Which river forms part of the border between Croatia and Serbia?

Which river forms part of the border between Namibia and Angola? Which river flows through Brazil, Peru, and Colombia? Which river forms part of the border between Thailand and Laos? Which river flows through Argentina, Bolivia, and Paraguay? 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 forms part of the border between the United States and Canada?