High Quality Content by WIKIPEDIA articles! The Okanogan River (called the Okanagan River in Canada) is a tributary of the Columbia River, approximately 115 mi (185 km) long, in southern British Columbia and north central Washington. It drains a scenic plateau region called the Okanagan Country east of the Cascade Range and north and west of the Columbia, and also the Okanagan Valley of British Columbia - which is also known as the Okanagan Country, but more usually as "the Okanagan".