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The Uruguay River is not navigable from the Atlantic Ocean to Brazil. Even before the construction of the Salto Grande Dam, there were rapids.
Encyclopædia Britannica states of the Uruguay River
Its 990-mile (1,593-km) course is interrupted by rapids between Salto (Uruguay) and the influx of the Quaraí River (Spanish: Río Guareim) near Monte Caseros; hence, its importance as a waterway is less than that of the Paraguay–Paraná river system.
That's the stretch between the Salto Grande Dam and Brazil - Monte Caseros is near the border.