They are Senanayake Samudra Sanctuary, Gal Oya Valley North-East Sanctuary and Gal Oya Valley South-West Sanctuary. Together these reserves and the national park cover approx 63,000 ha of land. The park and the three sanctuaries were established by the Gal Oya Development Board on the 12th of February 1954 and subsequently handed over to the Department of Wildlife Conservation in 1965.
Gal Oya is a valley, which has given refuge to several kings in the ancient past. In the 2nd Century BC, King Tissa sought refuge at the Digavapi, a place Lord Buddha visited in his third visit to Sri Lanka. The Digavapi Dagoba, built in the 2nd Century BC to mark the spot where the Lord Buddha sat on his last visit to Sri Lanka, attracts thousands of pilgrims even today.
About 45% of the vegetation is evergreen forest, 33% savanna, 09% grassland, 02% cheina cultivations and the balance is water bodies dominated by the Senanayake Samudraya. A host of medicinal shrubs and trees such as Aralu, Bulu, Nelli can be readily found in the Nilgala area, while a number of locally known trees such as Vevarana, Halmilla, Veera, Palu, Ebony and Mahogany are found in great numbers. The park with its thick green canopy is a haven for species of birds and nearer to the Samudraya even migratory birds such as painted storks, pelicans, cormorants and teals could be seen.
A host of local birds such as the Grey Dove, Malabar Horn Bill and Grey Horn Bill, Koel and a number of water birds are found in this jungle habitat. In addition to elephants, the park is home to leopards, bear, spotted deer, sambur, wild boar etc. Among other fauna are several species of monkeys, porcupine, a number of fish species, reptiles and four species of butterflies such as the Crimson Rose and Glassy Tiger have also been recorded.