Lake Chelan ( ) is a narrow, long lake in Chelan County, north-central Washington state, U.S. It is an overdeepened lake and resembles a fjord, with an average width of . Near its upper end, the lake surface lies more than below peaks less than away. Before 1927, Lake Chelan was the largest natural lake in the state in terms of both surface area and water volume. Upon the completion of Lake Chelan Dam in 1927, the elevation of the lake was increased by to its present maximum-capacity elevation of . With a maximum depth of , Lake Chelan is the third deepest lake in North America behind Crater Lake, the deepest, and Lake Tahoe, the second deepest. Because of overdeepening, the sides of this lake drop steeply to its bottom. The deepest part of Lake Chelan lies as much as below sea level. In places, the bedrock floor of the valley occupied by Lake Chelan, which is buried by Pleistocene glacial and lacustrine sediments, lies at least below sea level. Two communities lie on the southern end of the lake, and a third sits at the far north end, providing a gateway to the North Cascades National Park. Hydrology On an annual basis, an average of flow into the lake. Approximately 75% of the water that flows into the lake comes from two tributaries. The Stehekin River alone contributes 65% of all water to Lake Chelan, averaging annually. The other major tributary, Railroad Creek, averages annually. The remaining water is added via a number of smaller tributaries as well as direct rain and snowfall. With a maximum depth of , Lake Chelan is the third deepest lake in the United States, and the 25th deepest in the world. At its deepest, the lake bottom is below sea level. The total watershed of the lake is More than 90% of the watershed is forested land. The remainder of the basin is composed of the lake itself (5.6%) and agriculture (3.5%). Geology The fjord-like topography of the Lake Chelan valley results from repeated glacial erosion and deposition (maybe nine or ten times) during the Pleistocene Period. The last episode of glacial erosion and deposition in the basin occurred during the Last Glacial Maximum about 21,000 years ago. At that time, in the upper Similkameen River valley of British Columbia, the Skagit Lobe split from the Okanogan Lobe of the Cordilleran ice