tributaries. The Skagit River is the largest river that flows into Puget Sound to the west, and the dams located in Ross Lake National Recreation Area and impounding other lakes adjacent to the park supply nearly 90 percent of the electricity used in Seattle. Other important rivers that originate in the park include the Chilliwack, Nooksack and the Stehekin rivers. Glaciers With approximately 312 glaciers, North Cascades National Park has the most glaciers of any US park outside Alaska, and a third of all the glaciers in the lower 48 states. Counting a few glaciers in the adjoining National Recreation Areas, the North Cascades National Park Complex glaciers covered an expanse totaling as of 2009. Boston Glacier, on the north slope of Boston Peak, is the largest glacier in the park, measured in 1971 to have an area of . Other glaciers that were measured in 1971 to be larger than include East Nooksack and Sulphide Glaciers on Mount Shuksan, McAllister and Inspiration Glaciers on Eldorado Peak, Redoubt Glacier on Mount Redoubt, Neve Glacier on Snowfield Peak, and Challenger Glacier on Mount Challenger. The dense concentration and relative ease of access to the North Cascade glaciers brought about some of the earliest series of scientific studies regarding glaciology in the United States. Beginning in 1955, the University of Washington sponsored Richard C. Hubley to undertake annual aerial photography expeditions designed to capture images of the glaciers and to show any alterations that might be occurring. In 1960, Austin Post expanded the aerial coverage to include other regions and he also used ground-based imagery to augment the research. In 1971, based on the photographs and other data collected since 1955, Post and others wrote a report that documented the number and scale of glaciers in the North Cascades. At the time of Austin Post's inventory, their study concluded that some North Cascades glaciers had experienced a period of minor growth or equilibrium in the mid-20th century, after undergoing decades of retreat. The study concluded that annual glacial melt due to seasonal variations has a significant influence on river levels, accounting for about 30 percent of the late summer water flow, which directly impacted the supported ecosystems such as salmon fisheries. The National Park Service, United States Geological Survey (USGS) and glaciologists such as Mauri S. Pelto, who has led the North Cascades Glacier Climate Project since 1984, have continued research on