Kootenay Lake is a Canadian lake located between the Selkirk and Purcell mountain ranges in the Kootenay region of British Columbia. It is the largest natural lake in British Columbia, at approximately 100 km in length and 3-5 km in width. It is, in part, a widening of the Kootenay River, which in turn drains into the Columbia River system. Although oriented primarily in a north-south configuration, a western arm positioned roughly halfway up the length of the lake stretches 35 km to the town of Nelson. The lake originally tidally and seasonally flooded an approximately 80 km long marsh lying to the lake's south. However, this has been now been diked and converted to commercial agriculture.