The North Saskatchewan River originates at the Columbia Icefield in Jasper National Park and bisects the city. It sometimes floods Edmonton's river valley, most notably in the North Saskatchewan River flood of 1915. It empties via the Saskatchewan River, Lake Winnipeg, and the Nelson River into Hudson Bay. It runs from the southwest to the northeast and is fed by numerous creeks throughout the city, including Mill Creek, Whitemud Creek and Blackmud Creek; these creeks have created ravines, some of which are used for urban parkland. Edmonton is within the Canadian Prairies Ecozone. Aspen parkland surrounds the city and is a transitional area from the prairies to the south and boreal forest in the north. The aspen woods and forests in and around Edmonton have long since been reduced by farming and residential and commercial developments including oil and natural gas exploration.