The Bradshaw Model: How Rivers Change Downstream

📊 The Bradshaw Model: How Rivers Change Downstream

The Bradshaw Model summarises all the ways in which a river's characteristics change from source to mouth. It is one of the most important conceptual models in GCSE Physical Geography and is frequently tested with graph interpretation questions.

Variable	Change Downstream	Reason
Discharge	Increases	Tributaries add more water to the main channel with each junction
Channel width	Increases	More water = greater force pushing outward on banks; lateral erosion widens channel
Channel depth	Increases	Greater discharge carves deeper channel; less bedload to obstruct flow
Velocity	Increases	Smoother bed = less friction per unit volume of water; deep channel is more efficient
Gradient (slope)	Decreases	River approaches base level (sea level); long profile flattens out
Sediment size	Decreases	Attrition progressively breaks particles down; finer tributaries add smaller load
Sediment roundness	Increases	Attrition progressively smooths and rounds particles
Channel roughness	Decreases	Smaller, rounder particles create a smoother, less obstructed channel bed
Load quantity	Increases	More tributaries add more material; capacity increases with discharge

The Velocity Paradox — A Common Exam Trick

Common misconception: "Upper course rivers are faster than lower course rivers because the gradient is steeper." This is wrong.

In reality, lower course rivers typically flow faster than upper course rivers. Here is why: in the upper course, the channel is rough, shallow, and narrow — boulders on the bed create enormous friction, slowing the water despite the steep gradient. In the lower course, the channel is deep, wide, and has a smooth bed of fine silt — friction per unit volume of water is much lower. This is measured by the hydraulic radius (cross-sectional area divided by wetted perimeter): the larger the hydraulic radius, the more efficient the channel and the faster the flow for a given gradient.

Think of it this way: running through deep water on a sandy beach is very slow. Running through deep, clear water in a smooth swimming pool would be much faster. The lower course is more like the pool.