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A natural stream or river refers to a natural channel with natural water fed from upper terrains, which covers both perennial stream and river with water flowing throughout the year as well as intermittent ones with water-flow only during the wet season. The streambed is natural and not man-made, which could consist of mixtures of bedrocks, boulders, cobbles, gravels, sand, silt or clay. The banks are also largely natural and defined, covered with bank-side and riparian vegetation. Occupying < 1% of the Earth's surface, streams and rivers support a disproportionately large fraction of its biodiversity. They also function as important corridors for the movement of nutrients and organisms, as well as feeding grounds and habitats for a wide range of species.

The landscape of Hong Kong is dominated by hills, ravines and flat land is restricted to the low floodplains and coastal regions. In this hilly domain, hundreds of small rocky streams flow through steep ravines. These hill streams are typically fast flowing at their upland sources, and gradually reduce speed as they wind along hill cuttings to reach the lowlands. There are only a few long and meandering rivers in Hong Kong and are found only in the floodplains of the northern and northwestern New Territories.

There are over 2,500 km of natural streams and rivers in Hong Kong, mostly located in hillsides remote from developed areas. Many of these natural streams and rivers are good habitats supporting a variety of wildlife. They have important ecological functions and carry high aesthetic and landscape value.

Hau Tong StreamRiparian vegetation provides food for many aquatic organisms in the stream as well as a sheltering effect that is important for regulating the water temperature. Water-dragon (<em>Ludwigia adscendens</em>) is one of the common native plant found in riparian zones.<em>Stiphodon atropurpureus</em> is a freshwater fish species of conservation concern with restricted distribution. Males are metallic blue in colour. Macroinvertebrates such as crabs, shrimps, dragonflies, mayflies, stoneflies and caddisflies (see photo) spend their whole larval stages (some even spend their entire life cycles) in natural streams and rivers. They form an important part of the stream ecosystem and are commonly used as indicators of stream/river water quality. Hong Kong Clubtail (<em>Leptogomphus hongkongensis</em>) is endemic to Hong Kong and widely distributed in small wooded streams throughout Hong Kong.Giant Spiny Frog (<em>Quasipaa spinosa</em>) is the largest frog species in Hong Kong. It is restricted to mountain streams in Tai Mo Shan. Hong Kong Newt (<em>Paramesotriton hongkongensis</em>) inhabiting stream/river is the only salamander species found in Hong Kong. The pattern of the orange markings on the ventral side are unique for each individual. Three-banded Box Turtle (<em>Cuora trifasciata</em>), a Critically Endangered species in the IUCN Red List of Threatened Species, can be found in local mountain streams.

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