequate in 1991 to provide a maximum depth that was suitable. Again, adequate depth at this backwater was as much a function of previous flushing spring flows as it was of stage. Run Transects (Appendix Figs. III, N, VI, VIII and IX) Fast and slow runs were preferred habitats only during summer when flows were 391-931 cfs. Suitable depth for fast runs during low-water, summer periods was 2.0-2.7 ft, but this was based on a very small sample (n=3 locations). For slow runs, suitable depth was 3.3-3.9 ft (n=26 locations) during low-water, summer periods (Table 2). Transects Nos. 1-B and 12 (side channel Nos. 2 and 4) and transects Nos. 2 and 6 (main channel Nos. 1 and 2; Table 3 ) bisected channel segments that contained fast runs when flows were low (810 and 557 cfs). Maximum depth along these transects during low flows exceeded our estimate of suitable depth for fast runs. Average transect depth was less than suitable during low flows in most cases but averages often included other habitat types adjacent to the fast runs and thus may not be a valid measure for comparison. Average depth for transect No. 12 (side channel No. 4) did fall within the range of suitability but only in 1991 after spring flows that year had scoured and deepened the bed (Table 3). Transects Nos. 1-A and 10-N (side channel Nos. 1 and 3) and transect No. 11 (main channel No. 3) bisected channels consisting primarily of slow runs at low flows. Maximum depth was less than the suitable at flows of 810 and 557 cfs at transect No. 1-A but was more than suitable at transect Nos. 10-N and 11. Average depth was insufficient at the two side channel sites (TR Nos. 1-A and 10-N) but was within the range of suitability at the one main channel site (TR No. 11). Summary We were able to develop some insight into the relationship between stage and habitat depth by monitoring 13 different transects in the 15-mile reach. However, it is difficult to draw definitive conclusions about minimum flows needed to provide adequate depth due to our low number of transects across some key habitat types and the dynamic nature of substrata. From our observa- tions over the years, both through radiotelemetry contacts and catch rates, the two backwaters we sampled with transects are probably the most important, i.e., heavily used, backwaters in the 15- 42