River Run

The Bradshaw Model: how rivers change downstream

This free game is part of our Cambridge iGCSE Geography (0460) course for exams in EXAM YEARS 2027-2029.

What is the Bradshaw Model?

The Bradshaw Model shows how the characteristics of a river change as you travel from its source to its mouth. It is one of the most important models in IGCSE Geography river studies and a favourite for fieldwork questions.

In River Run, you fly a survey drone to three sites: the upper course, middle course and lower course. At each one you predict what has changed, take measurements, and build up the Bradshaw Model yourself.

What increases and what decreases downstream?

• Increase: width, depth, discharge, channel efficiency, velocity (usually) and the cross-sectional area of the channel.
• Decrease: gradient (the slope gets gentler), load particle size and channel roughness. Load also becomes more rounded and better sorted.

Velocity is the one that surprises students. Even though the gradient gets gentler, velocity usually increases because the wider, deeper channel is more efficient, with less friction from the bed and banks.

Calculating discharge

Discharge = cross-sectional area × velocity, where cross-sectional area = width × depth. Discharge is measured in cumecs (cubic metres per second, m³/s).

Velocity can be measured in fieldwork with a float: velocity = distance ÷ time. For example, a float that travels 10 m in 6.7 seconds has a velocity of about 1.5 m/s. A common mistake is to add the measurements instead of multiplying them. Always multiply all three.

Load: attrition and abrasion

As rocks are carried downstream, attrition (rocks knocking into each other) and abrasion (rocks scraping the bed and banks) make them smaller and rounder. In fieldwork, the Powers Scale of Roundness is used to record how rounded the stones are.

Evaluating the model

The Bradshaw Model is a useful simplification, but real rivers do not always follow it. Dams, channelisation, tributaries, bands of hard rock and waterfalls can all break the pattern. In the exam, good evaluation explains why a real river might not match the model.