8251

• UCREFRP Catalog Number: 8251
• Author: Rakowski, C. L. and J. C. Schmidt.
• Title: The Geomorphic Basis of Colorado Squawfish Nursery Habitat in the Green River Near Ouray, Utah.
• USFW Year: 1996.
• USFW - Doc Type: #93-1070,
• Copyright Material: NO

r-

Draft Final Completion Report to UDWR for Contract #93-1070, Amendment 3

39

Sediments were rearranged by 1993 - 1994 overwinter flows. A mid-channel bar was emergent between
cross-sections 11 and 12, and a low-elevation bar was emergent near the bank-attached bar margin above cross-section
7 during the March 1994 survey.

During passage of the 1994 snowmelt flood, the thalweg between cross-section 4 and 8, inclusive, filled 1 to 2

m during the ascending limb and subsequently scoured on the descending limb. After flood passage, the thalweg filled

to approximately the same elevation as had existed on November 20. 1993. The most downstream portions of the point

bar (cross-sections 1 to 3) scoured throughout the passage of the 1994 flood. The upstream part of the point bar

(between cross-sections 4 and 6) had approximately 05 m of fill.

Sediment bud2et within the 15-Ion reach
The average bed elevation was calculated at each cross-section and for the entire 15-Ion reach. The reach-
averaged bed-elevation change was calculated using the average cross-section bed elevation weighted by the area of
channel it represents (see Appendix A for calculation method). The weighting of cross-section bed elevation included
the effects of uneven cross-section spacing and variation of channel width; thus, the resultant reach-average bed
elevation indicated the change in sediment volume in the 15-Ion reach. The average bed elevation of the 15-Ion reach
increased 0.15 m for the period between November 1992 and October 1994. Net reach scale aggradation primarily
occurred during the 1993 flood and to a lesser degree during the 1994 flood (Figure 18).
Examination of the spatial distribution of the sediment on the bed and bars was more illuminating. Figure 19
shows the longitudinal distribution of the average elevation of the channel bed at.three different times. Average
elevation of the bed increased in the upstream end of the study reach because the bank-attached bar occupied most of the
channel.

Just as the average bed elevation did not constantly decrease through the reach because of the existence of
pools and shallows (Fig. 19), sediment storage within the reach was unevenly distributed (Fig. 20). The change in

sediment storage was calculated by multiplying each cross section's change in bed elevation by the area that cross

section represented (Appendix A). While figure 19 shows the calculated bed elevation at each cross section. figure 20

shows changes in sediment storage relative to a baseline (August 1993) and thereby accounts for the downstream

changes in width and cross-section spacing. The 1993 flood resulted in increased sediment storage in the upstream part

of the reach, and this was entirely due to aggradation of the compound bar, because the thalweg scoured at the same