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Eighteenmile Island is a 9.89 acre (4 ha) island on the Oregon side of the Columbia River at river mile 174 in Wasco County, Oregon, United States.
The river flows by Mission at about river mile (RM) 61 or river kilometer (RK) 98, leaves the Indian reservation, and reaches the city of Pendleton at about RM 56 (RK 90), passing under Oregon Route 11.
The streamflow of the Malheur and its tributaries is heavily influenced by a complex system of irrigation diversions, siphons, and canals, which begin near Malheur river mile 65, near Namorf and Harper, Oregon.