On Streams and Rivers by Hilaire Belloc — Read

There is a pass called the Bon Agua, and also Bon Aigo, which leads from the heights of the Catalans to those other heights of Aragon, or as some would say of Bearn, for the pass is from the south of the mountains to the north; on the northern side one knows why it is called Bon Agua, because one sees many thousands of feet below one the little bracelet, the little chain, of the young Garonne.

Do not mistake me, there are two sources of the Garonne. That which is most famous does the most famous thing; for it rises on the far side of the mountains and it plunges into a pond, quite a little pond. Then it cascades underground, through dark passages of which no one knows anything, and comes out beyond the main chain of the hills to join its other quieter sister from the Bon Agua.