⛓️ Why Each Bradshaw Variable Changes: The Cause Chain
Understanding the reason for each change is essential for exam questions that ask you to "explain" or "suggest why". Here is the chain of causes that drives the model:
More tributaries join as you move downstream — every time a smaller stream (tributary) joins the main channel, it adds water to the flow. This means discharge (the volume of water passing a point per second, measured in cumecs — m³/s) consistently increases downstream.
Greater discharge produces a larger channel — the increased volume of water has more energy, which drives both lateral erosion (widening the channel) and vertical erosion (deepening it). Over time, the river carves a bigger cross-section.
A larger, smoother channel has less friction — in the upper course, the river flows over a rough, rocky bed with steep banks. In the lower course, the channel is wider and the bed is lined with finer, smoother sediment. The hydraulic radius (the ratio of water volume to channel contact) increases, meaning less energy is lost to friction per unit of water.
Less friction means greater velocity — this surprises many students, who assume rivers must slow down as they get bigger. In fact, because efficiency increases faster than size, downstream rivers are typically faster than their upper-course counterparts despite having a gentler gradient.
More energy means more erosion and transport of sediment — the increased velocity and discharge give the lower course more energy to transport and rework sediment, including the pebbles and particles carried from upstream.
Transport breaks down pebbles over distance — as pebbles travel hundreds of kilometres from upland sources, attrition progressively reduces their size and increases their roundness. By the lower course, the coarse angular boulders of the upper course have been worn down to small, smooth, rounded pebbles.
