Fly fishing on the Green River, Utah

An angler casts his streamer into a green pool on the Green River, Utah. This portion of the river below Flaming Gorge Dam is a "tailwater" fishery. The reservoir above the dam traps all of the upper river's sediment and releases clear water. The dam also stops floods, allowing filamentous algae and macrophytes to proliferate in the river.

The cool waters released from the mid-level of the reservoir provide excellent temperatures for trout growth, and the river supports abundant populations of introduced brown and especially rainbow trout.

The negative influence of the dam is that native fishes, reliant on the warm summer temperatures of the river and needing migratory access to spawning areas, were extirpated from this section of the river and are now on the U.S. endangered species list. The insect diversity (primarily mayflies) was also impacted, primarily because the reservoir release does not allow temperatures to drop to near 0 C (32 F) in winter, and the absence of the cold water stops the insects from breaking diapause, and thus completing their life cycle. The benthic invertebrates are now dominated by amphipods (scuds) and dipterans (midges).

Photo: 5 November 2010

Limnology: The study of inland waters