188.8.131.52. Lakes/Streams, Rivers, and Glaciers
Lakes, streams, glaciers, and other freshwater ecosystems in Asia are highly
diversified in terms of plant and animal species. These freshwater ecosystems
have been stressed by environmental burdens, exploitation of natural resources,
transformation of lands, and recreational activities. There is growing concern
that climate change may accelerate the damage to freshwater ecosystems such
as lakes, marshes, and rivers. More than 50,000 ha of coastal territories, including
35,000 ha of delta in the Selenga River and 12,000 ha of delta in the Upper
Angara River, have been damaged during the past few years by precipitation and
riverflow increases (Anokhin and Izrael, 2000). With an increased amount of
precipitation likely in the future, more incidences of flooding and other adverse
impacts are possible. With a rise in temperature, a decrease in the amount of
snowfall in the Lake Biwa catchment in Japan is projectedwhich might exacerbate
the process of eutrophication (Fushimi, 2000a). Deterioration of lake water
quality also is suggested in Kasumigaura Lake in eastern Japan (Fujimoto et
al., 1995; Fukushima et al., 2000). The response of lakes and streams to climate
change will involve complex interactions between the effects of climate on areal
inputs, hydrology, and catchments and in-lake processes.
Many of the major rivers in Asia have long been targets for development projects
related to the hydroelectric, water supply, agriculture, industry, and navigation
sectors. As a consequence, there have been shifts between freshwater and estuarine
conditions as a result of high freshwater flows during the rainy season and
low to nonexistent freshwater flows in the dry season. Increasing literol vegetation
is causing health risks for local habitats in many countries of south Asia.
Changes in aquatic habitat also have affected fisheries in lower valleys and
deltas; the absence of nutrient-rich sediments has detrimental effects on fish
productivity. Reduced flows in lower valley catchments also have resulted in
eutrophication and poor water quality.
Many rivers originate from the glaciers in the Tianshan mountain range, which
create wide alluvial fans at the foot of the northern Tianshan. The Hindukush
Himalayan ranges are the source of some major rivers. The total amount of water
flowing from the Himalayas to the plains of the Indian subcontinent is estimated
at about 8.6 x 106 m3 per year. The Himalayas have nearly 1,500 glaciers;
it is estimated that these glaciers cover an area of about 33,000 km2
(Dyurgerov and Meier, 1997). These glaciers provide snow and the glacial meltwaters
keep major rivers perennial throughout the year. In recent decades, the hydrological
characteristics of watersheds in this region seem to have undergone substantial
change as a result of extensive land-use changeleading to more frequent
hydrological disasters, enhanced variability in rainfall and runoff, extensive
reservoir sedimentation, and pollution of lakes (Ives and Messerli, 1989). Almost
67% of the glaciers in the Himalayan and Tienshan mountain ranges have retreated
in the past decade (Ageta and Kadota, 1992; Yamada et al., 1996; Fushimi, 2000b).
The mean equilibrium-line altitude at which snow accumulation is equal to snow
ablation for glaciers is estimated to be about 50-80 m higher than the
altitude during the first half of the 19th century (Pender, 1995). Available
records suggest that Gangotri glacier is retreating by about 30 m yr-1.
A warming is likely to increase melting far more rapidly than accumulation.
As reported in IPCC (1998), glacial melt is expected to increase under changed
climate conditions, which would lead to increased summer flows in some river
systems for a few decades, followed by a reduction in flow as the glaciers disappear.