Lake Mapourika is located on the West Coast of New Zealand's South Island. It lies north of Franz Josef Glacier, and the out-flowing Ōkārito River drains it into the Ōkārito Lagoon. It is the largest of the West Coast lakes, a glacier formation from the last ice age. Since the water from glacial melts no longer drains into the lake, it is filled with fresh rain water which runs though the surrounding forest floor, collecting tannins, giving it its dark colour.