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.