With these rivers aforesaid, and many others, too, as its tributaries, the Ister becomes the greatest river of all, while river for river the Nahr an- Nil [31.1,30.166] (river), AfricaNile surpasses it in volume, since that owes its volume of water to no tributary river or spring.

But the Ister is always the same height in summer and winter, the reason for which, I think, is this. In winter it is of its customary size, or only a little greater than is natural to it, for in that country in winter there is very little rain, but snow everywhere.

In the summer, the abundant snow that has fallen in winter melts and pours from all sides into the Ister; so this snow-melt pours into the river and helps to swell it and much violent rain besides, as the summer is the season of rain.

And in proportion as the sun draws to itself more water in summer than in winter, the water that commingles with the Ister is many times more abundant in summer than it is in winter; these opposites keep the balance true, so that the volume of the river appears always the same.