Changing River Environments » Width, Depth, Velocity, Discharge, Wetted Perimeter and the River Channel

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. The shape of the channel and its wetted perimeter help explain why these changes happen, and why rivers look and behave differently as they flow downstream.

You met the upper, middle and lower courses in The River Journey: Long Profile, Source, Tributaries, Confluence and Mouth. This lesson looks at the measurable characteristics of the channel through those courses.

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).
• Wetted Perimeter: The length of the cross-sectional line that is "wet" or in contact with water in a river channel.
• Channel: The physical confine of a river or stream consisting of a bed and banks.
• Channel efficiency: How effectively a river channel can transport water.

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 abrasion, 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 because there is so little water the river itself stays shallow.

💦 More Water

As more tributaries join, the river carries a greater volume of water, so the channel needs to be deeper as well as wider.

💨 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.

Velocity: The Speed of Flow

Velocity generally increases from source to mouth, though it can vary significantly based on local conditions. Many students expect the steep upper course to be fastest, but the water there is often slow, at around 0.1-0.5 m/s, because so much of it is slowed by a rough, rocky channel.

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.

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 Formula

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

Width × depth gives the cross-sectional area (A), so this is often written as Q = A × V, where Q is discharge and V is velocity.

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

• Width gets greater
• Depth increases
• Velocity typically rises

Wetted Perimeter

The wetted perimeter is a crucial measurement in river studies. It tells us how much of the river channel is in contact with water at any given time. Imagine a cross-section of the river: the wetted perimeter is the length of the bed and both banks that lies under the water.

The River Channel: Form and Function

River channels are the pathways that water follows as it moves downstream. Every channel has a bed (the bottom) and two banks (the sides). Their shape and characteristics change from source to mouth, affecting how the river behaves.

Channel Characteristics

River channels have several key features that geographers study:

🛠 Channel Shape

Channels can be:

• V-shaped - common in upper courses
• U-shaped - found in middle courses
• Rectangular - typical in lower courses
• Braided - multiple channels separated by small islands
• Meandering - winding paths with distinct bends

🌊 Channel Efficiency

Efficiency depends on:

• Hydraulic radius - higher values mean greater efficiency
• Roughness - smoother channels are more efficient
• Gradient - steeper slopes increase velocity
• Wetted perimeter - affects friction and flow speed

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 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

How These Variables Work Together

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

For example, deforestation in a drainage basin can lead to increased runoff, which may cause channel widening and a larger wetted perimeter. This in turn can alter erosion patterns and flood risks downstream.

Fieldwork: Measuring a River Channel

For your fieldwork, you might measure these characteristics in a local river. Take measurements at several sites moving downstream so you can compare them. Here's how:

• Wetted perimeter: Lay a measuring tape or chain across the channel along the bed and banks, following the line where the water meets them.
• Channel cross-section: Plot your depth readings against distance across the river to draw the shape of the channel and work out its cross-sectional area.
• Discharge: Multiply the cross-sectional area by the average velocity.
• Hydraulic radius: Divide the cross-sectional area by the wetted perimeter to compare channel efficiency between sites.

With these measurements, you can calculate discharge and see for yourself how these characteristics change along a river's course! The pattern you would expect to find is summed up in The Bradshaw Model, which is the next lesson.

Summary

From source to mouth, rivers usually get wider and deeper, flow faster and carry a much greater discharge. Tributaries add water, lateral erosion widens the channel, and a smoother, more efficient channel with a relatively smaller wetted perimeter loses less energy to friction. Measuring these characteristics in the field lets you test these patterns for yourself.