Lake Päijänne is the second largest lake in Finland. The lake drains into the Gulf of Finland via the Kymi River. The major islands are from north to south Vuoritsalo, Muuratsalo, Onkisalo, Judinsalo, Edessalo, Taivassalo, Haukkasalo, Vehkasalo, Mustassalo, Virmailansaari and Salonsaari. The largest island is Virmailansaari. The word saari means an island. Salo once meant a great island, nowadays it means a great forest area.

The Kymi is a river in Finland. It begins at Lake Päijänne, flows through the provinces of Päijänne Tavastia, Uusimaa and Kymenlaakso, and discharges into the Gulf of Finland. The river passes the towns of Heinola and Kouvola. The town of Kotka is located on the river delta. The length of the river is 204 kilometres (127 mi), but its drainage basin of 37,107 square kilometres (14,327 sq mi) extends to almost 600 kilometres (370 mi) inside the Tavastia, Central Finland, Savonia and Ostrobothnia. The furthest source of the river is Lake Pielavesi, its furthest point being some 570 kilometres (350 mi) from the sea measured by flow route. The name of the river, itself, kymi, means "large river", in Old Finnish.