They could not travel upriver because several sets of cataracts blocked passage and they did not know that the waters traveled from the heartland of Africa.
The presence of multiple cataract sets prevented them from travelling upriver. 
mnli hypothesis: The presence of multiple cataract sets prevented them from travelling upriver.  premise: They could not travel upriver because several sets of cataracts blocked passage and they did not know that the waters traveled from the heartland of Africa.