rain shadow caused by the Cascade Range. Air from the Pacific Ocean rises over the western slopes, which causes it to cool and dump its moisture as rain (or snow in the winter). Precipitation increases with elevation. Once the moisture is wrung from the air, the air descends on the eastern side of the crest, which causes it to become warmer and drier. On the western slopes, precipitation ranges from annually, while precipitation over the eastern slopes varies from in the east. Temperature extremes reach in summers and during the winters. The Three Sisters have about 130 snowfields and glaciers ranging in altitude from with a cumulative surface area of about . The Linn and Villard Glaciers are north of the North Sister summit, while the Thayer Glacier is on its eastern slope. The Collier Glacier is nestled between North Sister and Middle Sister and flows to the northwest. The Renfrew and Hayden Glaciers are on the northwestern and northeastern slopes of Middle Sister, respectively, while the Diller Glacier is on its southeastern slope. The Irving, Carver and Skinner Glaciers lie between Middle Sister and South Sister. Finally, around the summit of South Sister, in a clockwise direction, are the Prouty, Lewis, Clark, Lost Creek, and Eugene Glaciers. The Collier Glacier, despite a retreat and a 64% loss of its surface area between 1910 and 1994, is generally considered to be the largest glacier of the Three Sisters at . Eliot Glacier on Mount Hood is now two-and-a-half times larger than the Collier Glacier. According to sources, the Prouty Glacier is sometimes considered to be larger than the Collier glacier. When Little Ice Age glaciers retreated during the 20th century, water filled in the spaces left behind, forming moraine-dammed lakes, which are more common in the Three Sisters Wilderness than anywhere else in the contiguous United States. The local area has a history of flash floods, including an event on October 7, 1966, caused by a sudden avalanche; this flash flood reached the Cascade Lakes Scenic Byway. Concerned about the hazard of similar flooding events, scientists in the 1980s from the United States Geological Survey (USGS) identified that Carver Lake on South Sister could flood and breach its natural dam, producing a large mudflow that could endanger wilderness visitors and the town of Sisters. Studies at Collier Lake and Diller Lake suggested that both had breached their dams in the