South Sister is reflected on the water of Sparks Lake on a foggy summer morning in Deschutes County, Oregon. South Sister, at 10,363 feet (3,159 meters) tall, is the tallest and youngest volcano in Oregon's Three Sisters group, last erupting about 2,000 years ago. Yellow spear-leaf arnica (Arnica longifolia) flowers bloom on a small island in Sparks Lake.