Cowichan Lake is a large freshwater lake located on southern Vancouver Island, British Columbia, Canada. It is located along the Cowichan Valley in the Cowichan Valley Regional District, with its eastern shore about 28 km (17 mi) west of Duncan, British Columbia. Cowichan Lake is about 30 km (19 mi) in length, running mostly from west to east, and is about 4 km (2.5 mi) wide at its widest point. The lake is also the source of the Cowichan River.