Thank you for posting your question on History Hub.
We searched in Record Group 115 - Records of the Bureau of Reclamation, 1889 - 2008 but were unable to locate maps of Marcus, Washington or identify relevant series of documents for the Coulee Dam that include maps.
We searched online as well and found several archival collections with photographs of the town before and after the dam construction, including the University of Washington Library and the Bureau of Reclamation, but did not find maps or plats of the area.
We recommend reaching out to our Cartographic Department to request a search of their holdings for the maps you are seeking.
The National Archives and Records Administration