Hong Kong has a warm, wet climate that encourages weathering and river erosion. Therefore, small streams plunge down most natural slopes across the territory-though many these are "ephemeral streams", becoming dry during the dry seasons. The many faults and joints in Hong Kong's bedrock also favours the formation of streams and rivers. The fact Hong Kong is small means there are no large rivers present. The larger rivers are mainly in the northwest, including: Shan Pui River, Shenzhen River, Kam Tin River, Sheung Yue River, and Ng Tung River.
Hong Kong is a mountainous area with little flat land. Precipitation is substantial, averaging 2,200 mm per year, so rivers and streams are densely distributed. Yet only the larger ones (such as Shan Pui River, Lam Tsuen River) and those at local tourism hotspots (such as Tai Shing Stream) have official names; most tributaries and streams are not named. Hence there are no formal statistics on the number of rivers in Hong Kong. According to the baseline survey on Hong Kong's ecology conducted by the Agriculture, Fisheries and Conservation Department since 2002, there are over 200 rivers, streams and nullahs in Hong Kong. Among these, 33 of them are designated as Ecologically Important Streams.