Water Pollution

Water pollution in downstream sections of rivers can become a serious problem as population, agriculture and industries increase. In less-developed areas, rivers are used as sewers - examples include the Ganges of India and Thailand's Mekong River.

Organic pollutants (such as livestock waste) or inorganic pollutants (such as heavy metals including mercury, cadmium and zinc) threaten living organisms in rivers. Human health will also be affected if people drink polluted water or eat river foods such as fish.

Dam Construction

Because of urbanization, and to decrease the impacts of floods and pollution, many rivers are channelized. Riverbeds and banks are dredged, so changing a river's depth, width, shape and course. They are then covered with concrete.

Natural river structure is complex, and provides varied habitats for different species. Channelization makes the river environment and hydrology homogenous, so far less suitable for plants and animals. Dams and weirs also change river structure, and may inhibit migration and range expansion by river animals. The impact is particularly serious for diadromous species (migrating between fresh and salt water).

A river has been converted to a concrete channel.


Exotic Species

Many animals are introduced to rivers, as food species, unwanted pets or even tools for biological control. Some may rapidly reproduce because of a lack of natural enemies, and become competitors for native species. Rivers can become blackspots for exotic species. For instance, some years ago Mosquito Fish (Gambusia affinis) were introduced into Hong Kong, to reduce the mosquito population, yet they also fed on species that are important foods for local fishes. Ecologists believe Mosquito Fish gradually outcompeted the native Rice Fish (Oryzias curvinotus). Today there are only few streams in Hong Kong without Mosquito Fish.

Apple Snail (Pomacea spp.) was introduced from South America as a culinary item. It has become common in local rivers and streams, feeding on aquatic plants.

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