narrowing

The shaft into which the river hurls itself is an immense chasm, lined by glistening coal-black rock, and narrowing into a creaming, boiling pit of incalculable depth, which brims over and shoots the stream onward over its jagged lip.

By deliberately narrowing the river channel, the embankments cleverly increased the velocity of the water, thereby helping to scour the riverbed clean of remaining sediment.