Rivers: Long Profile, Cross Profile & Processes
From Source to Mouth
Every river begins at a source — usually high ground where rainfall, snowmelt or a spring releases water. From there it flows downhill through a channel, joined by smaller streams (tributaries) at confluences, until it reaches its mouth at the sea or a lake. The total area of land drained by the river and its tributaries is called the drainage basin.
Geographers split the river into three sections: the upper course (close to the source, steep and narrow), the middle course (gentler gradient, wider valley) and the lower course (very gentle gradient, wide flat valley). The same river looks completely different in each section, because the dominant processes change as you move downstream.
The Long Profile
A long profile is a side-on diagram of the river from source to mouth. The line is steepest at the source (high gradient) and flattens out towards the mouth (low gradient), forming a smooth concave curve. The shape reflects how the river uses its energy: in the upper course energy fights friction with the rough bed and banks, so most goes into vertical erosion; further downstream the smoother channel needs less energy for friction, so the river can carry more water and a larger load with a much gentler slope.
The Cross Profile
The cross profile is the view you get by slicing straight across the valley. The shape changes dramatically downstream:
Upper course. Narrow, steep-sided V-shaped valley. The channel itself is narrow and shallow, with a rough bed of large angular boulders. Vertical erosion dominates because the river is cutting down towards sea level. Interlocking spurs jut from alternate sides of the valley.
Middle course. Wider valley with gentler slopes. The channel is broader and deeper. Lateral (sideways) erosion becomes more important as the river starts to swing across its valley. Meanders begin to form.
Lower course. Very wide, almost flat valley. The channel is wide and deep but the gradient is tiny. Deposition dominates — the river drops the load it has carried downstream. Wide flood plains and levees develop.
Upper course = vertical erosion (V-shaped valley, interlocking spurs). Middle course = lateral erosion (meanders). Lower course = deposition (flood plain, levees, estuary).
Erosion Processes
Rivers erode in the same four ways as the sea.
- Hydraulic action. Fast-flowing water hits the bed and banks, forcing air into cracks and prising them apart. Most powerful in upland streams during floods.
- Abrasion (corrasion). Sand and pebbles carried by the water scrape against the bed and banks, sandpapering them away. The dominant erosion process in most UK rivers.
- Attrition. The pebbles in the water knock against each other, becoming smaller, smoother and rounder. Attrition shapes the load, not the bed.
- Solution (corrosion). Slightly acidic river water dissolves alkaline rock such as limestone and chalk — slow but steady.
Erosion can also be split by direction. Vertical erosion deepens the channel and dominates in the upper course (helping form V-shaped valleys). Lateral erosion widens the channel by cutting sideways — dominant in the middle and lower courses, helping form meanders.
Transportation
A river carries its load in four ways:
- Traction. Large boulders and pebbles roll along the bed. Only happens during floods when the river has the most energy.
- Saltation. Smaller pebbles bounce along the bed in a series of hops, picked up briefly by the current and dropped again.
- Suspension. Fine sand, silt and clay are held up in the water column. This is what makes rivers look brown after heavy rain.
- Solution. Dissolved minerals, especially from limestone catchments, are carried invisibly in the water.
Deposition
Rivers deposit their load when they no longer have the energy to carry it. This happens when the river slows down — for example, when the gradient flattens, when the channel widens, when the river enters the sea, or after a flood when discharge drops. Larger material is dropped first (it needs more energy to carry); the finest particles travel furthest. This is why the upper course has angular boulders, the middle course rounded pebbles, and the lower course mud and silt.
Transport processes by particle size: traction (largest, rolled along the bed) → saltation (bounced) → suspension (held up in the water) → solution (dissolved). Bigger particles need more energy to move.
Discharge is the volume of water passing a point each second, measured in cubic metres per second (cumecs). It increases downstream as more tributaries join. The River Severn has a discharge of around 2 cumecs at its source on Plynlimon in mid-Wales but over 100 cumecs by the time it reaches Worcester — and over 600 cumecs during major floods. Higher discharge means more energy to erode and transport, which is why floods reshape rivers so dramatically.