9383

• UCREFRP Catalog Number: 9383
• Author: SWCA, I.
• Title: Recovery Goals for the four Colorado River Endangered Fish Species.
• USFW Year: 2000.
• USFW - Doc Type: Logan.
• Copyright Material: NO

forces are different from those experienced by the fish assemblage of the lower basin, and will
to be unique as long as these river basins remain segregated.
~ctivities in the upper basin are coordinated by the Upper Colorado River Recovery
Program (UCRRP). The UCRRP was initiated under a Cooperative Agreement on January 22,
1988, as a coordinated effort of state and federal agencies, water users, and environmental groups
to recover the four endangered fish species in the upper basin while allowing water development
to proceed (Evans 1993).
2.2 Lower do River Basin

comprises of the lower Colorado River and Gila River subbasins.
The lower Colorado River subbasin consists of the mainstem lower Colorado River and the Little
Colorado, Virgin, and Bill Williams rivers as major tributaries. From Glen Canyon Dam to the
international boundary with Mexico, there are eight mainstem dams (i.e., Hoover, Davis, Parker,
Headgate Rock, Palo Verde, Imperial, Laguna, and Morelos dams) on the lower Colorado River.
Numerous diversions and intakes withdraw large amounts of water from the river. These dams
and diversions have transformed the his river from a broad meandering river with high
springs floods and low base flows to f reservoirs and short intervening, contained
reaches of river, cooled and cleared by deep reservoir releases. The Gila River subbasin consists
of the Gila River and the Salt, Verde, San Pedro, and Santa Cruz rivers as major tributaries.
Numerous dams and small diversions on the Gila River subbasin have also transformed this
system into a regulated river with reduced flow and fragmented by dams and reservoirs.
The ecological setting of the lower basin was historic~fferent from the upper basin in
that it received higher spring floods and experienced higher ~Ihore prolonged seasonal
temperatures (Minckley 1985). Like the upper basin, this unique ecological setting has not only
been isolated but transformed to a greater degree than the upper basin by the numerous dams,
diversions, and the compounded influence of alien species, especially nonnative fishes. A
relatively small proportion of the natural riverine habitat remains in the lower basin, primarily as
short fragments of river, bounded upstream by dams and downstream by deep clear reservoirs.
Only fragments of the historic riverine structure and function remain in the lower bas' and there
are between 29 (Pacey and Marsh 1998) and 37 species of nonnative fishes establish

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