Today Borders goes back to Spain for another agreement with Portugal about rivers: 300 years after the rio Guadiana agreement, these are the documents on the negotiations for the free negotiations of the Douro (Duero) river, which has the mouth in Spain and the estuary in Portugal, provided by PARES and the Archivo Historico de la Nobleza.
You can find them archived on Archives Portal Europe here:
http://www.archivesportaleurope.net/ead-display/-/ead/pl/aicode/ES-D706/type/fa/id/ES-45168-AHNOB-UD-4034845/unitid/ES-45168-AHNOB-UD-4034845+-+ES-45168-AHNOB-UD-4036021/
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