86
the combined effects of glacial melting elsewhere in the world; subsidence of
coastal sediments because of removal of water, oil, and natural gas; and lack of
continued deposition of new sediment (Blum and Roberts 2009).
In summary, the cumulative effects of upland alteration, river engineering,
flow regulation, contaminants, and introduced species within the Mississippi
River drainage basin result in high rates of extinction for native freshwater species; the presence of an enormous dead zone in the coastal waters receiving the
river’s discharge; contaminated water; a shrinking delta; and a plethora of interrelated changes to tributary rivers throughout the drainage basin. Attempts to
reverse or mitigate these trends through management and restoration programs
focused on distinct portions of the river basin and delta together run into the
billions of dollars.
3.3.2 Australia’s Murray-Darling River Drainage Basin
The Murray and Darling Rivers together drain nearly 1.1 million square kilometers
of southeastern Australia (Fig. 3.15). The Darling drains a larger area but the Murray
contributes more discharge at the point where the rivers join before flowing into the
Pacific Ocean. Neither river, however, has much flow by world standards. The entire
basin produces an average flow of less than 800 cubic meters per second, compared
to nearly 17,000 for the Mississippi or 7000 for the Danube. The history of human
alteration of the Murray-Darling drainage basin exemplifies that of many of dryland
rivers around the world.
Like the Mississippi, the Murray-Darling underwent intensive human alteration
relatively recently compared to many rivers in Eurasia. Europeans explored the
drainage basin during the first half of the nineteenth century and intensive use of
natural resources quickly followed. Much of the Murray-Darling drainage is grassland or desert, but extensive riparian forests dominated by river red gum (Eucalyptus
camaldulensis) were historically present. These forests were cut to expand grazing
and crop lands and to provide fuel for steamboats (Fig. 3.16). Large wood was
removed from the river to enhance navigation and reduce flooding. Water was
diverted for irrigated agriculture starting during the 1860s. Diversions accelerated
through time, along with construction of locks, dams, and large reservoirs, and
channelization of the river (Wohl 2011).
Among the primary concerns in the Murray-Darling today are species loss, introduced species, salinization, and algal blooms (Walker and Thoms 1993). The drainage basin historically contained 34 fish species, half of which are now threatened as
a result of habitat loss, declining water quality and quantity, overfishing, and introduced species such as carp (Fisher 1996). Among the primary introduced species
that affect river process and form is willow (Salix spp.). Willows were planted along
the main channel of the Murray to distinguish it from secondary channels as part of
navigation improvements and to stabilize the banks of irrigation canals across the
floodplain. The willows have become invasive, excluding native vegetation. The
3 Human Alterations of Rivers
the combined effects of glacial melting elsewhere in the world; subsidence of
coastal sediments because of removal of water, oil, and natural gas; and lack of
continued deposition of new sediment (Blum and Roberts 2009).
In summary, the cumulative effects of upland alteration, river engineering,
flow regulation, contaminants, and introduced species within the Mississippi
River drainage basin result in high rates of extinction for native freshwater species; the presence of an enormous dead zone in the coastal waters receiving the
river’s discharge; contaminated water; a shrinking delta; and a plethora of interrelated changes to tributary rivers throughout the drainage basin. Attempts to
reverse or mitigate these trends through management and restoration programs
focused on distinct portions of the river basin and delta together run into the
billions of dollars.
3.3.2 Australia’s Murray-Darling River Drainage Basin
The Murray and Darling Rivers together drain nearly 1.1 million square kilometers
of southeastern Australia (Fig. 3.15). The Darling drains a larger area but the Murray
contributes more discharge at the point where the rivers join before flowing into the
Pacific Ocean. Neither river, however, has much flow by world standards. The entire
basin produces an average flow of less than 800 cubic meters per second, compared
to nearly 17,000 for the Mississippi or 7000 for the Danube. The history of human
alteration of the Murray-Darling drainage basin exemplifies that of many of dryland
rivers around the world.
Like the Mississippi, the Murray-Darling underwent intensive human alteration
relatively recently compared to many rivers in Eurasia. Europeans explored the
drainage basin during the first half of the nineteenth century and intensive use of
natural resources quickly followed. Much of the Murray-Darling drainage is grassland or desert, but extensive riparian forests dominated by river red gum (Eucalyptus
camaldulensis) were historically present. These forests were cut to expand grazing
and crop lands and to provide fuel for steamboats (Fig. 3.16). Large wood was
removed from the river to enhance navigation and reduce flooding. Water was
diverted for irrigated agriculture starting during the 1860s. Diversions accelerated
through time, along with construction of locks, dams, and large reservoirs, and
channelization of the river (Wohl 2011).
Among the primary concerns in the Murray-Darling today are species loss, introduced species, salinization, and algal blooms (Walker and Thoms 1993). The drainage basin historically contained 34 fish species, half of which are now threatened as
a result of habitat loss, declining water quality and quantity, overfishing, and introduced species such as carp (Fisher 1996). Among the primary introduced species
that affect river process and form is willow (Salix spp.). Willows were planted along
the main channel of the Murray to distinguish it from secondary channels as part of
navigation improvements and to stabilize the banks of irrigation canals across the
floodplain. The willows have become invasive, excluding native vegetation. The
3 Human Alterations of Rivers