Changing River Environments » River Characteristics and Drainage Basins

What you'll learn this session

Study time: 30 minutes

The key features of a drainage basin and river system
How to identify and explain river processes like erosion, transportation and deposition
The formation and characteristics of different river landforms
How to interpret drainage basin hydrographs
The factors affecting river discharge and flooding

Understanding River Systems and Drainage Basins

Rivers are dynamic systems that shape our landscapes and provide essential resources for human settlements. They form part of the water cycle, carrying precipitation from higher ground to the sea in a continuous journey.

Key Definitions:

Drainage Basin: The area of land drained by a river and its tributaries.
Watershed: The boundary between drainage basins (also called a divide).
Tributary: A smaller river or stream that flows into a larger river.
Confluence: The point where two rivers meet.
Source: The starting point of a river, often in upland areas.
Mouth: Where a river meets the sea, a lake, or another river.

🌍 Drainage Basin as a System

A drainage basin functions as an open system with:

Inputs: Precipitation (rain, snow, hail)
Processes: Interception, infiltration, surface runoff, throughflow, groundwater flow
Outputs: Evaporation, transpiration, river discharge
Stores: Vegetation, soil, groundwater, lakes

💧 The Water Cycle in Drainage Basins

The water cycle (hydrological cycle) connects all parts of the drainage basin:

Precipitation falls onto the drainage basin
Water moves through various pathways (surface and subsurface)
Water is stored temporarily in different locations
Water eventually leaves the system via the river mouth or evapotranspiration

River Processes and Characteristics

Rivers change their characteristics as they flow from source to mouth. These changes are driven by three main processes:

🚧 Erosion

The wearing away of the river bed and banks:

Hydraulic action: Force of water against banks
Abrasion: Scraping by sediment
Attrition: Sediment colliding and breaking down
Solution: Dissolving of soluble rocks

🚜 Transportation

The movement of eroded material:

Traction: Rolling of large particles
Saltation: Bouncing of pebbles
Suspension: Carrying of fine particles
Solution: Dissolved materials

🏡 Deposition

The dropping of material when energy decreases:

Occurs when velocity slows down
Heavier materials deposited first
Common at river bends, mouths and after floods
Creates distinctive landforms

River Course and Profile

A river's journey from source to mouth can be divided into three main sections, each with distinctive features:

The Changing River Course

⛰ Upper Course

Characteristics:

Steep gradient
V-shaped valleys
Fast-flowing, turbulent water
Vertical erosion dominant
Features: waterfalls, rapids, gorges

💑 Middle Course

Characteristics:

Gentler gradient
Wider, flatter valley floor
Mix of erosion and deposition
Lateral erosion becomes important
Features: meanders, oxbow lakes

🌊 Lower Course

Characteristics:

Very gentle gradient
Wide, flat floodplain
Slow-flowing water
Deposition dominant
Features: floodplains, levees, deltas

River Landforms

Rivers create distinctive landforms through the processes of erosion, transportation and deposition. Understanding how these landforms develop is crucial for geographical analysis.

💧 Erosional Landforms

Waterfalls and Gorges

Form when a river flows over bands of hard and soft rock. The process:

River erodes soft rock faster than hard rock
Creates a step in the river profile
Water falls over the step, creating a plunge pool
Undercutting causes the hard rock to collapse
Waterfall retreats upstream, forming a gorge

V-Shaped Valleys

Form in the upper course through vertical erosion, creating steep valley sides that are further shaped by weathering and mass movement.

📝 Depositional Landforms

Floodplains and Levees

Floodplains form when rivers flood and deposit sediment across the valley floor. Levees are raised banks formed when the heaviest material is deposited closest to the river channel during floods.

Deltas

Form at river mouths when the river deposits its load as it enters the sea or lake. Types include:

Arcuate (fan-shaped) - like the Nile
Bird's foot - like the Mississippi
Estuarine - where tides rework deposits

Meanders and Oxbow Lakes

Meanders are bends in a river that develop in the middle and lower courses. They form through a combination of erosion and deposition:

Faster flow on the outside of a bend causes erosion (creating a river cliff)
Slower flow on the inside leads to deposition (creating a slip-off slope)
The meander neck narrows over time
During a flood, the river may cut through the narrow neck
The abandoned meander becomes an oxbow lake

Case Study Focus: River Severn, UK

The River Severn is the longest river in Great Britain (354km) and shows typical features of a river system:

Upper Course: Begins in the Welsh mountains with steep gradients, waterfalls and V-shaped valleys
Middle Course: Flows through Shropshire and Worcestershire with well-developed meanders
Lower Course: Wide floodplain and estuary with extensive mudflats
Flooding: Regular flooding affects towns like Shrewsbury and Gloucester
Management: Various flood defences have been constructed, including embankments, flood walls and washlands

River Discharge and Hydrographs

River discharge is the volume of water flowing through a river channel measured in cumecs (cubic metres per second). It is affected by various factors and can be visualised using a hydrograph.

Understanding Hydrographs

A hydrograph is a graph showing how a river's discharge changes over time in response to rainfall. Key features include:

Rising limb: Shows increasing discharge after rainfall
Peak discharge: Maximum flow
Falling limb: Shows decreasing discharge as water drains away
Lag time: Time between peak rainfall and peak discharge
Base flow: Normal river level from groundwater

Factors affecting the shape of a hydrograph:

Physical factors: Drainage basin size and shape, rock type, soil type, vegetation cover, relief
Human factors: Urbanisation, deforestation, land use changes
Weather factors: Intensity and duration of rainfall, previous weather conditions

Quick Facts: Drainage Basin Characteristics

The world's largest drainage basin is the Amazon Basin (7 million km²)
Impermeable surfaces like concrete increase surface runoff and flood risk
Vegetation intercepts rainfall, slowing its journey to the river
Climate change is altering rainfall patterns, affecting river systems globally
Drainage density refers to the total length of streams per unit area

🧠 Test Your Knowledge!