Which river forms part of the border between Thailand and Laos?

Which river runs along the border between Romania and Bulgaria? Which river forms part of the border between Argentina and Uruguay? Which river forms part of the border between Namibia and Angola? Which river flows through Brazil, Peru, and Colombia? Which river flows through Argentina, Bolivia, and Paraguay? 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?