The Bradshaw Model: The Theory You Are Testing

🔍 The Bradshaw Model: The Theory You Are Testing

Before you go anywhere near a river, you need to understand the theory you are testing. The Bradshaw Model is a predictive model that describes how a river's characteristics change systematically as you move downstream — from the source (where the river begins, usually in upland areas) towards the mouth (where it meets the sea or a lake). It is one of the most important models in physical geography.

The model was developed by studying many rivers across different environments, and it predicts the following changes:

River Variable	Downstream Change	Why It Happens
Discharge	Increases	More tributaries join the main channel, adding extra water
Velocity	Increases	Greater discharge and a smoother, wider channel create less friction per unit of water — the river becomes more efficient
Channel width	Increases	Lateral (sideways) erosion by the river widens the channel over time
Channel depth	Increases	Vertical erosion (hydraulic action and abrasion) deepens the channel
Load size (bedload)	Decreases	Attrition — particles bash against each other as they travel and gradually break into smaller fragments
Load roundness	Increases	Attrition also smooths and rounds particles — sharp angular fragments become rounded pebbles over distance
Gradient	Decreases	The river erodes its bed towards base level (sea level), flattening its long profile over time
Wetted perimeter	Increases	Greater width and depth mean more of the channel is in contact with water
Hydraulic radius	Increases	The ratio of cross-sectional area to wetted perimeter increases — a more efficient channel shape

It is critical to understand that the Bradshaw Model is a generalisation, not a guarantee. Real rivers are messy. A tributary joining suddenly will spike discharge. A waterfall will slow velocity above it. A gorge will speed things up. Your fieldwork is testing whether your river broadly fits the model — and if any sites do not fit, you need to explain why using geographical reasoning. That is what separates a Level 3 answer from a Level 1 answer.