Letha is an unincorporated community and census-designated place in Gem County, Idaho The community is located 8 mi west of Emmett. The Payette River flows northeast of Letha. Letha was founded by W.W. Wilton and a Colonel Barnard and named for Wilton's daughter, Letha Wilton. It was built approximately midway along the railway running from Emm…Letha is an unincorporated community and census-designated place in Gem County, Idaho The community is located 8 mi west of Emmett. The Payette River flows northeast of Letha. Letha was founded by W.W. Wilton and a Colonel Barnard and named for Wilton's daughter, Letha Wilton. It was built approximately midway along the railway running from Emmett to New Plymouth, Idaho, with anticipation that it would become a major rail center; although this never occurred, Letha today remains a service center for the adjacent farms and ranches.