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.