How is earth's surface likely to change when a river flows over a steep landscape? its channel becomes wider than deeper. its channel forms curvy loops. a narrow valley is formed. a lake is formed.

Well, first of all its called a waterfall. 

The water that flows over the steep landscape does not create too much water, unless its been flowing for a long time. Which in that case it creates a lake and makes the surface of Earth to become deeper and wider.
But I don't think your question was implying that it went on for a long time, so in that case, the answer is a narrow valley is formed.

Hope I helped! :)

Ithink it takes sixty votes to approve a supreme court justice