River Processes and Landforms » Discharge Variations

What you'll learn this session

Study time: 30 minutes

What discharge is and how it's measured
Factors that cause river discharge to change over time
How to read and interpret hydrographs
The difference between storm hydrographs and annual hydrographs
Human activities that affect river discharge patterns
Real-world examples of discharge variations in UK rivers

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Introduction to River Discharge

River discharge is one of the most important concepts in understanding how rivers behave. It tells us how much water is flowing past a point in a river at any given time. But discharge isn't constant - it changes throughout the day, across seasons and from year to year. Understanding these variations helps us predict flooding, manage water resources and protect communities.

Key Definitions:

Discharge: The volume of water flowing past a point in a river per second, measured in cubic metres per second (cumecs).
Hydrograph: A graph showing how river discharge changes over time.
Peak discharge: The highest point of discharge during a flood event.
Lag time: The time between peak rainfall and peak discharge.
Base flow: The normal, steady flow of a river fed by groundwater.

💧 Calculating Discharge

Discharge = Cross-sectional area × Velocity. This means a wider, deeper, or faster-flowing river will have higher discharge. Rivers naturally adjust their channel size to cope with typical discharge levels.

Factors Affecting Discharge Variations

River discharge doesn't stay the same - it's constantly changing due to various factors. Some changes happen quickly (like during a storm), whilst others occur gradually over months or years.

Natural Factors

Several natural factors influence how much water flows in a river and how quickly discharge changes:

🌧 Precipitation

Rainfall intensity, duration and type all affect discharge. Heavy rainfall causes rapid increases, whilst light drizzle has minimal impact. Snow creates delayed discharge when it melts.

🌲 Seasonality

UK rivers typically have higher discharge in winter due to increased rainfall and reduced evaporation. Summer discharge is usually lower as plants use more water and evaporation increases.

🌳 Temperature

Higher temperatures increase evaporation and plant water uptake, reducing discharge. Cold weather can freeze water, temporarily reducing flow until thaw occurs.

Drainage Basin Characteristics

The physical features of a drainage basin significantly influence how quickly discharge responds to rainfall:

Basin size: Larger basins take longer to respond to rainfall but can produce higher peak discharges
Basin shape: Circular basins respond more quickly than elongated ones
Relief: Steep slopes cause faster surface runoff and quicker discharge response
Rock type: Impermeable rocks like granite cause rapid runoff, whilst permeable rocks like limestone allow water to soak in
Soil type: Clay soils become waterlogged quickly, increasing surface runoff
Vegetation: Forests intercept rainfall and slow water movement, reducing peak discharge

Case Study Focus: River Severn Discharge Patterns

The River Severn, Britain's longest river, shows classic seasonal discharge variations. Winter discharge averages 107 cumecs at Bewdley, whilst summer discharge drops to just 29 cumecs. The river's large drainage basin (11,420 km²) means it responds slowly to rainfall, with lag times of 24-48 hours. Major floods in 2007 and 2014 saw discharge exceed 500 cumecs, demonstrating how extreme weather can dramatically alter normal patterns.

Understanding Hydrographs

Hydrographs are essential tools for understanding discharge variations. They show how river flow changes over time and help us predict flooding and manage water resources.

Storm Hydrographs

Storm hydrographs show how discharge changes during and after a rainfall event. They typically show a sharp rise to peak discharge, followed by a gradual decline back to base flow.

⚡ Key Features

Rising limb: The steep increase in discharge as rainwater reaches the river. Peak discharge: The highest point of flow. Falling limb: The gradual decrease back to normal levels. Lag time: Delay between peak rainfall and peak discharge.

Factors Affecting Hydrograph Shape

Different drainage basins produce different hydrograph shapes:

Flashy hydrographs: Steep rising limbs, high peaks, short lag times - typical of urban areas or impermeable rock
Subdued hydrographs: Gentle rising limbs, lower peaks, long lag times - typical of permeable rock or forested areas

Annual Hydrographs

Annual hydrographs show discharge patterns over a whole year, revealing seasonal variations. UK rivers typically show:

High discharge in winter months (December-February)
Declining discharge through spring as rainfall decreases
Low discharge in summer (July-August)
Gradual increase through autumn as rainfall returns

Human Impacts on Discharge

Human activities significantly alter natural discharge patterns, often increasing flood risk and changing seasonal flow patterns.

Urbanisation Effects

Cities dramatically change how water moves through drainage basins:

🏢 Impermeable Surfaces

Concrete and tarmac prevent water soaking into the ground, increasing surface runoff and creating flashier hydrographs with higher peak discharges.

🚧 Drainage Systems

Storm drains and sewers rapidly channel water to rivers, reducing lag time and increasing flood risk downstream.

🏭 Reduced Vegetation

Removing trees and grass reduces interception and transpiration, meaning more water reaches rivers more quickly.

Agricultural Impacts

Farming practices affect discharge in various ways:

Ploughing: Creates furrows that channel water rapidly to rivers
Drainage: Field drains remove water quickly, increasing discharge
Deforestation: Removing trees reduces interception and increases runoff
Irrigation: Can reduce river discharge by extracting water

Water Management

Humans deliberately modify discharge through various methods:

Reservoirs: Store water during high flow periods and release it during dry periods, evening out seasonal variations
Dams: Control discharge downstream and can prevent flooding
Channel modifications: Straightening and deepening rivers increases flow velocity
Flood defences: Levees and flood barriers contain high discharges

Case Study Focus: River Thames Flood Management

The Thames Barrier, completed in 1984, controls discharge and prevents flooding in London. During high tides or storm surges, the barrier closes to prevent seawater backing up the river. The Thames also has numerous weirs and locks that regulate flow. Despite these controls, climate change is increasing winter rainfall, leading to higher peak discharges. The 2014 floods saw the Thames reach its highest levels in 60 years, demonstrating the ongoing challenges of managing discharge variations.

Climate Change and Future Discharge Patterns

Climate change is altering precipitation patterns across the UK, creating new challenges for managing river discharge:

Increased winter rainfall: More intense storms leading to higher peak discharges
Drier summers: Reduced base flow and increased drought risk
Extreme events: More frequent flooding and drought cycles
Seasonal shifts: Earlier spring snowmelt and changed growing seasons

Monitoring and Prediction

Understanding discharge variations helps us prepare for floods and droughts:

Gauging stations: Continuously monitor river levels and discharge
Weather radar: Tracks rainfall patterns to predict discharge changes
Computer models: Simulate how discharge will respond to different rainfall scenarios
Flood warnings: Alert communities when dangerous discharge levels are expected

Conclusion

River discharge variations are natural and essential processes that shape our landscape and affect human activities. Understanding these patterns helps us manage water resources, predict floods and adapt to climate change. As weather patterns become more extreme, monitoring and managing discharge variations becomes increasingly important for protecting communities and maintaining water supplies.

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