The Okanogan River flows through Osoyoos Lake, which extends across the international boundary, and continues southward to empty into the Columbia River near Brewster. However, an even greater inflow from Canada is from the Okanogan's major tributary, the Similkameen River. The Similkameen crosses the border west of the Okanogan and enters the Okanogan River near the south end of Osoyoos Lake. About 2.1 million acre-feet of water enters the watershed from Canada as streamflow; about 75 percent of this amount is from the Similkameen River. The outflow from the watershed at Brewster is estimated to be 2.2 million acre-feet.
This watershed is within the Columbia Basin, Cascades, and Northern Rockies ecoregions. The eastern and western boundaries of the basin are steep, jagged ridgelines at elevations ranging from 1,500 feet to more than 5,000 feet above the basin floor. The floodplain of the Okanogan River valley averages about a mile in width, and descends from an elevation of about 920 feet at the international boundary to about 780 feet at the river's confluence with the Columbia River. Osoyoos Lake occupies the northern most 4 miles of the valley floor and extends several miles into Canada.
The soils in the watershed include shallow to moderately deep sandy loam and silt loam. These soils are formed from volcanic ash and pumice (ejected from Glacier Peak to the west centuries ago), glacial till and outwash, alluvium, lake sediments, and wind-laid silts.
There are approximately 32,855 people living in the Okanogan Basin. The primary population centers are Omak and Okanogan. The majority of people live in unincorporated areas. The largest land uses in the basin are forested lands (51%) and agricultural lands (39%).