page_title: Three Sisters (Oregon)
section_title: Physical geography
hierarchical_section_title: Three Sisters (Oregon) / Geography / Physical geography
caption_attribution_description: English: Carver Lake is a moraine-dammed lake on the side of the South Sister volcano in Oregon
context_page_description: 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 more than 10,000 feet 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 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 has erupted in the last 14,000 years, and it is considered unlikely that either will ever erupt again.
context_section_description: 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, it descends on the eastern side of the crest, which causes the air to be warmer and drier. On the western slopes, precipitation ranges from 80 to 125 inches (200 to 320 cm) annually, while precipitation over the eastern slopes varies from 40 to 80 inches (100 to 200 cm) in the east. Temperature extremes reach 80 to 90 °F (27 to 32 °C) 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 feet (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 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-foot (1,500 m) retreat and a 64% loss of its surface area between 1910 and 1994, is generally considered to be the largest 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.
  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 early 1940s and in 1970, respectively. Other moraine-dammed lakes within the wilderness area include Thayer Lake on North Sister's east flank and four members of the Chambers Lakes group between Middle and South Sister.
  Before settlement of the area at the end of the 19th century, wildfires frequently burned through the local forests, especially the ponderosa pine forests on the eastern slopes. Due to fire suppression over the past century, the forests have become overgrown, and at higher elevations, they are further susceptible to summertime fires, which threaten surrounding life and property. In the 21st century, wildfires have been larger and more common in the Deschutes National Forest. In September 2012, a lightning strike caused a fire that burned 41 square miles (110 km²) in the Pole Creek area within the Three Sisters Wilderness, leaving the area closed until May 2013. In August 2017, officials closed 417 square miles (1,080 km²) in the western half of the Three Sisters Wilderness, including 24 miles (39 km) of the Pacific Crest Trail, to the public because of 11 lightning-caused fires, including the Milli Fire. As a result of the increasing incidence of fires, public officials have factored the role of wildfire into planning, including organizing prescribed fires with scientists to protect habitats at risk while minimizing adverse effects on air quality and environmental h
caption: Carver Lake is dammed by a glacial moraine and could produce a dangerous mudflow.
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