Pick up, carry, drop: erosion, transport and deposition

Every river does three jobs: it erodes (picks up material), transports (carries it) and deposits (drops it). Which job it is doing at a given moment depends mostly on two things: how fast the water moves and how big the grains are.

How a river erodes. Rivers wear away their beds and banks in four main ways.

• Hydraulic action: the force of moving water pushes into cracks and loosens material.
• Abrasion: rocks carried by the river scrape and grind the bed and banks, like sandpaper.
• Attrition: the carried stones knock against each other and break and round off.
• Solution: soluble rock, such as limestone, dissolves in the water.

Attrition is why pebbles in the lower river are smaller and rounder than the boulders upstream. Geographers call this downstream fining.

How a river transports. Four ways to carry material, from smallest to largest grains:

• Solution: dissolved ions that you cannot see.
• Suspension: fine silt and clay carried within the water, which makes it look muddy.
• Saltation: sand grains that bounce along the bed in short hops.
• Traction: large pebbles and boulders that roll or slide along the bottom, mostly in floods.

A flood can move boulders that a gentle stream could never shift. Many of the biggest changes to a river happen in just a few high-flow days.

Speed and grain size: the Hjulström curve. In 1935 the Swedish geographer Filip Hjulström plotted how fast water must flow to start moving grains of each size, and how slowly it must flow before grains settle out. His diagram is still used to teach the idea. It is a schematic (a rough guide, since real beds differ), but three messages are solid:

• Bigger grains need faster water to be picked up. Gravel and boulders need strong flows.
• Sand is the easiest to pick up. It needs only modest flow, a few tenths of a metre per second in the classic curve.
• Clay is the surprise. Clay and fine silt are tiny, but their particles stick together, so it takes a surprisingly fast flow to pick them up. Once they are in the water, however, they settle only when the water is almost still, so muddy water can stay muddy for a long way.

When flow slows down, the biggest grains drop first and the smallest last. The result is sorting: coarse gravel near the slowdown, sand beyond it, mud furthest away.