Glaciers and Related Features of Three Sisters
Glaciers mantled virtually the entire Three Sisters area during the last glacial maximum, but today the high stratovolcanoes support only about one dozen small glaciers, chiefly in cirques, and numerous patches of perennial snow and ice. During the past few decades, three named glaciers have shrunk to snowfields (Skinner and Carver on South Sister and Irving Glacier on Middle Sister), and Thayer and Villard Glaciers on North Sister and Renfrew Glacier on Middle Sister are nearing a similar fate. As the climate warms further, more and more rock will be exposed at the expense of ice. Although glaciation and other erosional processes have modified most of the area, many of the volcanoes are young enough that their original forms remain largely intact.
The highest concentration of lakes dammed by glacial moraines in the conterminous United States is in the Three Sisters (and Mount Jefferson) wilderness Areas in Central Oregon. Most of these lakes formed between 1920 and 1940 during a period of substantial warming and glacier retreat. The moraine dams are chiefly breached during the melt season (July-October) and have formed lahars that have extended up to 10 km downstream with sediment-laden flood flows continuing farther.