The Gin River and Nilwala Rivers are the two most important rivers in southern Sri Lanka, which are the nourish Kanneliya- Dediyagala- Nakiyadeniya (KDN) forest complex. This biosphere reserve harbors many endemic plants and animal species in Sri Lanka. This area is covered by a natural high forest eco system consisting of several floristically richest areas in Indomalayan.
The forest complex designated as a biosphere reserve in 2004 by UNESCO . The KDN complex is the last large remaining rainforest in Sri Lanka other than Sinharaja. Mainly lowland tropical forest. Total of about 5305.9 ha and core unit of the Kanneliya forest reserve is about 5108.2ha.
The KDN forest complex rich of floral endemic city, as 319 woody plants recorded in the area and 17 percent of lowland endemic floral species dominated here. Large number of medicinal plants are found in these forests.
KDN complex celebrated its faunal species including 41 endemic and home to the 86 species mammals. Majority of the 20 endemics birds in Sri Lanka are restricted to the rainforest of the southwest. Include four species of shrews, 5 rodents, one carnivore and 2 primates. Out 26 endemic birds of Sri Lanka 20 of them can be seen in KDN forest complex. Spur fowl, Jungle fowl, Grey Hornbill, Red-faced Malkoha, Orange-billed Babbler, and Blue Magpie are some of them. 20 percent of Sri Lanka's endemic freshwater fishes inhabit in the waters of Gin River and Nilwala River.
Snakes of Kanneliya forest: A total of 36 species snakes, includes 17 endemic forms belonging to six families have been recorded form Kanneliya further a total of 23 species of lizards including 13 endemic species have been recorded and 38 species of fresh water fishes belonging to 14 families have been recorded from KDN complex and among them are 20 endemic species. The Gin Ganga, Udugama Ela, Kanneliya Ela and their shady forest streams provide extremely suitable habitats for their fishes.