8112 - Laserfiche WebLink

Home Browse Search

108 mm depending on the site and year of sampling (Appendix; Tables 5-8). Only a slight difference in average particle size was detected between runs and riffles, with rifles having the smaller sizes (means were significantly different; medians were not [Table I]). One fairly consistent difference we noted, was that within sampling sites, the median particle size on the tops or upper flanks of newly exposed cobble bars (the channel margin), after high water receded, was smaller than in the bed submersed at base flow. For the study area as a whole, the adjusted mean rock width from upper-bank sites was 64 mm; from lower-bank sites, 85 mm. This difference was particularly acute in Stratum 8; there, the adjusted mean of upper- bank rock widths was 66 mm versus 92 mm for lower-bank rocks (Table 1). For the study area as a whole, particle size did not change from 1996 to 1997. Means and medians were very similar between years (Table 1). However, there were some changes at individual sites where sampling location was constant between years. All four of the significant changes (< 0.05) occurred at lower-bank sites (N = 12); no differences were detected between years in any of the upper-bank sites (N = 13). Lower-bank changes were evenly divided between size increases and size decreases. Average rock size was significantly larger in Stratum 8 (18-mile reach) than in Stratum 9 (15- mile reach). Interestingly, this can be attributed to a significant difference in lower-bank rock sizes between the strata (Table 1). There was no significant difference between strata in upper-bank rock sizes. Pitlick and Van Steeter (1998) reported the D-50 from two exposed bars in Stratum 9 and three in Stratum 8; the mean of their D-50's (56 mm) was similar to the mean of our upper-bank D-50's (59 mm). Embeddedness Results of embeddedness comparisons are presented in two ways: we first describe large- scale differences or similarities in embeddedness between various parameter types (run versus riffle, 1996 versus 1997, etc.); we then describe patterns of cleansing or deposition through time within runoff and base-flow periods. To make comparisons, we primarily used total depth-to-embeddedness (in mm) as our unit of measure. However, we also estimated relative depth-to-embeddedness, calculated as the mean number of median rock widths above the embeddedness layer. Because rock size distributions changed between years at some sites, patterns were more difficult to identify using relative DTE as the unit of measure for depth- to-embeddedness. Tables and additional figures of total and relative DTE at the individual sites are provided in the Appendix (Tables 5-8 and Figures 10-16). Comparisons Between Habitats, Flow Stages- Years and Reaches Mean DTE varied among sites and, within some sites, varied through time. From a total of 161 samples taken during the two-year period, mean DTE ranged from 80 to 171 mm. The adjusted mean changed little between years: 125 mm in 1996; 123 mm in 1997. During the two-year period, overall DTE across all sites was very similar between the two strata: 126 mm in Stratum 9; 122 mm in Stratum 8. Also, samples taken during runoff (upper-bank 14