In northern Asia, there are extensive productive wetlands along the floodplains of rivers. In western Siberia, the river Ob extends over 50 000 km2 and supports what is called the largest waterfowl breeding and moulting area in Euroasia. The Ob Valley is a labyrinth of intricately arranged channels and floodplain lakes. As in other seasonal floodplains, the region is a land of fluctuating water levels, with seasonal and annual fluctuations in river discharge and flooding patterns. This area avoided any serious human impacts for centuries, but oil and gas exploration has resulted in significant pollution and transformation of the landscape.
The Indus River has long been the lifeblood of arid Pakistan. In earlier times, people used the river's water to cultivate the floodplain, but during the last 100 years, the river has been dammed and diverted into one of the largest and most complex irrigation systems in the world. In the absence of a drainage system to remove irrigation water, evaporation leaves salt in the soil. As a result of this salinization of the soil, combined with waterlogging, over 400 km of irrigated land is lost each year.
Many of the large river systems in South Asia display considerable annual variation in discharge and, during the rainy season, may flood very large expanses of land (e.g., approximately 200 km on each side of the Ganges). In some cases, entire deltaic areas may be inundated. The prolonged, monomodal flooding promotes extensive spatial and temporal contact with floodplains and, consequently, dominates the socioeconomics of the large human populations near those systems. Agricultural activities often cause significant sediment export from upper reaches of many rivers and, as a result, delta tributaries may become clogged. Due to the subsequent reduced flow, salinity can increase in soils and alter species composition in the delta forests.
About 80% of Bangladesh (115 000 km2) is formed by floodplains of the Ganges, Brahmaputra, and Meghna rivers. In major floods, 57% of the country can be flooded. Availability of water during the dry season makes it possible to grow three crops a year in some areas. Deposition of waterborne sediments keeps the soils fertile and algal growth enriches the soil by fixing nitrogen. As in many parts of the world, forest vegetation of South Asian floodplains strongly reflects variations in hydroperiod and soil. In the Ganges and Brahmaputra River Valleys, within areas with heavy clay soils where flooding occurs for most of the year or permanently, forest vegetation may be only 5-10 m in height and occur in conjunction with numerous vines. However, combinations of similar flooding regimes and lighter, fertile soils may increase canopy heights by 10 m or more. Many of these riverine forests exhibit a prevalence of evergreen or semievergreen species, although at higher altitudes alders may dominate. Some lowlands, in particular many river deltas such as those of the Ganges-Brahmaputra and Irrawady, are occupied by mangrove forests.
In China, 95% of the population is concentrated in the eastern half of the country, mainly in the vast alluvial plains of the major rivers, the Yellow and Yangtze Rivers primarily. High population densities coupled with high growth rates, rapid urbanization, and industrialization play a major role in most Asian countries. Water resources in this region are under increasing pressure as the demand for domestic supplies, agricultural use, and hydroelectric power increases. Past water resource and agricultural management practices have resulted in rapid loss and degradation of natural wetlands throughout the region. The regulation of rivers and streams through embankments and dams has eliminated floodplains and reduced groundwater recharge. Changing hydrological regimes have increased flooding during the rainy season and reduced availability during dry periods. Water resource management has often resulted in numerous man-made wetlands such as reservoirs and paddy fields that have very different functions and values than natural wetlands, and are in no way a substitute for natural wetlands, particularly floodplain wetlands. In short naturally occurring floodplains in these regions are threatened by numerous human activities, including mining, aquaculture, unsustainable forestry or fisheries practices, and conversion of forests to urban or agricultural land.
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