(a) List three processes of river erosion (b) Outline four characteristics of each of the following sections of a river valley: (i) upper course (ii) middle...

Answer Details

(a) Processes of river erosion:

1. Corrasion (abrasion): the wearing away of the bed and banks by the load (sand, pebbles) the river carries and hurls against them.
2. Attrition: the load particles knock against one another and are broken into smaller, rounder pieces.
3. Hydraulic action: the sheer force of the moving water loosens and drags away materials from the bed and banks. (Also corrosion/solution - the chemical dissolving of soluble rocks such as limestone.)

(b) Characteristics of each section of a river valley:

(i) Upper course:

• Steep gradient and fast, turbulent flow.
• Narrow, deep V-shaped valley with vertical (downward) erosion dominant.
• Features such as waterfalls, rapids, gorges and interlocking spurs.
• Coarse, angular load and a small, swift channel.

(ii) Middle course:

• Gentler gradient and increased volume from tributaries.
• Valley widens; lateral (sideways) erosion becomes important.
• Development of meanders and a wider floodplain begins.
• Both erosion and deposition occur; load becomes finer.

(iii) Lower course:

• Very gentle gradient and slow, wide flow with large volume.
• Broad, flat floodplain with deposition dominant.
• Features such as large meanders, ox-bow lakes, levees, deltas and estuaries.
• Fine load (silt and mud) is carried and deposited.