Which river forms part of the border between the United States and Canada?

Which river forms part of the border between Croatia and Serbia? Which river flows through Germany, France, and Switzerland? Which river flows through China, North Korea, and Russia? Which river flows through both Germany and the Czech Republic? Which river flows through both Poland and Germany? What is the name of the river that crosses the border of Costa Rica and Nicaragua? Which river forms part of the border between Brazil and Peru? The Kosi River flows through which countries?