Yellowstone Lake is the largest high-elevation lake (above 7,000 ft) in North America, covering up to 139 square miles, with an average depth of 138 feet, and just over 12,000,000 acre-feet of water. The lake is covered by ice from mid-December to May or June. Entering Yellowstone Lake are more than 141 tributaries, but only one river. The Yellowstone River, which enters at the south end of the southeast arm, dominates the inflow of water and sediment flows out. The only outlet of the lake is at Fishing Bridge, where the Yellowstone River flows north and discharges 2,000–9,000 cubic feet per second.