What keeps Salva moving forward, even as he grieves Marial’s and Uncle’s deaths?

Devastated by the deaths of his uncle and Marial, Salva feels alone and without hope. He is determined to move forward, however, because that is what his Uncle Jewiir wanted. His determination to survive gives him the strength to remember his uncle's words of encouragement, that he should set small goals and follow through..... to walk to the next clump of bushes, and then just walk to the next rock.... until he reaches his destination.

Where was Rome located? How did they use the river ?