9385

• UCREFRP Catalog Number: 9385
• Author: Hawkins, J.
• Title: Responses by Flaming Gorge Technical Integration Team to April 4, 2000, Minority Report from John Hawkins
• USFW Year: 2000.
• USFW - Doc Type: Flow and Temperature Recommendations for Endangered Fishes in the Green River Downstream of Flaming Gorge Dam ‎(hereafter the Flow Report.
• Copyright Material: NO

The reduction in sediment load at Jensen from 6.21 to 3.21 million tons/yr
following the construction of Flaming Gorge Dam is attributed to a decrease in
effective discharge from 20,500 cfs (580 m3/s) pre-1963 to 11,500 cfs (325 m3/s)
post-1963. (Andrews 1986). The Yampa River has a comparable effective
discharge of 11,500 cfs (325 m3/s, OIBrien 1984). An analysis should be
conducted to determine if there has been a decrease in sediment supply from the
Yampa River, post-Flaming Gorge, because the Little Snake River contributes
approximately 60% of the sediment load at Jensen.

O'Brien (1987) analyzed historical sediment data to assess potential impacts
of flow reductions in the Yampa River downstream of the Little Snake River
confluence. This study analyzed the sediment supply from the Little Snake and
Yampa rivers and compared them to sediment loads predicted by sediment rating
curves derived by O'Brien (1984) and Elliott et al. (1984). The rating curves were
represented by a power regression relationship between sediment discharge as
and water discharge a:

a = a ab
s

where a is a regressed coefficient and b is the regressed exponent. This power
function was based on a log-log transformation using a least squares fit to the
data. This regression method under-predicts sediment loads and the inaccuracy
increases with rating curve data scatter and it therefore is necessary to apply a
bias correction factor to the regression coefficient (Ferguson 1986; Koch and
Smillie 1986). O'Brien (1987) re-computed annual sediment loads at the Lily and
Maybell gages for the entire period of record 1921 to 1984 with this correction
and predicted Little Snake River mean annual sediment load increased from
approximately 1.3 million tons per year to 2.0 million tons per year (Table 4). The
Yampa River predicted mean annual sediment load remained essentially unchanged
(407,000 tons per year measured to 389,000 tons per year based on the

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