Changing River Environments » Width, Depth, Speed of Flow and Discharge

What you'll learn this session

Study time: 30 minutes

How river width changes from source to mouth
Why river depth increases downstream
How and why velocity (speed of flow) varies along a river's course
What discharge means and how it changes downstream
The relationship between these variables and river processes
Real-world examples of these changes in river systems

Introduction to Changing River Characteristics

Rivers are dynamic systems that change dramatically from their source (where they begin) to their mouth (where they enter the sea or a lake). Four key characteristics that change along a river's course are width, depth, velocity (speed of flow) and discharge. These changes help explain why rivers look and behave differently as they flow downstream.

Key Definitions:

Width: The distance from one bank of the river to the other.
Depth: The vertical distance from the water surface to the river bed.
Velocity: The speed at which water flows downstream, measured in metres per second (m/s).
Discharge: The volume of water flowing past a point in a given time, measured in cumecs (cubic metres per second).
Channel efficiency: How effectively a river channel can transport water.

💧 The River System

A river system has three main courses:

Upper course: Near the source, steep gradient, narrow and shallow
Middle course: Moderate gradient, wider and deeper
Lower course: Near the mouth, gentle gradient, widest and deepest

📊 Bradshaw Model

The Bradshaw Model shows how river characteristics change from source to mouth. It predicts that width, depth, velocity and discharge all increase downstream as the river collects more water from tributaries and rainfall.

Width: How and Why Rivers Get Wider

River width typically increases from source to mouth. At the source, rivers are often just a few metres wide, but by the time they reach their mouth, they can be hundreds or even thousands of metres across.

Why Rivers Widen Downstream

Several processes contribute to river widening:

🔃 Erosion

Lateral (sideways) erosion becomes more dominant in the middle and lower courses. The river erodes its banks through processes like hydraulic action and corrasion, making the channel wider.

🚰 Tributaries

As tributaries join the main river, they add more water, requiring a wider channel to accommodate the increased volume.

🔄 Meanders

In the middle and lower courses, rivers develop meanders (bends). The outer bend erodes while the inner bend receives deposits, gradually widening the overall river valley.

Depth: The Vertical Dimension

River depth generally increases from source to mouth. In the upper course, rivers might be just centimetres deep, while in the lower course they can be several metres deep.

Factors Affecting River Depth

River depth is influenced by:

📍 Vertical Erosion

In the upper course, vertical erosion dominates as the river cuts downward into the riverbed. This creates V-shaped valleys but doesn't significantly deepen the water itself.

💨 Reduced Friction

As rivers flow downstream, the channel becomes more efficient. Smoother beds and banks reduce friction, allowing water to pile up rather than spread out, increasing depth.

Measuring River Depth

Geographers measure river depth using a measuring rod in shallow sections or echo sounding in deeper parts. They typically take measurements at several points across the channel to create a cross-section profile. The average depth can be calculated by dividing the cross-sectional area by the width.

Velocity: The Speed of Flow

Velocity generally increases from source to mouth, though it can vary significantly based on local conditions. Typical velocities range from 0.1-0.5 m/s in the upper course to 3-5 m/s in the lower course during normal flow.

What Affects River Velocity?

📈 Gradient

Steeper slopes in the upper course should create faster flow, but this is often counteracted by greater friction from the rough channel.

🛠 Friction

Rough, rocky beds in the upper course create friction that slows water down. Smoother channels downstream reduce friction, increasing velocity.

📏 Channel Shape

Deeper, more efficient channels in the lower course have less water in contact with the bed and banks, reducing friction and increasing velocity.

Case Study: River Severn Velocity

The River Severn in the UK shows clear velocity changes. Near its source in the Welsh mountains, average velocity is around 0.5 m/s. By Shrewsbury in the middle course, it increases to about 1.2 m/s. Near Gloucester in the lower course, normal velocity reaches 2.5 m/s. During floods, velocities can double or triple these values, causing significant erosion and flooding risks.

Discharge: The Volume of Water

Discharge is perhaps the most dramatic change along a river's course. It typically increases from just a few litres per second at the source to thousands of cubic metres per second at the mouth.

Understanding Discharge

Discharge is calculated using the formula:

Discharge (m³/s) = Width (m) × Depth (m) × Velocity (m/s)

This explains why discharge increases downstream - all three components generally increase:

Width gets greater
Depth increases
Velocity typically rises

💦 Tributaries and Discharge

Each tributary that joins the main river adds its discharge to the total. The Amazon River, for example, has over 1,100 tributaries, which is why it has the world's largest discharge at around 209,000 cubic metres per second - enough to fill 84 Olympic swimming pools every second!

☔ Rainfall and Discharge

Discharge varies seasonally and after rainfall events. During heavy rain, discharge increases rapidly, sometimes leading to flooding. This is why geographers study river hydrographs, which show how discharge changes over time after rainfall.

How These Variables Work Together

Width, depth, velocity and discharge don't change in isolation - they're all connected. Understanding these connections helps explain why rivers behave as they do.

The Hydraulic Radius

An important concept that links these variables is the hydraulic radius - a measure of channel efficiency. It's calculated as:

Hydraulic Radius = Cross-sectional Area ÷ Wetted Perimeter

As rivers flow downstream:

The cross-sectional area increases (width × depth)
The wetted perimeter (the length of bed and banks in contact with water) increases, but not as quickly
This means the hydraulic radius increases, making the channel more efficient
More efficient channels have less friction, allowing faster flow

Case Study: The River Thames

The River Thames demonstrates these changes clearly. Near its source in the Cotswolds, it's just 2-3 metres wide and 30cm deep, with a discharge of about 1 cubic metre per second. By the time it reaches London, it's approximately 250 metres wide and 9 metres deep, with an average discharge of 65 cubic metres per second. During floods, this can increase to over 500 cubic metres per second. These changes reflect all the processes we've discussed - tributary inputs, changing channel efficiency and the balance between erosion and deposition.

Practical Investigation Skills

For your GCSE fieldwork, you might measure these variables in a local river. Here's how:

📏 Measuring Width

Use a tape measure stretched across the river from bank to bank. Take measurements at several sites moving downstream.

📏 Measuring Depth

Use a meter stick at regular intervals across the channel to create a cross-section. Calculate average depth from multiple readings.

🕐 Measuring Velocity

Time how long it takes a float to travel a set distance. Repeat several times and calculate the average. Remember that surface velocity is faster than average velocity (multiply by 0.85 for a correction).

With these measurements, you can calculate discharge and see for yourself how these variables change along a river's course!

🧠 Test Your Knowledge!