Up until recently I thought that the reason none of the ancient/medieval civilizations that controlled the Nile valley went upstream to discover the river's source was the number of cataracts (white currents) the river had up until the modern times.
But now I am learning that even the Romans reached the Sudd swamps (now in South Sudan) that proved to be so difficult to pass through that even the modern explorers had to resort to making land expeditions over the wast jungles of Central Africa in order to discover the source of Nile
My question is: why? I understand that swamps are obviously very difficult to pass through, but neither the Romans nor the 19-century explorers were exploring the Nile by moving on it's banks, right? They were exploring it on ships, going on water. So why did the swamps provide any difficulty exploring whatsoever?
EDIT: Found the answer to the question - see comments.
Size, disease and logistics. The Sudd is tens of thousands of square miles of muck and swamp that is difficult to navigate, especially if you don't know the best route besides "south". It's frequently a few inches of water on top of feet of muck. It's hard to get a boat through and harder to wade through.
The swamps were a haven for disease. Biting flies and mosquitos carried malaria and other disease which cut down porters and livestock. For each porter that got sick they needed to transport the porter's goods plus the porter on one less back.
That combines for logistical nightmares. It's hard to forage for food, it's hard to keep your expedition alive and able to carry the supplies you had, and even if you knew where to go finding a passable route. Then the deeper you travel guarantees that your return will be that much harder. The book "Explorers of the Nile" by Tim Jeal mentions the Roman expeditions before focusing on Victorian European expeditions. It will give you a healthy appreciation of how hard it was to travel and explore Africa even then.