John Day River in Northwestern Oregon

Mouth of the John Day River on the Columbia, spanned by a railroad bridge

The John Day River is a tributary of the Columbia River, approximately 281 mi (452 km) long, in northeastern Oregon in the United States. One of two rivers in Oregon to bear this name, it is by the longer and more well-known. The other John Day River is a small tributary of the Columbia in Clatsop County.
The river was named for a member of the overland party that was funded in part by John Jacob Astor, John Day (1771-1819), who wandered lost through this part of Oregon in the winter of 1811-1812. Through its tributaries it drains much of the western side of the Blue Mountains, flowing across the sparsely populated arid part of the state east of the Cascade Range in a northwest zigzag, then entering the Columbia upstream from the Columbia River Gorge. The flows through  its entire length, the river is the second longest free-flowing river in the United States. Despite the extensive use of its waters for irrigation, its free-flowing course still furnishes an exceptional habitat for diverse species, including prolific wild salmon runs.
It rises in the Strawberry Mountains in eastern Grant County, in the Malheur National Forest. It flows initially north, then west past John Day. At Dayville in western Grant County it is joined from the south by the South Fork John Day River, then flows north, past the Sheep Rock Unit of the John Day Fossil Beds National Monument. At Kimberly in northwestern Grant County it is joined from the east by the North Fork John Day River, then flows west across Wheeler County. At the border county line with Jefferson County it flows north, past the Clarno Unit of the John Day Fossil Beds National Monument. As it approaches the Columbia in north central Oregon it flows in an increasingly meandering course, forming the boundary between Sherman County to the west and Gilliam County to the east. It joins the Columbia from the southeast approximately 10 mi (16 km) northwest of Biggs. The mouth of the river is on the narrow Lake Umatilla reservoir, formed on the Columbia by the John Day Dam, approximately 2 mi (3 km) downstream from the mouth of the John Day.
The river is not navigable. In its lower course its water is used for cropland irrigation and for ranching. In 1988, the United States Congress designated 147.5 mi (239 km) of the river from Service Creek to Tumwater Falls as the John Day Wild and Scenic River, as part of the National Wild and Scenic River program. The segment of the river is a popular destination for anadromous steelhead and warm-water bass fishing, as well as whitewater rafting.
In addition to wild salmon and bass, the river furnishes habitat for redband trout, bull trout, and west slope cutthroat trout.