River Landscapes & Processes

From Source to Sea

Every river in the UK starts as rain falling on high ground, gathers into a channel, and flows downhill to the sea. Along the way it changes shape and behaviour. The upper course in the hills is steep and narrow; the middle course winds through gentler country; the lower course nears the sea is broad and flat. The journey of the River Severn or the Thames captures these changes in miniature — and the same processes shape every river worldwide.

Four Erosion Processes

A river wears away its bed and banks through four erosion processes. Hydraulic action: the sheer force of moving water pushes air into cracks, which eventually break apart. Abrasion (or corrasion): the river drags rocks and pebbles along, and these scour the bed and banks. Attrition: rocks crash into one another and break into smaller, rounder fragments. Solution (or corrosion): rivers flowing through limestone or chalk dissolve the rock chemically.

Transport and Deposition

A river carries its load in four ways. Traction: large rocks roll along the bed. Saltation: smaller pebbles bounce. Suspension: fine clay and silt are carried in the water column, giving rivers their muddy colour. Solution: dissolved minerals travel invisibly. Deposition happens when the river loses energy — on the inside of meander bends, on flat floodplains, or where the river enters a lake or the sea.

Upper Course Landforms

In the steep upper course, vertical erosion dominates. Rivers cut downwards into the bedrock, creating V-shaped valleys with interlocking spurs (the river weaves between projecting hills). Where a band of hard rock sits above softer rock, a waterfall forms: water drops over the resistant lip, eroding a plunge pool below; the soft rock beneath undercuts, and the lip eventually collapses, retreating upstream. High Force on the River Tees in Co. Durham is a classic example. As waterfalls retreat, they leave a steep-sided gorge.

Middle Course Landforms

In the middle course, gradient eases and lateral erosion takes over. The river starts to swing from side to side, forming meanders. On the outside of a bend, fast-flowing water erodes a steep river cliff. On the inside, slower water deposits sand and gravel as a slip-off slope. Over time the loops widen until adjacent meanders meet during a flood; the river takes the shorter route and abandons the loop. The cut-off, banana-shaped lake left behind is an ox-bow lake.

Lower Course Landforms

Near the sea, the river is wide, deep and slow. Floodplains form on either side: when the river floods, it spreads out, slows down and deposits silt, building up flat fertile land. Levees are natural or built-up banks of coarser sediment that the river drops at its edges during floods, slightly raising the channel above the surrounding plain. At the sea, where the river drops the rest of its load, a delta can form — a fan of sediment criss-crossed with channels (the Nile and the Mississippi are classic deltas).