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River fragmentation,
which is the interruption of a river’s natural flow by dams,
inter-basin transfers, or water withdrawal, is an indicator of the
degree to which rivers have been modified by humans. Freshwater
systems have been altered since historical times, but such modifications
skyrocketed in the mid-1900s and continue today in many developed
and developing countries. These changes have improved transportation,
provided flood control and hydropower, and have boosted agricultural
output by making more land and irrigation water available. At the
same time, these physical changes in the hydrological cycle have
significantly impacted freshwater ecosystems and species. Dams disconnect
rivers from their floodplains and wetlands and slow water velocity
in riverine systems, converting them to a chain of connected reservoirs.
This, in turn, impacts the migratory patterns of fish species and
the composition of riparian habitat, opens up paths for exotic species,
changes coastal ecosystems by limiting sediment and nutrient loads,
and contributes to an overall loss of freshwater biodiversity and
fishery resources.
This map portrays
an indicator of the extent to which dams and canals have fragmented
river basins and how water withdrawals have altered river flows.
Of the 227 large river basins assessed, 37 percent are strongly
affected by fragmentation and altered flows, 23 percent are moderately
affected, and 40 percent are unaffected. Large river systems are
defined as rivers with a virgin mean annual discharge equal to or
above 350 m3 per second. Strongly affected systems include those
with less than one quarter of their main channel left without dams,
where the largest tributary has at least one dam, as well as rivers
whose annual flow patterns have changed substantially. Unaffected
rivers are those without dams in the main channel of the river and,
if tributaries have been dammed, river discharge has declined or
been contained in reservoirs by no more than 2 percent. In this
analysis, strongly or moderately fragmented systems account for
nearly 90 percent of the total water volume flowing through the
rivers. All river systems with parts of their basins in arid areas
or that have internal drainage systems are strongly affected.
The only remaining
large free-flowing rivers in the world are found in the tundra regions
of North America and Russia, and in smaller coastal basins in Africa
and Latin America. It should be noted, however, that considerable
parts of some of the large rivers in the tropics, such as the Amazon,
the Orinoco, and the Congo, would be classified as unaffected rivers
if an analysis at the subbasin level were done. The Yangtze River
in China, which currently is classified as moderately affected,
will become strongly affected once the Three Gorges dam is completed.
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