Effects of Glen Canyon Dam on Fish in the Colorado River

Humpback chub

Razorback sucker

Native Fish

Before Glen Canyon Dam was built in the late '50's and early '60's, eight native and several non-native fish species inhabited the Colorado River system.

Today,

• three native species--the Colorado squawfish, bonytail chub, and roundtail chub--have disappeared from the Grand Canyon,
• two native species--the humpback chub and razorback sucker--are listed as endangered,
• one native species--the flannelmouth sucker--is a candidate for listing under the Endangered Species Act, and
• two native species--the bluehead sucker and speckled dace--remain relatively common in tributaries and certain sections of the river.

The reasons for declines are undoubtedly complex, but principal known factors are competition and predation by non-native fish, habitat changes, and a fragmented ecosystem brought about by construction and operation of Glen Canyon Dam.

A controlled flood was held in late March and early April of 1996, and another in November 2004. Researchers hope to find that the flood cleaned silt out of backwater channels used as habitats by native fish.

Non-Native Fish

Although coldwater trout species make up the majority of non-native fish in the Colorado River through Glen and Grand Canyons, other species have been introduced through the years.

Anecdotal evidence suggests that channel catfish and carp have been present in the Colorado River system since the late 1800's, and sparse pre-dam data (from Glen Canyon only) suggest that warmwater non-natives dominated natives by the late 1950's.

Since completion of Glen Canyon Dam in 1963, non-native carp and channel catfish have declined, while trout have increased. In fact, post-dam conditions now support a highly successful rainbow trout fishery in the Glen Canyon reach below the dam.

It is likely that some trout eggs, fry, and young were lost downstream during the controlled flood, a temporary loss that can be mitigated by restocking.

Related Wildlife

Bald eagles--which only passed through Grand Canyon before the dam--now stop during winter at sites along the river to feed on spawning trout and fish stranded by fluctuating flows.

See suggestions for further reading for the sources of information on this page.