River Landforms & Processes
From Source to Mouth
A river starts in upland country as a thin trickle and ends at the coast as a wide, slow channel. Along the way it carves valleys, rolls boulders, sorts sand and dumps silt. Geographers split this work into three jobs: erosion, transport, and deposition. Different processes dominate in different parts of the river’s course, and that is why the landforms change as you move downstream.
Rivers erode in four ways: abrasion (load scrapes the bed), attrition (stones knock together and round off), hydraulic action (water force compresses air in cracks) and solution (slightly acidic water dissolves rock).
Upper Course — Cutting Down
In the uplands the river is small but steep. Most of its energy is used cutting downwards, called vertical erosion. Hydraulic action and abrasion grind a narrow slot into the rock. Weathering on the valley sides loosens material that mass movement carries down to the river. The result is a steep-sided, narrow V-shaped valley. Examples include the upper River Tees in the North Pennines and the head of the Wye in mid-Wales.
Waterfalls form when a river crosses a layer of hard rock that lies on top of softer rock. The softer rock erodes faster, leaving the harder rock as a lip. Plunge pools form below as falling water drills into the softer layer through hydraulic action and abrasion. Over centuries the lip is undercut and collapses, so the waterfall retreats upstream. The narrow steep-sided valley left behind is called a gorge. High Force on the River Tees has retreated to leave a gorge about 700 metres long.
Middle Course — Bending Sideways
As gradient slackens, the river begins to use its energy sideways rather than downwards. This is called lateral erosion. Slight bends become exaggerated as faster water on the outside of each curve scours the bank away while slow water on the inside drops sediment. The river develops a string of meanders.
Each meander has a deep, fast outer channel cut into a river cliff, and a shallow, slow inner channel where a slip-off slope of sand and gravel builds up. Over time meanders migrate sideways and downstream, carving a wide, flat valley floor.
On a meander, the strongest flow swings to the outside of the bend, eroding a steep river cliff. On the inside, slow water drops material as a slip-off slope of sand and gravel.
Lower Course — Spreading Out
By the lower course the river is wide, slow and heavy with load. It often flows across a broad floodplain built up over thousands of years. When the river overtops its banks, water slows, drops its coarsest sand close to the channel and carries fine silt across the field beyond. Repeated floods raise low ridges of sediment alongside the channel called levees, while the floodplain itself becomes a thick layer of fertile alluvium.
As meanders grow, the necks between two bends get narrower and narrower. A flood eventually slices through the neck, leaving the old loop cut off as an oxbow lake. Sediment seals the ends and the oxbow gradually fills in to become a marshy crescent in the field. The River Severn near Shrewsbury and the lower Trent are dotted with old oxbows.
Transporting the Load
Whatever the course, every river carries material. There are four ways. Traction rolls heavy boulders along the bed. Saltation bounces pebbles in short hops. Suspension lifts fine clay and silt to give the water its brown colour. Solution dissolves chemicals such as calcium carbonate. The mix changes downstream: traction dominates upland streams; suspension dominates lowland rivers like the Trent.
Why Energy Controls Everything
A river only does work when it has energy. Energy comes from gradient (slope) and discharge (volume per second). High in the hills the slope is steep, so even a small flow has enough energy to roll boulders and grind potholes. In the lowlands the slope is gentle, so most of the energy is needed just to keep the water moving, and the river starts to drop its load. After heavy rainfall, discharge spikes and even a sluggish lowland river can briefly carry boulders. The relationship between energy and load explains why the same river can erode in winter and deposit in summer.
Erosion, transport and deposition do not happen separately — they pass material along like a relay. A boulder loosened in the upper course is broken into pebbles by attrition in the middle, suspended as silt in the lower, and finally dropped on the floodplain or carried out to sea. The landforms (V-shaped valley, meander, oxbow, floodplain) are simply the visible record of this continuous, energy-driven journey from upland source to coastal mouth.