Karjaanjoki basin

Lohjanjärvi (; also known as Lake Lohja) is a lake located in the Uusimaa region of Finland. The majority of the lake is located within the borders of the city of Lohja and smaller parts in the municipality of Karjalohja and in the city of Raseborg. The lake drains into the Gulf of Finland through Mustionjoki (also known as Karjaanjoki, ). Together with Hiidenvesi, which drains into the gulf from the north, Lohjanjärvi is a part of the Karjaanjoki basin.

Hiidenvesi (; ) is the second largest lake in the Uusimaa region in Finland. The biggest part of the lake is located in the municipality of Vihti and smaller parts in the town of Lohja. The lake drains through the Väänteenjoki River into lake Lohjanjärvi. Both lakes are part of the Karjaanjoki () basin that drains into the Gulf of Finland. 300px|none|thumb|Hiidenvesi in Retlahti, Pusula, Lohja. To the south.