Columbia River Basalt Group
- Grande Ronde River (Wikipedia)
The Grande Ronde River (/ɡrænd rɑːnd/ or, less commonly, /ɡrænd raʊnd/) is a 210-mile (340 km) long tributary of the Snake River, flowing through northeast Oregon and southeast Washington in the United States. Its watershed is situated in the eastern Columbia Plateau, bounded by the Blue Mountains and Wallowa Mountains to the west of Hells Canyon. The river flows generally northeast from its forested headwaters west of La Grande, Oregon, through the agricultural Grande Ronde Valley in its middle course, and through rugged canyons cut from ancient basalt lava flows in its lower course. While it joins the Snake River upstream of Asotin, Washington, more than 90 percent of the river’s watershed is in Oregon.
Columbia River Basalt Group map shows main regions of basalt exposure in Washington, Oregon, Idaho and Nevada, USA. (usgs.gov)