Correct Option
The correct option is (c).
Explanation
A waterfall is an erosional landform typically associated with the youthful stage of a river's course. It represents a sudden vertical drop in the longitudinal profile of a river, resulting from significant changes in gradient or bedrock resistance.
Statement-wise Analysis
- Statement 1 is Incorrect. Waterfalls do not form when a river flows through a flat plain. Flat plains are characteristic of the old or mature stage of a river, where the gradient is gentle, velocity is low, and depositional features (like floodplains and deltas) dominate. Waterfalls require a steep gradient or a sudden break in slope.
- Statement 2 is Correct. Waterfalls typically occur where a band of very hard (resistant) rock lies horizontally or vertically across the river channel. The river erodes the softer rock downstream faster than the hard rock (differential erosion), creating a ledge or precipice over which the water plunges.
- Statement 3 is Correct. Waterfalls can form down a steep valley side. This often occurs in glaciated regions where tributary streams flow from "hanging valleys" down into a deeper main valley, or along fault scarps where tectonic activity has created a steep vertical face.
Key Takeaway: Waterfalls are erosional features formed by the differential erosion of hard and soft rocks or by tectonic faulting, primarily occurring in the upper course (youth stage) of a river, not in flat depositional plains.