1.2 - River processes and downstream changes
Rivers erode, carry and deposit material while weathering and gravity reshape their valley sides. Learn why the River Tees changes downstream, and how to use profiles, geology and measurements without mistaking a general trend for an unbroken rule.
Erosion and valley-side change
Erosion removes material. River water exerts force on beds and banks, particularly where it enters cracks (hydraulic action). Carried stones scrape and strike surfaces (abrasion). Soluble minerals can dissolve in water (solution). Attrition occurs when transported particles collide, becoming smaller and rounder; it changes the load rather than directly cutting the bed.
Vertical erosion deepens a channel, while lateral erosion widens or shifts it. Both can occur in one reach. Steep upland channels often have much vertical erosion, whereas bend migration becomes especially important in wider lowland valleys. High energy alone does not determine the rate: resistant rock, fractures and the availability of abrasive sediment also matter.
Weathering breaks down rock where it lies. Water freezing in cracks can expand and help fragment rock; chemical reactions can dissolve or weaken susceptible minerals; roots can widen cracks. Mass movement transfers slope material downslope under gravity. Rockfalls, slides and slow soil creep supply sediment to channels. Weathering loosens material; gravity and river flow then move it. These are linked processes, not interchangeable words.
Transport and deposition
A river carries coarse stones by traction, rolling or sliding them along the bed. Smaller particles may bounce by saltation. Fine particles remain within the water as suspended load; dissolved minerals travel in solution. One river can use all four modes simultaneously. A particle's mode can change as flow conditions change.
A flow needs sufficient force to move a particle. Larger particles generally need stronger flows than smaller loose grains, although cohesive clay complicates a simple size rule. When velocity or flow energy falls, some load is deposited. The largest particles that can no longer be moved are often left first, while fine material may travel farther. Deposition can follow reduced discharge, entry into slower water, spreading over a floodplain, or a locally sheltered part of the channel.
Imagine a pebble resting on the bed at low flow. A storm increases discharge and it begins to roll. Farther downstream, the water spreads onto a floodplain and slows, so the pebble is deposited. The pebble has not become heavier: the transporting conditions changed.
Controls on river processes
Climate influences water supply, flood frequency and weathering. Temperature affects freeze-thaw opportunities and chemical reactions, while intense rain can supply water and sediment rapidly. Slope affects the downslope force on water and loose material. A steep slope can increase erosion potential, but a small headwater flow may still carry less total water than a large lowland river.
Geology controls rock resistance, jointing and permeability. Resistant beds can create steps; permeable rocks can store water and sustain flow between storms. Altitude changes temperature, precipitation and the likelihood of snow or ice. Aspect, the direction a slope faces, affects solar heating, evaporation, snowmelt and vegetation. In the northern hemisphere, a sunny south-facing slope may lose snow earlier than a shaded north-facing one. These effects depend on local conditions rather than working identically everywhere.
A geological map overlaid with a river's long profile lets you test a proposed explanation for a sudden steepening. If the step coincides with a resistant rock outcrop, differential erosion is plausible. A match supports the explanation; map scale, faults, glacial history and human structures may also need investigation.
The BGS Geology Viewer provides a real map to apply this method around High Force, Teesdale. Compare bedrock with superficial deposits and use the legend to identify the Whin Sill and neighbouring sedimentary rocks. Place the fall on the river profile before linking the local step to contrasting resistance. A coloured geological polygon locates a mapped unit; its colour alone does not measure erosion rate.
The River Tees from source to sea
The River Tees, northern England, rises in the North Pennines near Cross Fell, passes through Teesdale and reaches the North Sea east of Middlesbrough. Its upper course has high relief, steep sections and a relatively narrow valley. High Force is a local break in slope where resistant Whin Sill dolerite overlies less resistant sedimentary rocks. It shows why a named river's profile must be checked against geology rather than drawn as a perfectly smooth curve.
Downstream through the lower Tees valley, relief and the overall gradient decrease. The valley becomes broader and meanders are prominent, for example near Yarm. Tributaries add water, so discharge generally increases downstream. Channels generally become wider and deeper as they accommodate that greater flow. Lower reaches and the estuary have also been modified by people; the Tees is a real managed river, not a laboratory model.
| Characteristic | General downstream change on the Tees | Reason and qualification |
|---|---|---|
| Channel width and depth | Increase overall | More discharge usually requires a larger channel; local banks and structures interrupt the trend. |
| Valley cross profile | Narrower/steeper-sided upland valley to broader lowland valley | Side-slope processes and lateral migration enlarge the valley over time. |
| Long-profile gradient | Decreases overall | Falls such as High Force create local steps. |
| Mean velocity | Commonly increases overall | A larger, deeper channel can have less friction relative to its water volume, despite gentler slope. Rapids and tides create local variation. |
| Discharge | Generally increases | Tributaries and a larger contributing area add water; abstraction/storage modify it. |
| Sediment size and shape | Bed material tends to become smaller and rounder | Attrition and sorting act during transport; tributaries can add fresh coarse or angular material. |
Do not claim that every downstream observation must be faster or deeper than the previous one. Compare similar flow conditions and methods. A shallow riffle and deep pool at nearby sites can differ more than two reach averages.
Using profiles and discharge evidence
A long profile follows channel elevation from source to mouth. A channel cross profile cuts across the water and bed at one site; a valley cross profile includes the wider slopes. Confusing the last two can lead to a claim about an entire valley from a few channel depths.
Discharge in m³/s equals channel cross-sectional area in m² multiplied by mean velocity in m/s.
For an illustrative site, a width of 4 m and mean depth of 0.5 m give an approximate area of . If mean velocity is 0.8 m/s, discharge is . A single maximum depth would overestimate area if used as though the whole bed were that deep. A surface float is also not automatically the cross-section's mean velocity.
Use GIS to place observations on the named river, then compare height, geological and channel layers. Keep a record of date, flow conditions and measurement limitations. The calculation above is simulated practice, not a measured Tees discharge or a record of completed fieldwork.