Western rivers in the United States are prime examples
of how flow manipulation can lead to multiple
damages to riverine and riparian processes and communities.
Dampening of natural flow variability by
managing for only minimum flows has contributed to
the widespread loss of native fish species (Moyle and
Light 1996), and to the regeneration failure of native
cottonwoods that support a diverse riparian community
(Scott et al. 1996, 1999). Since the completion of the
Glen Canyon Dam (Arizona, USA) in 1963, measurable
flows of the Colorado River to its mouth at the Gulf
of California have occurred only six times. The wetland
area at the mouth of the river has decreased to 5800–
63 000 ha (depending on the year), compared with
250 000 ha of original wetlands (Glenn et al. 1996,
Postel et al. 1998). The lack of freshwater inflows has
contributed to the endangerment of a large number of
species in the Sea of Cortez, and the abundance of
bivalve mollusk populations has dropped 94% from
1950 values due to loss of benthic productivity (Postel
et al. 1998, Kowalewski et al. 2000)