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1 . Draft Final Completion Report to UDWR for Contract #93-1070, Amendment 3 38 base flows. Throughflow was initiated in the secondary channel a discharge less than 170 m3 Is in 1993 and less than 136 m3/s in 1994 (the 94.5 and 94.25 m elevations, respectively) (Fig. 14e, f). These discharges, and hence throughflow, occurred only during the floods. Analysis of sedimentary structures of the newly aggraded parts of the bar showed that the low-amplitude bar near cross-section 8 was superimposed on the previous year's deposit. The lateral extent of the bar platform was greater at this location in 1993. This area of the bar scoured to below base-flow stage, and the low amplitude bar was superimposed on the scoured deposit. The low amplihlde bars had superimposed 3-dimensional downstream-migrating dunes and ripples. Geomorphic Characteristics and Changes in the l.5-km Study Reach The 1.5-Ian study reach was composed of the bank-attached compound bar, transient mid-channel bars, and point bar. The bank-attached bar described above is located between cross-sections 7 and 12, and was truncated in its downstream extent by the shift of the thalweg from left to right bank. The point bar was highly variable in its extent at base flow from year to year. Although detailed topographic maps were not developed for the point bar, cross-sections 1 through 6 describe this feature. During and after flood passage, the thalweg meandered from river left to river right through the length of the reach, and the cross-over point occurred between cross-sections 6 and 8. The meandering of the thalweg was more complex after the overwinter base flows of 1993 - 1994, although a cross over point persisted near cross-section 7 throughout this study. No field observations of thalweg location were made after the 1992-1993 overwinter flows but prior to the passage of the 1993 flood. During passage of the 1993 flood, the channel scoured at least 3 m at cross-section 2 between May 21 and June 10, a period extending from near the flood peak of570 m3fs to mid-way on the descending limb of the flood (422 m3fs). Aggradation of 1.5 m occurred on river left near cross-sections 1 and 3 when split flow with a thread on river left became single-threaded flow on river right. Between June 10 and June 22, aggradation continued at the downstream bar. Between June 22 and August 15, the point bar and the upstream bank-attached bar became emergent. The mid- channel bar between cross-sections 2 and 3 was removed prior to the return to base flow. The thalweg filled during this period everywhere in the reach except the cross-over area where 2 m of scour was measured. Filling of the thalweg, increased thalweg meandering, and lateral erosion of the bank-attached bar occurred during the remainder of the year.