The Uruguay River forms the border between Uruguay & Argentina & most of the border between Argentina & this country
It is located 
on the Ro del Valle de Catamarca, a river between the two ... which forms the 
border with Paraguay (north), and by the Uruguay River... [DOC] [TLE] ArgentinaUruguay border - Revolvy [PAR] As of the census of 2011, it is the fourth most populated city of the country.