Rivers: processes, landforms, opportunities and hazards

How rivers shape the land

A river begins as a small stream in the hills and ends at the sea. Along the way it erodes, carries and drops material, and builds landforms such as waterfalls and meanders. This lesson covers the drainage basin, river processes, landforms, and why people live near rivers despite the danger.

The drainage basin

A drainage basin is the area of land drained by a river and its tributaries. Its edge is the watershed. Inputs: precipitation. Stores: interception by plants, surface water, soil water and groundwater. Flows: surface run-off, infiltration, throughflow and groundwater flow. Outputs: evaporation, transpiration and river discharge.

River discharge

Discharge is the volume of water passing a point in a river each second. It rises with heavy rain, impermeable rock, steep slopes, and urban areas, where water runs off quickly. Deforestation increases run-off, so discharge rises faster. A large reservoir near the source stores water, so discharge downstream is lower and steadier.

Erosion

Hydraulic action: the force of water breaks rock apart. Abrasion: stones carried by the river scrape the bed and banks. Attrition: stones knock together and become smaller and rounder. Solution: some rocks, such as limestone, dissolve in the water. Vertical erosion deepens the channel, mainly in the upper course; lateral erosion widens it, mainly lower down.

Transport and deposition

Traction: large boulders roll along the bed. Saltation: pebbles bounce. Suspension: fine material is carried in the water. Solution: dissolved material. Deposition happens when the river slows down and loses energy, such as on the inside of a bend or at the mouth. The largest material is dropped first, and the finest last.

How a waterfall forms

A waterfall forms where hard rock lies over softer rock. The softer rock erodes faster, so the hard rock is undercut and left overhanging. Eventually the overhang collapses, and the waterfall moves back upstream, leaving a steep gorge. A plunge pool forms at the foot, where falling water erodes the bed.

Other landforms

Upper course: steep V-shaped valleys and interlocking spurs, formed by vertical erosion. Middle course: meanders, with erosion on the outside of bends and deposition on the inside; ox-bow lakes form when a meander is cut off. Lower course: flood plains, levees, and deltas where the river meets the sea and deposits sediment.

Opportunities

Fertile flood plains for farming, as floods leave rich silt. Water for drinking, irrigation and industry; transport routes; and fish. Flat land for building, and hydro-electric power from dams.

Flooding: the hazard

Physical causes: heavy or long rainfall, snowmelt, steep slopes, and impermeable ground. Human causes: deforestation and building on flood plains, which increase run-off. Effects: deaths, damage to homes and crops, disease, and disrupted transport. Responses: dams, levees, flood warnings, and land-use planning.

Mistakes that lose marks

Mistake one: confusing attrition with abrasion. Mistake two: explaining a waterfall without the hard and soft rock, or without a labelled diagram when asked. Mistake three: describing floods without causes and effects. The fix: the process name, the sequence, and a labelled diagram.

The full-marks tip

The full-marks tip: explain a landform as a sequence of processes, and draw and label a diagram when asked. For discharge, explain how each factor changes the speed water reaches the river.

On one card

So, on one card: rivers. Keep it for revision.