Despite its relatively small land area, Hong Kong has a large number of rivers and streams, often with their catchments forming in the mountainous inland areas of the New Territories, Hong Kong Island and Lantau Island. There are also a number of significant stillwater bodies, in particular man-made reservoirs which provide a source of freshwater and which are often located within the territory's Country Parks. Figure 5.3a illustrates the major freshwater features in the territory.

Through land use controls such as the designation of water gathering grounds (see Section 3.4 for further discussion on freshwater resources), reservoir catchment areas should not receive any polluting point source discharges and there is no regular monitoring of the water quality of stillwaters in Hong Kong by EPD (although WSD undertake samples for compliance with drinking water quality at storage and service reservoirs). This section therefore focuses upon the assimilative capacities of rivers since the quality of the larger watercourses is regularly monitored by EPD and these watercourses are subject to the most significant pollutant loadings. The current environmental quality of the freshwater systems is reviewed in the context of assimilative capacity enhancers and impacts upon water quality. However, the effects of changes in water quality arising from these impacts are addressed in other relevant sections of the report. For example changes in freshwater quality on ecological resources are addressed in Paragraph 4.3.4.5 - Impacts on Terrestrial Habitats and Species.

ThRivers and streams play an important role in assimilation of pollutants though the assimilative capacity of each will depend on its physical, chemical and biological characteristics. For example the assimilative capacity will be influenced by the flow (dilution) and flushing capacity of the stream, its natural chemical balance (which may be altered by changes in pH from polluting inputs) and the tolerance of its aquatic faunal communities to pollution. In addition to assimilation of pollutants, since these watercourses drain much of the land area of Hong Kong, they also provide assimilation of stormwater flows to prevent flooding of the land.