Green River Flow Regulation and Piscivorous Garter Snakes

Andrew L. Nichols

Downstream electrical power demands force large daily fluctuations in late spring and summer water releases from Flaming Gorge Dam. Between 13 and 22 June 2006, daily Green River discharges (measured immediately below the dam) varied from 900 cubic feet per second (cfs) to 2000 cfs, causing rapid changes in water levels, which alternately inundated and isolated channel floodplains. This dramatic (and unnatural) hydrologic variability can both assist and hinder terrestrial and aquatic species occupying floodplain habitat.


Figure 1. Discharge hydrograph for Green River immediately below Flaming Gorge Dam (from USGS 2006)

A visit to Trailer Draw (River Mile 238.9) on the morning of 17 June 2006 provided a poignant example of how one species can benefit while another can suffer from anthropogenic hydrologic variability associated with the operation of Flaming Gorge Dam. On this morning we observed a garter snake feeding upon a dead sculpin stranded upon the active floodplain following the passing of the previous day’s dam-release flood wave. While the sculpin was clearly harmed by the quick recession of water levels, the piscivorous garter snake was able to take advantage of the rapid abandonment of the upper floodplain by feeding on the stranded fish.


Figure 2. Garter snake feeding on a dead sculpin at Trailer Draw (photograph by Rene Henery)

Our observations at Trailer Draw show that while some species (i.e. the garter snake) have successfully adapted to daily fluctuations in dam releases, other species (i.e. the sculpin) find themselves harmed by a flow regime managed for human needs.