The Ponderosa Pine/Bitterbrush Woodland ecoregion has a terrain dominated by high, undulating volcanic plateaus and canyons, with permanent, medium gradient streams. Elevation varies from . Stream flow is consistent year-round, due to the volcanic hydrogeology. The well-drained, frigid soils are derived from Mazama Ash, which was produced by the catastrophic eruption of Mount Mazama about 6,845 years ago, and support nearly homogenous stands of ponderosa pine. Historically, frequent fires burned undergrowth, creating open groves of pines. Lodgepole pine is largely absent here. Understory vegetation varies with elevation; at lower elevations, antelope bitterbrush is important winter browse for deer. At higher elevations, greenleaf manzanita and snowberry are found. Riparian areas support mountain alder, stream dogwood, willows, and sedges. The region covers in Oregon, east of Mount Jefferson, Three Fingered Jack, and the Three Sisters, in the Deschutes National Forest and on the Warm Springs Indian Reservation.