The lower Grande Ronde River where it meets, (confluence), the Snake River in southeast Washington State. The Grande Ronde is considered a premier river for sea-run rainbow trout, or steelhead. The Grande Ronde River is a tributary of the Snake River, 212 miles long, in northeastern Oregon and southeastern Washington in the United States. It drains an area southeast of the Blue Mountains, and northwest of the Wallowa Mountains, on the Columbia Plateau. It flows through the agricultural Grande Ronde Valley in its middle course and through a series of scenic canyons in its lower course.