River landscapes · GCSE Geography

The long and cross profiles

The long profile shows how a river's height changes from its source to its mouth: steep in the upper course and gentle in the lower course.
The cross profile changes too: a narrow, steep-sided V-shaped valley in the upper course, a wider valley with a floodplain in the middle course, and a very wide, flat floodplain in the lower course.
The river's discharge, width and depth all increase downstream.

Erosion: hydraulic action, abrasion, attrition and solution. Vertical erosion deepens the channel, mainly in the upper course. Lateral erosion widens it, mainly further downstream.
Transportation: traction, saltation, suspension and solution.
Deposition happens when the river loses energy, such as on the inside of bends, during floods and at the mouth.

Upper course: interlocking spurs, and waterfalls. A waterfall forms where hard rock lies over softer rock: the soft rock is eroded, undercutting the hard rock, which collapses. The waterfall retreats upstream, leaving a gorge. Example: High Force on the River Tees, where the river drops about 21 m over hard whinstone.
Middle course: meanders. Fast water on the outside of a bend erodes a river cliff; slow water on the inside deposits a slip-off slope. An ox-bow lake forms when the narrow neck of a meander is cut through, usually during a flood, and the old bend is sealed off by deposition.
Lower course: floodplains (flat land built from deposited silt), levees (raised banks, built up because floods drop their heaviest sediment next to the channel) and estuaries (tidal river mouths with mudflats and salt marsh).

Physical factors that increase flood risk: heavy or long-lasting rain, impermeable rock, saturated or frozen ground, and steep slopes.
Human factors: urbanisation (hard surfaces, and drains that carry water quickly to rivers), deforestation and some farming.
A flood hydrograph shows a river's discharge after a storm. The lag time is the gap between peak rainfall and peak discharge. A short lag time and a high peak mean a higher risk of flooding.

Hard engineering: dams and reservoirs (store water and may generate electricity, but are expensive and flood land), straightening the channel (water drains away faster, but flooding may increase downstream), embankments (raised banks) and flood relief channels (take away extra water).
Soft engineering: flood warnings and preparation, floodplain zoning (limiting building on the floodplain), planting trees and river restoration (returning a river to its natural course).
Example: Banbury (2012). An £18.5 million scheme built a 2.9 km earth embankment, creating a flood storage area that holds about 3 million m³ of water. It also added a pumping station, raised the main A361 road and created new wildlife habitats.

Long profile: A side view of a river's height from its source to its mouth.
Cross profile: A view across a river's valley and channel.
Discharge: The volume of water flowing past a point each second.
Meander: A bend in a river.
Ox-bow lake: A horseshoe-shaped lake left when a meander is cut off.
Levee: A raised bank along a river, built up by deposition during floods.
Floodplain: The flat land beside a river that floods.
Lag time: The time between peak rainfall and peak discharge.
Hydrograph: A graph showing a river's discharge over time.
Floodplain zoning: Controlling what can be built in different parts of a floodplain.