This inland Ecologically Significant Unit (ESU) occupies the Snake River Basin in Washington, Oregon, and Idaho and includes all naturally spawned populations of spring and summer Chinook in the Snake River and its tributaries. The ESU was originally listed as threatened on April 22, 1992 and the listing was reaffirmed on June 28, 2005 (U.S. Office of the Federal Register 1992, 2005). A draft Recovery Plan is being developed for the Oregon unit of the Snake River Chinook ESU to guide recovery of the Oregon populations. The Snake River Basin encompasses an area of approximately 277,128 sq km, of which the northeast Oregon unit comprises 12,639 sq km. Three major rivers in Oregon, along with their tributaries, flow into the Snake River drainage: the Grande Ronde, Imnaha, and Wallowa rivers. The region is located in the Columbia plateau of northeastern Oregon and is characterized by a rolling, semi-arid landscape that is bordered by the plush terrain of the Blue Mountains.
The Snake River ESU contains five Major Population Groups (MPGs) - Lower Snake River, Grande Ronde / Imnaha Rivers, South Fork Salmon River, Middle Fork Salmon River, and Upper Salmon River. Collectively the five MPGs contain 28 extant (exiting), 2 extirpated (locally extinct), and 2 functionally extirpated (not enough fish or habitat in suitable condition to support a fully functional population) populations. The Oregon portion of the ESU includes 6 extant populations within the Grande Ronde/Imnaha MPG - Wenaha River, Wallowa/Lostine rivers, Minam River, Grande Ronde River upper mainstem, Catherine Creek, and Imnaha River; one extirpated population - Lookingglass Creek; and one functionally extirpated population – Big Sheep Creek. Status assessments were developed for the populations residing in the currently accessible sections of the Snake River basin (ICTRT 2010).