With appropriate diagrams, outline three conditions under which the following features can be formed; (a) river capture (b) waterfall

Conditions under which the following features are formed

(a) River capture

River capture (river piracy) occurs when a more powerful stream diverts and takes over the headwaters of a weaker, neighbouring stream. The conditions include:

• Two rivers flowing in adjacent valleys separated by a narrow watershed, where one river lies at a lower level than the other.
• One river being more powerful (with a steeper gradient and greater erosive power) so that it can cut its valley faster, especially by headward (backward) erosion.
• Rapid headward erosion by the more powerful stream until it cuts back through the watershed and reaches the valley of the weaker stream, diverting its upper course. The point of diversion is the elbow of capture, the beheaded river below it is the misfit stream, and the dry valley left behind is the wind gap.

(See diagram: two parallel rivers, the captor cutting back to the elbow of capture, with wind gap and misfit stream marked.)

(b) Waterfall

A waterfall is a sudden vertical fall of a river over a steep rock face along its course. It forms where:

• A band of hard (resistant) rock lies across the river's course over softer rock; the soft rock downstream is worn away faster, leaving a step over which the water falls.
• There is a sudden change of gradient or a fall in the level of the land, for example where a river crosses a fault line, a plateau edge or an escarpment.
• The river plunges over a steep cliff or edge, for example at the edge of a hanging valley, a coastal cliff, or where a hard rock ledge (like a lip) causes a vertical drop, e.g. Farin Ruwa or the Zambezi at Victoria Falls.

(See diagram: river crossing a hard rock band over softer rock, with the fall, plunge pool and undercutting shown.)