The Green River is the largest tributary of the Colorado River, originating in the Wind River Range of Wyoming and flowing south through Wyoming, Utah, and Colorado. Regulated by several dams, most notably Flaming Gorge Dam, the river transitions from a cold-water mountain stream to a warm-water desert river. This shift supports a highly diverse population of fish, including world-famous sport fish, unique native species, and numerous introduced species. The specific composition of fish changes dramatically depending on the river section.
Primary Game Fish and Recreational Targets
The Green River below Flaming Gorge Dam, particularly the 30-mile stretch in Utah, is renowned as a blue-ribbon fishery for its dense populations of trout. This tailwater fishery releases consistently cold water (40 to 60 degrees Fahrenheit) year-round. These stable temperatures, combined with the river’s high productivity, create an ideal environment for salmonids to thrive.
The highest concentrations of fish are found in the uppermost section, where trout populations can reach an estimated 15,000 to 20,000 fish per mile. The primary species sought by anglers are the non-native Rainbow Trout (Oncorhynchus mykiss) and Brown Trout (Salmo trutta). Rainbow Trout, often found in the 15-inch range, dominate the population immediately downstream of the dam, thriving in the clear, highly oxygenated water.
Brown Trout are known for their larger size, averaging 15 to 16 inches, with trophy-sized fish reaching up to 34 inches in the lower “B” and “C” sections. The river also supports a smaller population of Cutthroat Trout (Oncorhynchus clarkii), including hybrid Cutbows. The native Mountain Whitefish (Prosopium williamsoni) is abundant in the upper river, benefiting from the cold dam releases.
Unique Native and Protected Species
Further downstream, where the river warms and becomes more turbid, the focus shifts to species endemic to the Colorado River system. The Green River provides habitat for four federally listed species often referred to as the “Big Four.” These fish are adapted to the historical conditions, which featured warm water, high sediment loads, and massive spring flood pulses.
The Colorado Pikeminnow (Ptychocheilus lucius) is the largest minnow in North America, historically reaching lengths of up to six feet and functioning as the river’s top native predator. The Humpback Chub (Gila cypha) is identifiable by the pronounced fleshy hump behind its head and was downlisted from endangered to threatened status in 2021. Both the Bonytail Chub (Gila elegans) and the Razorback Sucker (Xyrauchen texanus) are listed as endangered, with recovery efforts focused on stocking hatchery-raised fish to boost their extremely low wild populations.
These four species require the warmer, lower sections of the Green River, generally downstream of the Yampa River confluence. The cold, regulated flows of the upper river have significantly reduced the native fishes’ ability to successfully reproduce. Consequently, conservation programs are actively managing water flows and non-native fish populations to help restore the natural river conditions necessary for their survival.
Generalist and Introduced Fish Populations
Beyond the trout fishery and protected native species, the Green River system is home to a wide array of generalist fish, many of which are non-native species introduced over the last century. These introduced fish occupy various niches throughout the river, from the cold upper reaches to the warm lower sections.
Common non-native fish include the Common Carp, a widespread bottom feeder, and the Channel Catfish, which thrives in the warmer, slower-moving water of the lower river. Several non-native predatory species are also established and pose a threat to the endangered natives, most notably Smallmouth Bass (Micropterus dolomieu) and Northern Pike. In some areas, anglers are encouraged to harvest these non-native predators upon capture to help protect native fish.
The river also supports several native suckers, such as the Flannelmouth Sucker (Catostomus latipinnis) and the Bluehead Sucker (Pantosteus delphinus). These species are not game fish but play an important role as bottom feeders.