Hydrology/Geomorphology of the
Green River Basin
A discussion of the flow needs of endangered fishes in the Green River system would be incomplete without an understanding of runoff patterns, runoff rates, water temperature, and sediment loading as affected by the climate, geology, physiography, and vegetative cover of the Green River Basin.
These factors strongly influence native species, which are adapted to the particular watershed characteristics in which they evolved. High flows are particularly important for creation and maintenance of habitats as these flows reshape sediment deposits, scour vegetation, and flush accumulated fine sediment from portions of the streambed. Occasional flooding temporarily creates productive floodplain habitats that help sustain aquatic food webs.
Most of the total annual streamflow in the Green River Basin is provided by snowmelt. Because of this, natural flow is very high in late spring and early summer and diminishes rapidly in midsummer. Although flows in late summer through autumn can increase following rain events, natural flow in late summer, autumn, and winter is generally low. Also, water and sediment inputs to the Green River and its tributaries are not uniformly distributed across the basin. The principle water sources are high elevation areas, especially in the northeast portion of the basin. Conversely, the semiarid parts of the basin at lower elevations contribute most of the sediment.
Capable of storing approximately twice the annual inflow to the reservoir, Flaming Gorge Dam has the largest effect on Green River flow patterns. The dam modifies the flow pattern in the Green River to meet demands of irrigation, power generation, recreation, and other uses.
As a result, there is a substantial reduction in the magnitude of spring peak flows, and the peak flows now occur earlier in the year than prior to regulation.
Flaming Gorge Dam has affected the quantity of sediment transported by a given flow at downstream locations. Alterations of the channel morphology and a decrease in the availability of sediment within the channel have been observed and linked to changes in flow pattern and sediment availability.
In the past, Flaming Gorge Dam has been operated to provide for a full reservoir while maximizing power revenue and avoiding the use of the bypass tubes or spillway. Depending on snowpack and monthly runoff forecasts, an appropriate winter drawdown is selected to avoid spills.
Minimum reservoir elevation usually occurs prior to April each year; and refilling of the reservoir occurs during spring runoff, reaching the maximum reservoir level in late July of each year. Releases during the remainder of the year generally are patterned to meet energy demands with peak demand for electrical power occurring during summer and winter.
The channel-forming consequences of Flaming Gorge Dam release patterns and other characteristics of the Green River flow were investigated by studying relationships between sediment transport and channel morphology over a range of flows in different channel settings. By describing details of channel morphology, hydraulics, and sediment transport that are important considerations in describing habitats of the endangered fishes, this research led to a refinement of operations at Flaming Gorge Dam.
The proposed operations address linkages between flow, sediment transport, channel morphology, and the habitats of the endangered fishes with the goal of restoring habitat complexity in the Green River through flow and temperature management, improving prospects for recovery of endangered fish populations.