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<br />Recommended Flows
<br />
<br />The goal of the recommended flows is to improve the habitat
<br />and enhance the populations of the endangered fishes by
<br />providing annual and seasonal patterns of flows and
<br />temperatures in the Green River below Flaming Gorge Dam.
<br />
<br />The table on the opposite page summarizes the flow
<br />recommendations contained in the July 2000 Flow and
<br />Temperature Recommendations for Endangered Fishes in
<br />the Green River Downstream of Flaming Gorge Dam. The
<br />table contains recommendations for three reaches of the
<br />Green River for the spring runoff and summer-to-winter base
<br />flow periods under a variety of hydrological conditions,
<br />including:
<br />
<br />. The recommended flows for the entire Green River below
<br />the dam which are increasingly influenced by other
<br />tributaries such as the Yampa, White, and Duchesne
<br />Rivers as you move downstream.
<br />
<br />. A wide range of hydrologic conditions occurring in the
<br />Green River Basin-from drought to flood.
<br />
<br />. Flows for different seasons of the year, from spring runoff
<br />to summer, fall, and winter flows.
<br />
<br />The recommended flows are adjusted for each of the three
<br />reaches and seasons (spring runoff and base flow), depending
<br />on projections of how wet or dry the season is expected to be
<br />(hydrologic condition) in a given year. Hydrologic
<br />conditions are defined using exceedance intervals-or how
<br />often a forecasted runoff is expected to exceed a certain
<br />volume.
<br />
<br />For example, a 0-10% exceedance year would be a year in
<br />which the forecasted volume of runoff flows would be
<br />expected to be higher only 0-10% of the time, or a very wet
<br />year. In contrast, in a 90-100% exceedance year, the
<br />forecasted runoff is expected to be higher 90-100% of the
<br />time, or a very dry year. Either wet or dry years could be
<br />expected to occur 10% of the time, or I in every 10 years.
<br />Average years (30-70% exceedance) would be expected to
<br />occur 40% of the time, or 4 out of 10 years. Simply put,
<br />exceedance intervals are used to depict wet or dry conditions
<br />expected in a given year.
<br />
<br />For example, if the snowpack is relatively high and the
<br />forecasted runoff is expected to be in the moderately wet
<br />category (10-30% exceedance), which will happen on an
<br />average of 2 out of every 10 years, this is considered a
<br />moderately wet runoff year. The flow recommendations
<br />suggest that peak flows in Reach 1 (Flaming Gorge Dam
<br />
<br />to the Yampa River) be greater than or equal to 4,600 cubic
<br />feet per second (cfs), while base flow during late summer,
<br />fall, and winter would be between 1,500 and 2,500 cfs.
<br />Peak flows in Reach 2 (from the Yampa River to the White
<br />River) should reach peaks of greater than or equal to 20,300
<br />cfs and be maintained, if possible, at 18,600 cfs for at least
<br />2 weeks. The base flow in the reach could vary between
<br />2,400 and 2,800 cfs. Finally, in Reach 3 during this
<br />moderately wet year, peak flows should be greater than or
<br />equal to 24,000 cfs and be maintained at a level of 22,000
<br />cfs or greater for at least 2 weeks. Recommended base flow
<br />for Reach 3 during a moderately wet year should range
<br />between approximately 2,700 and 4,700 cfs.
<br />
<br />In summary, during a moderately wet year, releases from
<br />Flaming Gorge Dam would be 4,600 cfs or greater for at
<br />least 2 weeks in duration to supplement inflows from the
<br />downstream tributaries to flows in Reaches 2 and 3 of the
<br />Green River in order to meet the target ranges specified for
<br />those reaches.
<br />
<br />In contrast, during a dry year (90-100% exceedance) which
<br />would occur I in 10 years on average, peak flows in Reach
<br />1 would be similar (4,600 cfs) but of a much shorter
<br />duration than the moderately wet year and the
<br />recommended base flow would be lower. In Reach 2,
<br />recommended peak flows and the duration of peak flows
<br />would be lower and shorter than during a moderately wet
<br />year (8,300 cfs for 2 days versus a peak of 20,300 cfs with
<br />at least 2 weeks above 18,600 cfs). The base flow would be
<br />lower (900-1,100 cfs versus 2,400-2,800 cfs).
<br />Recommended flows for Reach 3 would be similarly
<br />reduced under dry conditions. Put another way, minimum
<br />releases from Flaming Gorge Dam in a very dry year would
<br />be approximately 4,600 cfs for at least 2 days compared to.
<br />a minimum release of 4,600 cfs or greater for at least 2
<br />weeks in moderately wet years.
<br />
<br />The flow and temperature recommendations also cover
<br />levels of day-to-day changes in water elevations on the
<br />Green River at Jensen, Utah, due to hydropower fluctu-
<br />ations; variations in the base flow during the summer, fall,
<br />and winter; and temperature recommendations during the
<br />summer.
<br />
<br />The detailed flow and temperature recommendations are
<br />available on the internet at: www.us.usbr.gov. Select
<br />Environmental Programs, then select Flaming Gorge EIS,
<br />then Flow Recommendations.
<br />
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