Nile River
And the name of the second river is Gehon: the same is it that compasseth all the land of Ethiopia.
(Douay-Rheims)
According to Adam Knobler, the association of the Ethiopian negus with the ability to control the Nile River dates back to the reign of the Fatimid sultan al-Mustansir Billah (r. 1036-1094). As Knobler notes (p. 204),
when a flood in Ethiopia failed to have any effect on the Nile. The sultan, in desperation, sent Patriarch Michael of Alexandria to Ethiopia to request a restoration of the river’s flow which, following the breaking of a mound, they did. From this point onward, the Ethiopians often claimed power over the Nile’s flow.
See the list of texts below that comment on Prester John's ability to control the flow of the Nile.