Janmashtami is the sacred day when the birthless, deathless Supreme Consciousness took birth as a human child in the city of Mathura. In the Bhagavatam it says that at the time of the Lord’s birth, a bright light pervaded, clearing darkness in all directions. People’s minds became suddenly joyful, and plants and trees bloomed. The shackles on Vasudeva’s feet fell away, and the prison doors opened by themselves. The Yamuna River parted to make way for Vasudeva, who was carrying the infant Krishna.
There is a teaching in this: Whenever God incarnates, along with the external changes the light of knowledge will spread within us. When we, like Vasudeva, move forward, holding God close to our hearts, all obstacles on our path will fall away and we will attain our goal.
It is to be borne in mind that Yamuna was a smaller and shallower river then. Sarasvati was a much larger, flowing river then.
Tamas was then a tributary of the Sarasvati, before it turned east to join the Yamuna much later in time.
Even now in the Mathura Agra belt, Yamuna is a shallow river. This is noticeable before summer, when the water is lowest in the river. It is sometimes easy to ford the river then, on foot.
Picture Credit : Google