The Nelson River, 2,575 km long, flows north northeast out of Playgreen Lake, at the northwest tip of Lake Winnipeg. It spills out into a number of lakes, including Cross, Sipiwesk, Split and Stevens, flowing east from the latter into Hudson Bay. Its main tributaries are the Burntwood River, on which Thompson, Manitoba, is located, and the Grass River. Sir Thomas Button wintered at its mouth 1612 and named it for Robert Nelson, a ship's master who died there. The entrance to the river became the scene of a bitter struggle for the fur trade, and York Factory was established at Marsh Point, a peninsula separating the Nelson and Hayes River. However, the Hayes, not the Nelson, became the main route inland. The Nelson takes an often turbulent course through the granitic Shield; as the last link in the long Saskatchewan River system and as a conduit for the waters of the Red and Winnipeg rivers as well, it has a mean flow at its outlet of 2066 m3/s. Today the Nelson's drop and huge volume are exploited for hydroelectric power. The largest developments are Kettle Rapids, built 1970-74 (1272 MW), Long Spruce, built 1977-79 (980 MW), and Kelsey, built 1960-72 (224 MW), to provide power for nickel smelters at Thompson.