Which of the following conditions can produce a waterfall?

Analyze the question: The question asks which condition can produce a waterfall. A waterfall forms where a river drops vertically over a steep step in its course.

Explain the concept: Waterfalls form when a river flows over rocks of differing resistance, or where there is a sudden change in the gradient of the river bed. Common causes include:

• A hard rock layer lying over a softer layer. The soft rock erodes faster and leaves a step.
• A fault (a sharp break in the land), which makes one side higher than the other.
• Rejuvenation, where a sudden drop in base level creates a knickpoint.
• A river flowing over a plateau edge or escarpment, such as the Kyoto/Gurara Falls in Nigeria or the Erin-Ijesha (Olumirin) Waterfall.

Step-by-step evaluation:

• A. Sharp break in the bed of a river: This is a sudden change in gradient (a step or knickpoint). Water plunges over it, forming a waterfall. ✔
• B. Elbow of capture of young rivers: This is a sharp bend formed by river capture (piracy). It changes the river's direction, not its vertical profile, so it does not create a waterfall by itself. ✘
• C. Backward erosion: Also called headward erosion, it lengthens a river's source upstream. It helps a waterfall retreat upstream once one exists, and it is involved in river capture, but it is not the condition that produces the waterfall in the first place. ✘
• D. Lateral erosion: This widens the valley and creates meanders and floodplains. It is not linked to vertical drops. ✘

Answer: The direct condition that produces a waterfall is a sudden, sharp break in the river bed's gradient, which is option A.

Exam tip: Think "waterfall = sudden drop." Whenever you see "break," "step," "fault," or "hard rock over soft rock" in the options, that is usually the answer. Remember that backward (headward) erosion makes a waterfall retreat, but it does not create it.