U.S. Historic Range of Westslope Cutthroat trout "Oncorhynchus clarki lewisi". Extracted from "Status Review for Westslope Cutthroat Trout in the United States US Fish and Wildlife Service, September 1999 Caption on Map: The historic distribution of westslope cutthroat trout in the United States (modified from Behnke 1992). The large region consists primarily of the upper Columbia River and upper Missouri River basins; some waters in the eastern part of this region may not have been occupied historically (MTFWP, in litt. 1998). Also shown are the Lake Chelan and Methow River drainages in Washington and the John Day River drainage in Oregon.