Wildlife and Anthropogenic Changes in the Arid Southwest - Design with the Desert: Conservation and Sustainable Development - page 176

FIGURE 9.9
Glen Canyon Dam on the Colorado River. (Courtesy of U.S. Geological Survey photo, Reston, VA.)
changes in hydrologic regimes associated with impoundment operations. 55 Other authors
have stressed that direct modification of aquatic habitats is the most prevalent threat to
native fishes in North America. 56,57 Invasion by non-native species, a form of habitat altera-
tion, is recognized as second only to the loss of habitat and landscape fragmentation as a
threat to global biodiversity. 58 To date, no North American fish species have been sufficiently
recovered to permit removal from the U.S. endangered species list.
The Colorado River drainage encompasses portions of all four North American des-
erts and has undergone extensive habitat alteration. The Colorado River basin is typically
divided into “upper” and “lower” basins, with the division at Lees Ferry, Arizona (see
Chapter 4). The upper basin alone has 82 man-made reservoirs, each with a capacity of over
5,000 ac ft (6,030,744 m 3 ), and has 43 diversions to export water from the basin. The largest
of these dams include Glen Canyon Dam on the Colorado River, Flaming Gorge Dam on
the Green River, the Wayne N. Aspinall Units on the Gunnison River, and Navajo Dam on
the San Juan River. These developments have caused significant changes in streamflow,
water temperatures, sediment loads, total dissolved-solids, and channel morphologies. 59,60
Glen Canyon Dam adversely affected the aquatic ecosystem of the Colorado River and
resulted in permanently fragmented fish populations between the upper and lower basins
(Figure 9.9). Cold water releases from reservoirs have disrupted or prohibited mainstream
reproduction of native fishes and destabilized rearing habitats because of daily water level
fluctuations in the river to meet power demands. 61 Additional threats to water quality and
quantity in the upper basin would be posed by the development of an oil shale indus-
t r y. 62,63 The lower basin is equally, if not more developed. 64,65
All of these impoundments along the Colorado River have altered large portions of
the watersheds. Historically, these were free flowing rivers, largely characterized by high
spring runoff and turbid waters with warmer summer water temperatures. Hundreds of
miles of these rivers now function as lakes or as altered river habitat generally charac-
terized by year-round cold water temperatures with low turbidities. As a result of these
activities, native fish populations have declined from historic levels in the Colorado River
basin. 66,67 Just as construction of impoundments and resulting habitat alteration favor some
introduced amphibians over native forms, introduced fish often flourish in the unnatural
lake habitats associated with arid land reservoirs.
The humpback chub (Figure 9.10), a morphologically unique fish endemic to the
Colorado River basin, was listed as endangered in 1967. The species is a member of a rel-
ict native fish community, many of which are locally extinct or declining. For example,