What is the chasm? What does Four say the chasm reminds the Dauntless of? Why?

The chasm is a deep, break in the earth. At the bottom of the chasm is a river.

As we approach the railing, I hear a roar—water, fast-moving water, crashing against
rocks.
I look over the side. The floor drops off at a sharp angle, and several stories below us is a river. Gushing water strikes the wall beneath me and sprays upward. To my left, the water is calmer, but to my right, it is white, battling with rock.