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The International Prester John Project: How A Global Legend Was Created Across Six Centuries

Mountains of the Moon

Brooks describes the antiquarian origin of this notion (p. 35):

Among the geographical features consistent between Ptolemy’s Geographia and the legend of Prester John was the idea of a snow-capped mountain range that served as the source of the Nile River. Ptolemy referred to these as the “Mountains of the Moon,” and this magical landform was a regular feature of many of the accounts of the kingdom of Prester John. In the following passage Ptolemy described the importance of these hypothetical mountains:
Around this bay the Aethiopian Anthropophagi dwell, and from these toward the west are the Mountains of the Moon, from which the lakes of the Nile receive snow water; they are located at the extreme limits of the Mountains of the Moon.

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George Abbot's Geography
Description of the Whole Worlde (1599)

A sober view of the geographical knowledge of time, George Abbot's Geography went through nine editions between its publication in 1599 and 1664.

Brooks summarizes (p. 154)
Abbot noted that visitors to the kingdom of Prester John would find the fabled Mountains of the Moon, which he considered to be the source of the Nile River. This belief is typical for the sixteenth and seventeenth centuries, and the search for the Mountains of the Moon would continue into the nineteenth century with the expedition of John Hanning Speke and Richard Burton to reach the southern shores of Lake Victoria. Abbot also claimed that Prester John was able to extort a sizeable tribute through innovative water management techniques; in the following passage Abbot briefly explained the supposed history of Prester John’s manipulation of the flow of the Nile River:
The Princes of Ægipt have paid vnto the gouernor of the Abisines, a great tribute time out of mind: which of late, the great Turke supposing to be a custome needelesse, did deny: till the people of the Abisines by commandment
of their Prince did breake downe their dammes: and drowning Egipt, did intórce the Turke to continue his pay,
and to giue much money for the new making of them very earnestily, to his great charge, desiring a peace.
The works of Abbot and his late sixteenth century contemporaries in the field of geography continued to influence Europeans into the seventeenth century and beyond. Despite the growing body of knowledge from European explorers, merchants, and colonial officials, there continued to exist in the minds of many Europeans a distinct fascination with the fantastic kingdom of a distant Christian priest-king, and even the mounting lack of direct evidence related to Prester John’s kingdom could shake this entrenched belief.

Brooks discusses how Abbot defamiliarizes Prester John and his kingdom from Catholicism (p. 153):

Abbot described Prester John as a “a prince absolute,” and that he also had “a priestlike, or patriarchall functió, & iurisdiction among [the Abyssinians].” In Abbot’s estimation, Prester John was a “verie mightie prince, & reputed to be one of the greatest Emperors of the world.” Ever keen to understand the true religious persuasion of his subjects, the Anglican cleric assured readers that Prester John “in no sorte acknowledge[ed] any supreame prerogatiue of the B. of Rome.” This was bit of information was likely quite important to English readers, given their own recent history of estrangement with the Vatican. (153-154)

In other words, Abbot, the future Archbishop of Canterbury, capitalizes on the long history of failures among Catholics (from missionaries to Popes) to convert Prester John to Catholicism. In this way he is able to skew Prester John's reputation for iconoclasm as a means to frame Prester John as an ally to England and to Anglicanism.

Read the text.
Nile River Part of the European fascination with the Nile River comes from its reputation as one of the four biblical rivers mentioned in Genesis. Along with the Euphrates, Hiddekel (Tigris), P(h)ison (Ganges), there was the Gehon, which due its biblical association with Ethiopia (Genesis 2:13), came to be understood as the Nile (Hamilton, p. 239).
And the name of the second river is Gehon: the same is it that compasseth all the land of Ethiopia.
(Douay-Rheims)
Over the medieval period, the Nile gathered a nearly mythical reputation. It's headwaters were located at the so-called Mountains of the Moon.
And, of course, there was the notion that the Ethiopian negus, often associated with Prester John, could control the flow of the river, with the implied or explicit threat of diverting water from Egypt or flooding the country.

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.
New, Plaine, & Exact Mapp of Africa
Peter Heylyn's 1652 Cosmographie included a world map by Nicolas Visscher that depicted the kingdom of Prester John. Despite the fact that the Dutch cartographer came from a family of mapmakers possessed of the most contemporary cartographic information, Visscher relied heavily on classical and biblical traditions in his mapmaking, which can be see in his depiction of Prester John's kingdom.

That is not to say that his map of Africa is completely inaccurate. The shape of Africa was one of the most accurate for its time and the tropics and equator were fixed precisely. Along the border of the map, Visscher depicted various African rulers, including Prester John, depicted as a youngish crowned black man and titled "King of Abissines." As Brooks notes (p. 209), the ornamentation of Prester John is compoaratively more European than that of the other African kings depicted on the map.

Prester John's Abyssinia takes up roughly a full third of Africa, hearkening back to the tradiition established by Fra Mauro in his mappaemundi.

From Brooks (p. 209):
It is also within the imagined boundaries of the kingdom of Prester John that Visscher included quite a vareity of mythical and legendary items beleived by late medieval and early modern Europeans to exist in Africa. Visscher included a "Zair Lake" from which the Congo and Nile Rivers supposedly emanated, which was a body of water that Visscher claimed was "where ye Tritons and Mermaids are said to be."

The southern borders of the laand of Prester John are the location of the Mountains of the Moon, also believed by Europeans to be the source of the Nile River.
Explore “New, Plaine, & Exact Mapp of Africa”

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Michael Brooks