(a) Describe the three stages of development of a river along its course. (b) Highlight two ways in which rivers are important to man.

Answer Details

(a) The three stages of development of a river along its course:

• Youthful (upper) stage: here the river is near its source in the highlands. The gradient is steep and the river flows swiftly, so it erodes chiefly vertically (downward). It carves a deep, narrow V-shaped valley with interlocking spurs, waterfalls, rapids, gorges and potholes. The channel is narrow and turbulent and the load is coarse.
• Mature (middle) stage: the gradient is gentler and the river has a larger volume from tributaries. Erosion is now both vertical and lateral (sideways), so the valley widens. The river begins to meander, forming river cliffs on the outer bends and slip-off slopes on the inner bends, and a wider valley floor develops.
• Old (lower) stage: near the mouth the gradient is very gentle and the river flows sluggishly over a broad, flat flood plain. Lateral erosion and deposition dominate: the river forms large meanders, ox-bow lakes, levees, and finally deposits its load to build a delta or estuary at the sea.

(b) Two ways in which rivers are important to man:

• Water supply and irrigation: rivers provide water for drinking, domestic use, industry and for irrigating farmland in dry areas.
• Hydro-electric power and transport: fast-flowing sections are dammed to generate electricity, while navigable stretches serve as cheap routeways for boats and for fishing. (Rivers are also important for fishing and fertile flood-plain soils.)