Which river forms part of the border between Brazil and Peru?

Which river flows through both Germany and the Czech Republic? Which river forms part of the border between the United States and Canada? Which river flows through both Poland and Germany? What is the name of the river that crosses the border of Costa Rica and Nicaragua? The Kosi River flows through which countries? The Paraná River forms part of the border between which countries? The Tigris River flows through which countries? The Okavango River flows through which countries?