8188

• UCREFRP Catalog Number: 8188
• Author: Muth, R. T., et al.
• Title: Flow and Temperature Recommendations for Endangered Fishes in the Green River Downstream of Flaming Forge Dam.
• USFW Year: 2000.
• USFW - Doc Type: \
• Copyright Material: NO

Final Report

3-21

September 2000

to annual hydrologic conditions, and, as would be expected, base flow under wet and moderately wet
conditions is greater than base flow under average or moderately dry to dry conditions (Table 3.8).

Within-year variability expressed here as the median within-year coefficient of variation5
(CV) among mean daily flows was considerably higher during the pre-dam period (1946 to 1962)
than the post-dam period (1963 to 1996; Table 3.8). For the August through February base-flow
period, the median CV was approximately 48% pre-dam and 25% post-dam. Mean daily flows at the
Jensen gage during 1947 to 1950 base-flow periods are plotted in Figure 3.10 to illustrate the
variability that occurred prior to construction of Flaming Gorge Dam. Variability during both
pre-dam and post-dam periods was less in the winter (December through February) than in the
summer and autumn (August through November). During the pre-dam period, the level of variability
within a year was dependent on annual hydrologic conditions, with lower variability observed in
drier years (Table 3.8).

Between-day variability also was assessed on the basis of mean daily flow values at the
Jensen gage. The percent change in flow between days was calculated by finding the difference
between flow values for day t and the previous day and dividing the result by the mean daily flow
on day t. Median between-day differences during the base-flow period were about 3% (range, 0 to
68%) pre-dam and 5% (range, 0 to 139%) post-dam.

To determine natural levels of within-day flow variability, instantaneous flow
measurements are needed. These data are not readily available because the USGS archives flow data
as mean daily values. Only the most recent data are available as instantaneous flow values. For this
study, instantaneous unregulated flow measurements were not available for the Jensen gage.
Consequently, values at the Deerlodge gage on the essentially unregulated Yampa River were used
instead to estimate natural levels of within-day variability during the base-flow period in the Green
River basin. Instantaneous flow values from this gage for the 1997-1998 and 1998-1999 base-flow
periods are plotted in Figure 3.11. These graphs illustrate the degree of variability that occurs within
each year and within days. Statistical analyses of these data indicate that the median percentage
change within days (calculated as daily maximum minus daily minimum divided by the mean daily
flow) is 9.6% (range, 0 to 94.4%); the mean percent within-day change is 14.3%.

The degree of variability in flows within the base-flow period in the Green River system
are higher than values that have been inferred for this system. Much of the within-year variability
can be attributed to weather patterns and events. The within-day variability may be attributed to daily
freeze-thaw cycles in higher-elevation snowpacks and subsequent variation in runoff and
groundwater discharge. The observed level of variability in the Green River system contrasts with
"natural" levels of variability that have been suggested for this system by Stanford (1994). In his
review of the 1992 Biological Opinion, Stanford suggested that base flows should be stable and
limited to pre-regulation conditions "as reflected in the Yampa River hydrographs over the period

5The CV is calculated as the standard deviation divided by the mean and is expressed as a percentage. The CV
allows comparison of within-year variation among years that have different mean-flow values.

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