River Landscapes in the UK

The Drainage Basin System

A drainage basin is the area of land drained by a river and its tributaries. Key features:

• Watershed — the boundary (ridge of high land) separating one drainage basin from another
• Source — where a river begins (usually in upland areas, fed by springs or rainfall)
• Mouth — where the river enters the sea or a lake
• Tributary — a smaller river joining the main river
• Confluence — the point where two rivers meet
• Floodplain — the flat area of land either side of a river that floods periodically

The drainage basin is an open system with inputs (precipitation), stores (soil, vegetation, groundwater), transfers (infiltration, throughflow, groundwater flow, surface runoff), and outputs (evaporation, transpiration, river discharge into the sea).

Processes of Erosion

Process	Description
Hydraulic action	Force of water breaking rock particles from the river bed and banks
Abrasion	Rocks carried by the river scrape along the bed and banks, wearing them away
Attrition	Rocks and stones carried by the river collide, becoming smaller and rounder downstream
Solution	Slightly acidic river water dissolves soluble rocks (e.g. limestone, chalk)

Processes of Transportation

• Traction — large boulders rolled along the river bed
• Saltation — pebbles and gravel bounced along the bed
• Suspension — fine silt and clay carried within the water (makes rivers look brown)
• Solution — dissolved minerals carried invisibly in the water

Deposition occurs when the river loses energy — on the inside of meanders, on floodplains during floods, at the river mouth, and where the gradient decreases.

The Long Profile and Cross Profile

Long profile: A graph of the river's altitude from source to mouth. Typically concave — steep in the upper course, gradually flattening towards the mouth.

Cross profile: A slice across the river valley at any point:

• Upper course — V-shaped valley, narrow and shallow channel, large angular boulders
• Middle course — wider valley with floodplain developing, wider and deeper channel
• Lower course — very wide flat floodplain, wide and deep channel, fine sediment

River Landforms

Upper Course Landforms

V-shaped valleys and interlocking spurs:

• The river erodes vertically (downward) through hydraulic action and abrasion
• The valley sides are steepened by weathering and mass movement
• The river winds around ridges of hard rock that interlock like the teeth of a zip

Waterfalls and gorges:

1. River flows over a band of hard rock overlying soft rock

2. Soft rock erodes faster by hydraulic action and abrasion, creating an overhang of hard rock

3. A plunge pool forms at the base where the water erodes downward

4. The overhang becomes unsupported and collapses

5. The waterfall retreats upstream, leaving a steep-sided gorge

Example: High Force, River Tees — the Whin Sill (hard dolerite) overlies softer limestone and shale; a 21-metre waterfall with a gorge stretching 700 m downstream.

Middle Course Landforms

Meanders:

• The fastest flow (thalweg) swings to the outside of bends
• Erosion (abrasion and hydraulic action) on the outside creates a river cliff (steep, concave bank)
• Deposition on the inside (slower flow, less energy) creates a slip-off slope (gentle, convex bank with sand/gravel)
• Meanders gradually migrate downstream and sideways, widening the floodplain

Oxbow lakes:

1. The neck of a meander narrows as erosion continues on the outer banks

2. During a flood, the river breaks through the narrow neck, taking a straighter, shorter course

3. The old meander loop is cut off and sealed by deposition

4. The abandoned loop becomes an oxbow lake, which gradually silts up

Lower Course Landforms

Floodplains and levees:

• The floodplain is the wide, flat area either side of the river built up by repeated flooding
• When the river floods, it slows down as it spreads across the floodplain, depositing layers of fine alluvium (fertile silt)
• The coarsest, heaviest material is deposited immediately beside the river channel, building up natural raised banks called levees

Estuaries:

• Where the river meets the sea; the channel widens considerably
• Tidal influence creates mudflats and saltmarshes
• Fine sediment deposited in the calm tidal waters

Flooding: Causes and Management

Physical Causes

• Prolonged heavy rainfall saturating the ground
• Snowmelt in spring
• Impermeable rock (e.g. clay, granite) increases surface runoff
• Steep slopes accelerate flow into rivers

Human Causes

• Urbanisation — concrete and tarmac are impermeable; drains channel water quickly into rivers
• Deforestation — removes interception and reduces transpiration, increasing surface runoff
• Agriculture — ploughing compacts soil, reducing infiltration

Flood Management

Hard engineering:

• Dams and reservoirs — control river flow; store excess water (expensive but effective)
• Channel straightening — speeds up flow through a town but increases flood risk downstream
• Embankments (artificial levees) — raise the river banks to contain more water
• Flood relief channels — divert excess water around a settlement

Soft engineering:

• Floodplain zoning — restricting development on floodplains
• Afforestation — planting trees to intercept rainfall and increase infiltration
• River restoration — returning rivers to natural courses; allowing natural flooding
• Flood warnings — Environment Agency issues warnings so people can prepare