The Guadiana River is an international river defining a long stretch of the Portugal-Spain border, separating Extremadura and Andalucia (Spain) from Alentejo and Algarve (Portugal). It marks the “natural border” between Spain and Portugal, and it has been at the centre of many treaties between the two countries.
These are the documents from the 1501 commission presided by Dr. Rodrigo de Neira, at the request of Álvaro de Zúñiga, Duke of Béjar, to negotiate with the King of Portugal the common use of the Guadiana river for the loading and shipping of wine and other merchandise that go out in ships to the sea: in fact, the registration of charges and the collection of rights were made in the Portuguese city of Alcoutim, ruled by Álvaro de Meneses, Marquis de Villarreal, to the detriment of the city of Sanlúcar, in Spain.
We don’t know how the dispute was solved, but you can find all the documentation at the Archivo General de Simancas and Portal de Archivos Españoles: PARES, and of course at Archives Portal Europe!