Here is a great book for anyone interested in the County or city of Zapata. It was written by Virgil N. Lott and Mercurio Martinez as a work to compile the history of Zapata county commissioned by the County Commissioners of Zapata county back in 1951.
The reason for this is mentioned in the book as follows,
The action was decided upon due to the inevitable inundation of a part of the county from the waters of the Falcon Dam which will destroy many of the historic landmarks of the county, including the county seat, Zapata, and other villages, many of which were founded in the early eighteenth century by colonists and their children, who were settled on this frontier by Escandon, the great colonizer of all time. The first colony on the north banks of the Rio Grande was at Dolores, Zapata County, Texas, in 1751, and from this establishment many of the present population of Starr, Zapata and Webb counties originated.