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.
Cataracts were built thousands of years ago on the river.  
mnli hypothesis: Cataracts were built thousands of years ago on the river.   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.