Freshwater stream habitat is an extensive and continuous habitat. Along terrestrial habitats and wetland, the freshwater flows from the water source to the oceans. A little impact on a section may cause a far-reaching consequence to the entire continuum. The flowing water can facilitate the inflow of food, outflow of waste and the dispersal of species. There is an interconnection of substances between the stream, valleys, wetlands and the underground water. Hong Kong's hilly terrain has given rise to many hill streams, many of which are protected inside country parks. These streams support many threatened species such as Chinese Three-striped Box Turtle (Cuora trifasciata), Hong Kong Newt (Paramesotriton hongkongensis), Thick-lipped Barb (Acrossocheilus parallens), etc. However, many of the lowland ones, for example, Tuen Mun River and Sheung Yue River, are polluted or channelized under urbanisation.