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.
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