Write explanatory notes on any two of the following; (a) Flood plain (b) Hydrological cycle (c) River erosion

Explanatory notes (any two)

(a) Flood plain

A flood plain is the flat, low-lying land bordering a river on either side in its lower course, which is regularly covered with water when the river overflows its banks during floods. It is built up of fine sediment (alluvium) dropped by the river each time it floods, so it is very fertile. It is formed by the deposition of alluvium and the lateral (sideways) swinging of meanders, which widens the valley floor. Features found on it include levees, meanders, ox-bow lakes and marshes. Flood plains are valuable for farming (e.g. rice) and settlement. (Diagram: a river winding across a broad flat plain with levees and an ox-bow lake.)

(b) Hydrological cycle

The hydrological (water) cycle is the continuous circulation of water between the sea, the atmosphere and the land. Water evaporates from oceans, seas, rivers and lakes (and transpires from plants) to form water vapour. The vapour rises, cools and condenses to form clouds. The moisture falls back to the earth as precipitation (rain, snow, hail). Some water flows over the surface as run-off into rivers that return it to the sea, some sinks into the ground as infiltration/ground water, and the cycle repeats endlessly. The total amount of water on earth stays the same. (Diagram: sea, arrows for evaporation up, condensation to cloud, precipitation, run-off and infiltration back to sea.)

(c) River erosion

River erosion is the wearing away and removal of the bed and banks of a river channel by the moving water and its load. It takes place in four main ways: corrasion (abrasion) - the load rubs and grinds against the bed and banks; hydraulic action - the sheer force of moving water dislodges loose material; corrosion (solution) - the water dissolves soluble rocks such as limestone; and attrition - the load particles knock against one another and break into smaller pieces. Erosion may act vertically (deepening the valley, chiefly in the upper course) or laterally (widening it). It produces features such as V-shaped valleys, gorges, waterfalls, potholes and rapids. (Diagram: a V-shaped valley in the upper course with a waterfall.)