7630

• UCREFRP Catalog Number: 7630
• Author: Pucherelli, M. J., R. C. Clark and R. D. Williams.
• Title: Mapping Backwater Habitat on the Green River as Related to the Operation of Flaming Gorge Dam Using Remote Sensing and GIS.
• USFW Year: 1990.
• USFW - Doc Type: Report No. R-90-18,
• Copyright Material: NO

Sand Wash

Flow at Sand Wash was difficult to determine because the Duchesne and White Rivers enter the
Green River between Sand Wash and the USGS Jensen gauge. Additionally, examination of flow
data indicated that the 46 m3/s flow had not reached the Sand Wash site when photography was
acquired and the actual flow at this time was probably less than 37 m3/s. Regression at Sand Wash
[rZ = 0, backwater area = 9,150 + (99.8) flow] indicated there was no relationship between flow and
area. However, the smallest backwater area at Sand Wash occurred at the highest flow studied
(table 4). Sand Wash was unique because backwater number was maximized at the highest flow.
Furthermore, backwater numbers were much more evenly distributed among the various size classes
than at the upper sites. Generally, the Sand Wash site had substantially fewer backwaters and less
backwater area per kilometer than the three upper sites.

Mineral Bottom

Backwater area and number at Mineral Bottom were maximized at 98 m3/s (table 5). Mineral
Bottom had substantially fewer and smaller backwaters than all other sites and is indicative of sites
on the lower river.

DISCUSSION

The use of the USGS 'Jensen gauge to estimate flows may have complicated the results at Ouray
and Sand Wash. It is apparent that flows at Island Park, which is 30 kilometers above the gauge,
and Jensen, which is only 10 kilometers below the gauge, were accurately reflected by the Jensen
gauge. There are no tributaries and no irrigation diversion that occur between the gauge and Island
Park and no tributaries occur between the gauge and Jensen. This may explain why the regressions
of flow and backwater area at these sites are relatively significant. However, Ouray and Sand Wash
are much farther from the gauge and flows at these sites may be affected by ground water recharge
and irrigation diversion and return. Flows at Sand Wash are further complicated by the Duchesne
and White Rivers which enter the Green River between Ouray and Sand Wash. Their combined
flows varied from 72 (May 30, 1987) to 20 m3 (July 7, 1987) which produced a substantial affect on
flows at Sand Wash. This may explain why it is increasingly difficult to correlate flows with
backwater area as they move from Jensen to Ouray and then to Sand Wash. Site specific gauges
would provide a more accurate estimate of flows at Ouray and Sand Wash.

Given the problems associated with gaging, the data indicated a range of flows which produced
maximal backwater area and numbers under the conditions of the study. When the four upper sites
were considered, a range of flows from 37 to 55 m3/s produced substantially greater backwater area
and higher numbers than did 142 and 71 m3/s flows. The large differences in backwater area from
46 to 45 m3/s at each site may be attributed to the chronological order of these flows. Backwaters
at 45 and 37 m3/s were allowed to form during a gradually descending hydrograph. The 46 m3/s
flow was obtained immediately after the lowest flow of the season (approximately 34 m3 / s).
Furthermore, this flow only occurred for 1 day and was not allowed to stabilize which very likely
affected backwater availability.

The Ouray and Sand Wash sites had more backwater area at slightly higher flows. Closer analysis
of individual backwaters at Ouray indicated that a few extremely large backwaters (5,000 to
10,000 m2) at 53 m3/s did not occur at lower flows which reduced backwater area substantially.

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