The Park is on flat gently undulating land on the Albertine Rift floor. Altitudinal range is 67 - 760 m. It is drained by three main streams: Nyahuka, Rwigo and Kirumia. The landscape is underlain by rocks of old lake deposits and basement complex granites. Soils are rift valley sediments, grey alluvial clay soils that tend to be alkaline and of poor fertility. The wettest months are March to May, and September to December, with average rainfall of 1250 mm. Annual mean temperature ranges from 18 C minimum to 30 C maximum.
The park is considered an easterly extension of DRC's Ituri Forest as its flora and fauna show strong resemblance to the Congo Basin species. The forest is classified as moist semi-decidous and is dominated by the Ironwood tree, Cynometra alexandrii.7% of the park is swamp forest dominated by Mytragyna rubrostipulosa, the palm Eliaeis guineensis, and the figFicus vogeliana.
It has a rich biodiversity likely to be part of the Pleistocene refugium supporting fauna characteristic of the 'Gunea-Congolian' and the Lake Victoria Regional Mosaic' centres of endemism in Africa.
Semliki National Park has at least 331 tree species. At least 441 bird species have been recorded here, of which 2 are restricted range species. The park has at least 86 mammal species of which 9 species or subspecies are threatened (Plumptre et al. 2003). Bates pygmy antelopeNeotragus batesi is only found in this park in East Africa, and the park is also particularly important for its population of mona monkeys (Cercopithecus pogonias denti) which is found nowhere else in Uganda and de Brazza monkeys (Ceropithecus neglectus). Larger mammals include elephant, forest buffalo, hippo, leopard and the chimpanzee (currently about 50 individuals, Plumptre et al 2003a). With eight species of diurnal primates in Semliki.
Semliki is especially important for conservation of species whose range is limited to this part of East Africa and the Congo Basin. Semuliki National Park's Sempaya hot springs are a great attraction to most visitors that begin their Uganda travel to the Western areas of the country.