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<br />9
<br />
<br />5,706 m2 (1,902 mttmile). Backwater area at the 3,119 and the 1,889 ft3/s
<br />flows was 2,752 m (917 m2/mile) and 4,411 m2 (1,470 m2/mile), respectively.
<br />Backwater number at Ouray ranged from 10 at 4,359 ft3/s to 6 at 3,119 ft3/s.
<br />Backwater area/mile was generally greatest at Oura~.
<br />
<br />Backwater area at WjJ]ow Creek was maximized at~~19 ft~ with 2,307 m2
<br />(721 m2/mile), was sli~htly less at 1,889 ftj/s Wl h 1,862 m2 (582 m2/mile),
<br />and lowest at 4,359 ft /s with 1,133 m2 (354 m2/mile). Backwater numbers
<br />were maximized at the lowest flow with 12. Backwaters numbers for the 4,359
<br />and 3,119 ft3/s flows were 7 and 8, respectively (table 3). Backwater
<br />area/mile was consistently lowest at Willow Creek. Isolated pool formation
<br />almost always increased with decreasing flow. -----
<br />
<br />A flow of 1,500 ft3/s was requested to further define the range of flows
<br />needed to maximize backwater habitat and a fourth set of aerial photography
<br />was acquired on December 4, 1986, almost 3 months after the initial
<br />1,889 ft3/s flow was photographed. Flow on this date was nearly identical
<br />to the September 12, 1986, 1,889 ft3/s flow. Upon examining the
<br />Ph.otograPhs, it was noted that sandbar configuration and backwaters had .~
<br />changed significantly. and bar area decreased b an avera e of 30 ercen
<br />during the second 1,889 ft /s flow photographed. 0 a backwater area for
<br />the three sites increased by 57 percent. At Ouray, a large backwater was
<br />transformed into a side channel.
<br />
<br />1987 Study - Island Park
<br />
<br />The areas of interpreted classes at each flow for Island Park are given in
<br />table 4. An inverse relationship existed between flow and backwater area at
<br />Island Park. Backwater area consistently increased as flow was decreased
<br />from 5,260 to 1,101 ft3/s (table 5). Two substantial increases occurred:
<br />while flows dropped from 2,423 to 1,773 ft3/s total backwater area
<br />increased 30 percent, from 8,575, to 11,160 m~; a flow decrease from It430
<br />to 1,381 ft3/s resulted in a 50-percent increase in backwater area..., from
<br />13,349 to 20,070 m2. Backwater area was maximized at 1,101 ftJ/s flow
<br />(22,153 m2). Isolated pool area generally increased as flow decreased,
<br />however, a substantial decrease occurred at 1,430 ft3/s (table 3). This
<br />decrease in isolated pools, as well as the large increase in backwater area
<br />from 1,430 to 1,381 ft3/s may be attributed to the chronological order of
<br />these flows. Backwaters at 1,381 and 1,101 ft3/s were allowed to form
<br />during a gradually descending hydrograph. The 1,430 ft3/s flow was obtained
<br />immediately after the lowest flow of the season (approximately 1,000 ft3/s -
<br />see Appendix A). Furthermore, this flow was not allowed to stabilize and
<br />only occurred for one day which very likely affected backwater and isolated
<br />pool formation and availability.
<br />
<br />The relationship between riverflow and backwater number was not as clear as
<br />the flow/area relationship. However, backwater numbers were maximized at
<br />1,381 and 1,101 ft3/s, with 56 and 52 backwaters, respectivell' The lowest
<br />number of backwaters occurred at 2,423 ft3/s (32). The >20-m <200-m2 size
<br />class consistently represented the highest number of both bank and channel
<br />backwaters. Very large backwaters (>1,000 m2) were more abundant at the
<br />lower flows. Channel backwaters were slightly more abundant than bank
<br />
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