Justice is like a still lake, and justice is like a surging river. How to understand it?

This is a classic legal proverb:

From the Jewish prophet Amos in the 8th century BC-Ah, Israel, be fair as a calm lake and just as a raging river.

It means: upholding justice is like a calm lake (in our words, a bowl of water is flat), and upholding justice is like a surging river.