Grande Ronde Valley

The Grande Ronde Valley is a valley in Union County in northeastern Oregon, United States. It is surrounded by the Blue Mountains, and is drained by the Grande Ronde River. La Grande is its largest community. The valley is 35 miles (56 km) long, north to south, from Pumpkin Ridge to Pyles Canyon, and 15 miles (24 km) wide, east to west, from Cove to the Grande Ronde River's canyon. Its name, fittingly, means, "great circle."

The Grande Ronde Valley is part of the Columbia River Plateau. The Columbia River Plateau was created by a series of basalt flood eruptions. These happen when a long crack in the Earth's crust spews floods of lava. The molten rock then hardens and creates layers of basalt. The Grande Ronde Valley floor began to sink from this plateau at a rate of about .07-.18 millimeters per year about nine million years ago.

The Grande Ronde Valley floor is at about 2,700 feet (820 m) above sea level. Its horizon is dominated by the Blue Mountains. Eagle Cap Wilderness is to the southeast. The highest nearby mountains are Mount Fanny to the east, with an elevation of 7,136 feet (2,175 m); Mount Harris to the northeast, elevation 5,335 feet (1,626 m), and Mount Emily to the northwest, elevation 6,110 feet (1,860 m). Communities within the valley include Cove, Imbler, Island City, La Grande, Summerville, and Union. The Grande Ronde River flows through the valley; its tributaries include Catherine, Fir, Ladd, Little, Mill, Pyles, and Willow creeks.

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