The USGS Central Region Energy Team assesses oil and gas resources of the United States. The onshore and State water areas of the United States comprise 71 provinces. Within these provinces, Total Petroleum Systems are defined and Assessment Units are defined and assessed. Each of these provinces is defined geologically, and most province boundaries are defined by major geologic changes. The Eastern Oregon-Washington Province is located in eastern Oregon and eastern Washington, encompassing all or parts of Baker, Clackamas, Crook, Deschutes, Gilliam, Grant, Harney, Hood River, Jefferson, Linn, Malheur, Marion, Morrow, Sherman, Umatilla, Union, Wallowa, Wasco, Wheeler Counties in Oregon and all or parts of Adams, Asotin, Benton, Chelan, Columbia, Douglas, Ferry, Franklin, Garfield, Grant, King, Kittitas, Klickitat, Lewis, Lincoln, Okanogan, Pend Oreille, Skagit, Skamania, Snohomish, Spokane, Stevens, Walla Walla, Whatcom, Whitman, Yakima Counties in Washington. The main population centers within the study area are Heppner, John Day, Mitchell and Union in Oregon; and The Dalles, Richland, Spokane and Yakima in Washington. The main highways, I-84 and I-90, generally traverse the area from east to west. The Columbia River and their tributaries drain the area. The province boundary was drawn to include the geologic structures generally considered to be in or bounding the Eastern Oregon-Washington Basin.