Lake Vesijärvi is part of the Kymijoki water system and empties into southern Lake Päijänne along the Vääksyjoki river and Vääksy Canal. The total length of Lake Vesijärvi is 25 kilometers. The deepest point of Lake Vesijärvi is 40 meters. The total area of the lake is 108 km2 and the biggest basins are Enonselkä (Lahti), Komonselkä and Laitialanselkä (Hollola) and Kajaanselkä (Asikkala).

Lake Vesijärvi has been managed for over thirty years, and nowadays the Lake Vesijärvi Foundation combines public and private resources to secure funding for research, maintenance, and management efforts focusing on Lake Vesijärvi and its catchment area.

In the summertime, a number of cafés and restaurants operate by Lake Vesijärvi in Lahti. At the harbour, you can also find one of the most interesting sights in Lahti: the Sibelius Hall. Lake Vesijärvi acts as a gateway to Lakeland Finland, and the clear and drinkable southern Lake Päijänne can be reached via Vääksy Canal. Several cruises on Lake Vesijärvi start from Lahti Harbour.