Which river flows through Peru, Brazil, and Colombia before emptying into the Amazon River?

The Aras River forms part of the border between which countries? The Limpopo River forms part of the border between which countries? The Sava River forms part of the border between which countries? The Ubangi River forms part of the border between which countries? Which river flows through Russia, Belarus, and Ukraine before emptying into the Black Sea? Which island nation is home to the famous waterfall, Gullfoss? Which famous rock formation is located in Uluru-Kata Tjuta National Park, Australia? What is the name of the largest hot spring in the United States, located in Yellowstone National Park?