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RESULTS Flow regimes Embeddedness was sampled in 1996 on three dates during runoff and four dates during base flow. In 1997, sampling was done on three dates during runoff and two dates during base flow (Fig. 7). Runoff sampling had to be done after the peak flow had occurred so that sampling was physically possible, i.e., site selection could not be made nor sampling conducted when all bars were under deep, fast-flowing water. On the descending limb of the hydrograph enough bars became exposed to enable site selection and sampling. In 1997, there were some sites that could not be sampled on the first date because of excessively high water. Two runs and two riffles were sampled in reaches 9-B, 9-C and 8-13; only one riffle and one run were sampled in reach 8-A due to time constraints. In both years, peak flows exceeded the thresholds necessary to initiate bed movement in both the 15- and 18-mile reaches. Van Steeter (1996) studied two sites in the 15-mile reach and calculated that initial bed movement (characterized by sporadic movement of a few particles somewhere on the bed) occurs at flows exceeding 250 cros (8,825 cfs) at one site and 350 cros (12,350 cfs) at the other. Significant mobilization of the bed (characterized by near- constant motion of nearly all particle sizes nearly everywhere on the bed) occurs at 525 cros (18,530 cfs) at the first site and 725 cros (25,600 cfs) at the second. In the 18-mile reach, initial bed movement occurs at flows exceeding 420 cros (14,825 cfs) at one site and 575 cros (20,300 cfs) at another. Significant motion of the bed occurs at these two sites as flows exceed 1,100 cros (38,800 cfs) and 1,300 cros (45,900 cfs), respectively. Peak flow in the 15-mile reach was 566 cros (20,000 cfs) in 1996. We can therefore assume that initial bed movement occurred throughout the 15-mile reach for an extended period and significant motion of the bed likely occurred at select sites. In the 18-mile reach, the peak flow reached 847 cros (29,900 cfs) in 1996. Some bed movement therefore likely occurred throughout much of the 18-mile reach, but significant, widespread, mobilization of the bed probably did not occur. In 1997, the peak flow in the 15-mile reach was 750 cros (26,500 cfs) allowing for full mobilization of the bed. This was not the case in the 18-mile reach in 1997 when the daily flow peaked at 1042 cros (36,800 cfs), somewhat less than that required for significant, widespread bed mobilization. However, this was clearly enough to provide some bed movement throughout the reach. Thus, peak flows in both reaches during both years were adequate to move much of the bed. Also, in 1995, the year prior to our initial sampling year, peak flows were 838 and 1,362 cros (29,600 and 48,100 cfs) in the 15- and 18-mile reaches, respectively, clearly enough to produce significant, widespread mobilization of the bed in both reaches. Substrate Particle Size Substrate particle size of the surface layer varied among sites, as expected, but also varied within sites both spatially and temporally. Overall, median particle size ranged from 35 to 12