 It also lies within the Inland Northwest, centered on Spokane, and the Columbia Plateau ecoregion near the Okanogan Drift Hills.The Okanogan River, a tributary of the Columbia River, flows through the central portion of the city, and receives Omak Creek from the east just outside municipal boundaries.Known for its balancing Omak Rock, the Omak Lake above sea levelis the largest saline endorheic lake in Washington.