'These rivers, my son, run, the eastern (like the Gangâ) toward the east, the western (like the Sindhu) toward the west. They go from sea to sea (i.e. the clouds lift up the water from the sea to the sky, and send it back as rain to the sea). They become indeed sea. And as those rivers, when they are in the sea, do not know, I am this or that river,

In plain words: The father points out that rivers like the Ganga flow east and the Sindhu (Indus) flows west. Their water rises from the sea as clouds, falls as rain, and returns to the sea. In the sea, the rivers no longer know 'I am this river' or 'that river.'