Longest Rivers in Sri Lanka Name of River  Length  Mahaweli Ganga  335 km Aruvi Aru  164 km Kala Oya  148 km Kelani Ganga  145 km Yan Oya  142 km Deduru Oya  142 km Walawe Ganga  138 km Maduru Oya  135 km Maha Oya  134 km Kalu Ganga  129 km Kirindi Oya  117 km Kumbukkan Oya  116 km Menik Oya  114 km Gin Ganga  113 km Mi Oya  109 km Gal Oya  108 km   Rivers of Sri Lanka Rivers Mahavali Rever (Ganga) is the largest of the 103 river basins found in Sri Lanka. It covers about 16% of the island's total area. The river itself has a winding course, rising about 50 km south of Kandy and flowing north then north-east to the sea near Trincomalee covering a distance of 320 km. It is the only perennial river to cross the dry Zone. Sri Lanka’s all perenial revers are called Ganga,while seasonal streams are called Oya in Sinhala or Aru in Tamil. A number of rivers have now been developed both for irrigation and power. The Victoria project on Mahavali Ganga is one of the largest in Asia.