The Three Sisters are closely spaced volcanic peaks in the U.S. state of Oregon. They are part of the Cascade Volcanic Arc, a segment of the Cascade Range in western North America extending from southern British Columbia through Washington and Oregon to Northern California. Each over 10,000 feet (3,000 meters) in elevation, they are the third-, fourth- and fifth-highest peaks in Oregon. Located in the Three Sisters Wilderness at the boundary of Lane and Deschutes counties and the Willamette and Deschutes national forests, they are about 10 miles (16 kilometers) south of the nearest town, Sisters. Diverse species of flora and fauna inhabit the area, which is subject to frequent snowfall, occasional rain, and extreme temperature variation between seasons. The mountains, particularly South Sister, are popular destinations for climbing and scrambling. Although they are often grouped together as one unit, the three mountains have their own individual geology and eruptive history. Neither North Sister nor Middle Sister have erupted in the last 14,000 years, and it is considered unlikely that either will ever erupt again. South Sister last erupted about 2,000 years ago and could erupt in the future, threatening life within the region. After satellite imagery detected ground inflation near South Sister in 2001, the United States Geological Survey improved monitoring in the immediate area.
Geography
The Three Sisters are at the boundary of Lane and Deschutes counties and the Willamette and Deschutes national forests in the U.S. state of Oregon, about 10 miles (16 kilometers) south of the nearest town of Sisters. The three peaks are the third-, fourth-, and fifth-highest in Oregon, and contain 16 named glaciers. Their ice volume totals 5.6 billion cubic feet (160 million cubic metres). The Sisters were named Faith, Hope and Charity by early settlers, but are now known as North Sister, Middle Sister and South Sister, respectively.
Wilderness
The Three Sisters Wilderness covers an area of 281,190 acres (1,137.9 km2), making it the second-largest wilderness area in Oregon. Designated by the United States Congress in 1964, it borders the Mount Washington Wilderness to the north and shares its southern edge with the Waldo Lake Wilderness. The area includes 260 mi (420 km) of trails and many forests, lakes, waterfalls, and streams, including the source of Whychus Creek. The Three Sisters and nearby Broken Top account for about a third of the Three Sisters Wilderness, and this area is known as the Alpine Crest Region. Rising from about 5,200 ft (1,600 m) to 10,358 ft (3,157 m) in elevation, the Alpine Crest Region features the wilderness area's most-frequented glaciers, lakes, and meadows.
Physical geography
Weather varies greatly in the area due to the 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 80 to 125 in (200 to 320 centimeters) annually, while precipitation over the eastern slopes varies from 40 to 80 in (100 to 200 cm) in the east. Temperature extremes reach 80 to 90 degrees Fahrenheit (27 to 32 degrees Celsius) in summers and −20 to −30 °F (−29 to −34 °C) during the winters. The Three Sisters have about 130 snowfields and glaciers ranging in altitude from 6,742 to 10,308 ft (2,055 to 3,142 m) with a cumulative surface area of about 2,500 acres (10 km2). 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 4,900 ft (1,500 m) 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 160 acres (0.65 km2). 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.
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