The Lake Washington watershed covers approximately 540 square miles in the greater Seattle area. The hydrology of the watershed is influenced by the Cascade Mountains to the east and the Olympics to the west, which provide a significant amount of precipitation. In the winter, snowpack in the Cascades can account for up to 60% of the annual water supply for the watershed. The surface water of the watershed includes Lake Washington, which is the second-largest natural lake in Washington state. The reservoir constituents of the lake include phosphorus, which is a concern for water quality and can lead to harmful algal blooms. Interesting climatic facts include the fact that the Seattle-Tacoma-Bellevue metropolitan area receives more rainfall than any other major U.S. city, with an average of 38 inches per year. Trends in the watershed include efforts to reduce pollution and improve water quality, as well as adaptation to the effects of climate change, such as increased frequency of extreme weather events.