Medina Lake is a reservoir on the Medina River in the Texas Hill Country of the United States. It is operated by the Bexar/Medina/Atascosa County Agricultural District. Medina Dam was completed in 1913 in a privately financed project, creating the lake to supply irrigation water for local agricultural use. Lake Medina is in northeastern Medina County and …Medina Lake is a reservoir on the Medina River in the Texas Hill Country of the United States. It is operated by the Bexar/Medina/Atascosa County Agricultural District. Medina Dam was completed in 1913 in a privately financed project, creating the lake to supply irrigation water for local agricultural use. Lake Medina is in northeastern Medina County and southeastern Bandera County, about 40 miles northwest of San Antonio.