6 Survey creation; you will see the power of heat reigning over all that is born and perishes. On account of it comes all the water spread over the earth, as well as that which is beyond our sight and is dispersed in the depths of the earth. On account of it are abundance of fountains, springs or wells, courses of rivers, both mountain torrents and ever flowing streams, for the storing of moisture in many and various reservoirs. From the East, from the winter solstice flows the Indus, the greatest river of the earth, according to geographers. From the middle of the East proceed the Bactrus, the Choaspes, and the Araxes, from which the Tanais detaches itself to fall into the Palus-Mæotis. Add to these the Phasis which descends from Mount Caucasus, and countless other rivers, which, from northern regions, flow into the Euxine Sea. From the warm countries of the West, from the foot of the Pyrenees, arise the Tartessus and the Ister, of which the one discharges itself into the sea beyond the Pillars and the other, after flowing through Europe, falls into Euxine Sea. Is there any need to enumerate those which the Ripæan mountains pour forth in the heart of Scythia, the Rhone, and so many other rivers, all navigable, which after having watered the countries of the western Gauls and of Celts and of the neighbouring barbarians, flow into the Western sea? And others from the higher regions of the South flow through Ethiopia, to discharge themselves some into our sea, others into inaccessible seas, the Ægon the Nyses, the Chremetes, and above all the Nile, which is not of the character of a river when, like a sea, it inundates Egypt. Thus the habitable part of our earth is surrounded by water, linked together by vast seas and irrigated by countless perennial rivers, thanks to the ineffable wisdom of Him Who ordered all to prevent this rival element to fire from being entirely destroyed.
However, a time will come, when all shall be consumed by fire; as Isaiah says of the God of the universe in these words, “That says to the deep, Be dry, and I will dry up your rivers.” Reject then the foolish wisdom of this world, and receive with me the more simple but infallible doctrine of truth.
Source: Nine Homilies of Hexaemeron (New Advent)