"Wallowa River and Valley" details the history of the Wallowa Valley region in northeastern Oregon from approximately 1805 through 1950. The document includes extensive details on the origins of many place names in the region. The manuscript also includes a history of the Nimiipuu (Nez Percé) people and their cultural traditions, which Horner wrote in collaboration with Otis Halfmoon. Topics covered in the manuscript include Chief Joseph and the events of the Nez Percé war of 1877, settlement of the Wallowa Valley region, and local participation in World War I and World War II.