The Blue Mountains are a mountain range in the western United States, located largely in northeastern Oregon and stretching into southeastern Washington.The range has an area of 4,060 square miles (10,500 km 2), stretching east and southeast of Pendleton, Oregon, to the Snake River along the Oregon-Idaho border..S. Route 26 crosses the southern portion of the range, reaching a summit of 5,098 feet (1,554 m) at Blue Mountain Pass. Much of the range is included in the Malheur National Forest, Umatilla National Forest, and Wallowa–Whitman National Forest.