Lake and river systems comprise of large lakes and the rivers that connect them. The water of a lake and river system flows into the sea via a specific main river. Lake and river systems have their own catchment areas. 

Watersheds are located on the borders of lake and river systems. They divide the flow of rainwater in different directions. They are often flat and boggy areas that are located at high elevations.

The main watersheds of Finland are called Maanselkä and Suomenselkä. The Maanselkä watershed divides Finnish rainwater between the catchment areas of the Baltic Sea and the Arctic Ocean. The Suomenselkä watershed, on the other hand, divides Finnish rainwater between the cathcment areas of the Gulf of Bothnia and the Gulf of Finland. 

The lake and river systems of Finland have been named according to their main rivers:

• the Paatsjoki water system (flows into the Arctic Ocean),
• the Kemijoki water system (flows into the Gulf of Bothnia),
• the Kokemäenjoki water system (flows into the Gulf of Bothnia),
• the Oulujoki water system (flows into the Gulf of Bothnia),
• the Vuoksi water system (flows into the Gulf of Finland via Lake Ladoga) and
• the Kymijoki water system (flows into the Gulf of Finland).

The main rivers of lake and river systems are called outlets. They are larger than other rivers of the system. The longest Finnish river is Kemijoki, which is 550 km long and flows into the Gulf of Bothnia. The largest Finnish lake is Saimaa, which is a part of the Vuoksi water system. The Saimaa channel (over 43 km long) connects Lake Saimaa to the Gulf of Finland.