When you look at the painting, you can see a man who is staring at the distance. We can see that he looks pensive - he is deeply in meditation, thinking about the flow of water, perhaps, as the name of the painting suggests. He looks melancholic and immersed in his thoughts, and is mulling over the forces of nature and its effects. Maybe he is thinking about life itself, and it passing by like water in a stream. The artist designed the image the way he did maybe to make us stop and think about life and nature ourselves, and not rush living our daily, perhaps meaningless lives.
This is my take on the painting, at least. :)
I’d say this is most likely oil paint
