What is a river? Parts of a river and its basin
A river is fresh water flowing in a channel, always downhill. It starts at the source (a spring, a lake, melting ice or rain on hills) and ends at the mouth, where it enters a sea, a lake or another river.
- Tributary: a smaller river that joins a bigger one.
- Confluence: the point where two rivers meet.
- Drainage basin: all the land that sends its water to one river.
- Watershed: the high edge that separates one basin from the next.
Lakes and inland waters
A lake is a large body of still water in a hollow. Many lakes are fed by an inflow river and drained by an outflow river, so the water is fresh. A lake with no outflow loses water only by evaporation, so salts build up and it becomes salty (for example the Caspian Sea or the Dead Sea). Rivers, lakes, groundwater, marshes and reservoirs together are a country's inland waters: we use them for drinking, farming, power, transport and fishing.
River processes: erosion, transportation, deposition
A river does three jobs.
Erosion (wearing away)
- Hydraulic action: the force of water pushes into cracks and breaks rock.
- Abrasion: stones carried by the river scrape the bed and banks like sandpaper.
- Attrition: stones knock into each other and become smaller and rounder.
- Solution: water dissolves some rocks, such as limestone.
Transportation (carrying)
Big boulders are rolled (traction), pebbles bounce (saltation), fine silt floats (suspension) and dissolved minerals are invisible (solution).
Deposition (dropping)
When the river slows down, it drops its load: heaviest first, finest last. This happens on the inside of bends, on floodplains and at the mouth.
Long profile and cross profile: the three courses
The long profile is the river seen from the side, source to mouth: steep at first, then gentler, then almost flat. The cross profile is a slice across the valley.
| Course | Slope and cross profile | Main process | Landforms |
|---|---|---|---|
| Upper | Steep; narrow V-shaped valley | Vertical erosion (cutting down) | V-valleys, interlocking spurs, waterfalls, gorges, rapids |
| Middle | Gentler; wider valley | Lateral erosion (sideways) and some deposition | Meanders, oxbow lakes |
| Lower | Almost flat; wide, flat valley | Deposition | Floodplains, levees, deltas, estuaries |
How a waterfall forms
Hard rock lies on top of soft rock. The soft rock wears away faster, the hard rock is undercut and an overhang forms. It collapses, and the waterfall moves back upstream, leaving a steep gorge.
How a meander and an oxbow lake form
On the outside of a bend the water is fastest, so it erodes a river cliff. On the inside it is slowest, so it deposits a slip-off slope (point bar). The bend grows. In a flood the river cuts across the narrow neck; the old loop is sealed off by silt and becomes an oxbow lake.
Floodplains, levees and deltas
When the river floods, it drops silt over the flat land: a fertile floodplain. The coarsest silt drops right next to the channel and builds natural banks called levees. At the mouth, if the sea is calm, silt piles up faster than waves remove it and builds a delta. The Yellow River (Huang He) carries so much yellow loess silt that it raises its own bed above the plain; the Yangtze is used for shipping, irrigation and hydro power (the Three Gorges Dam).
Floods, hydrographs and flood management
A flood happens when a river's water rises above its banks.
Causes
- Physical: very heavy or long rain, fast snowmelt, steep slopes, rock or soil that cannot soak up water (or is already soaked).
- Human: cutting trees (rain runs off faster), concrete and roads in towns, building on floodplains.
Flood hydrograph
A hydrograph is a graph of river discharge (how much water passes per second, in m³/s) against time, with rainfall shown as bars. Key words: peak rainfall, peak discharge, lag time (time between the two peaks), rising limb and falling limb. A short lag time and a high peak mean a "flashy" river that floods easily (towns, steep bare slopes). A long lag time and a low peak are safer (forests, gentle slopes, soaking soil).
Managing floods
- Hard engineering (built structures): dams and reservoirs, raised embankments (artificial levees), straightening channels, flood relief channels. Strong but costly and can move the problem downstream.
- Soft engineering (working with nature): planting trees, floodplain zoning (no houses on the riskiest land), letting some fields flood, warning systems. Cheaper and greener but needs space and time.
Example of a scheme: a town on a floodplain may combine a flood wall in the centre (hard) with tree planting upstream and a flood-warning app (soft). Judge any scheme by cost, how many people it protects and its effect on the environment.
Try it: build a mini river
Take a tray, fill one end with damp sand and raise that end on a book. Pour water slowly from a cup at the high end. Watch: a channel forms, sand is carried down, and a small fan (a delta) builds where the water spreads out. Now pour faster: the channel overflows (a flood). Then press a line of sand along the channel (levees) and try again.
Key formulas and definitions
- Discharge (m³/s) = cross-section area (m²) × velocity (m/s)
- Lag time = time of peak discharge − time of peak rainfall
- Upper course → vertical erosion; middle → lateral erosion; lower → deposition
Worked examples
1. A river channel is 10 m wide and 2 m deep, and the water flows at 1.5 m/s. Find the discharge.
Area = 10 × 2 = 20 m². Discharge = 20 × 1.5 = 30 m³/s.
2. Peak rainfall is at 06:00 and peak discharge at 18:00. What is the lag time, and is this river flashy?
Lag time = 18:00 − 06:00 = 12 hours. This is fairly long, so the river responds slowly; it is not very flashy.
3. Explain why a meander's outer bank is steep and its inner bank is gentle.
Water is fastest on the outside of the bend, so it erodes and undercuts the bank, making a steep river cliff. Water is slowest on the inside, so it deposits sand and gravel, making a gentle slip-off slope.
4. A town builds a concrete flood wall. Upstream, a village floods worse than before. Why?
The wall stops water spreading on the town's floodplain, so the water backs up and flows faster elsewhere. Hard engineering can move the flood problem to other places.
Common mistakes
- Thinking the mouth of a river is where it starts. The source is the start; the mouth is the end.
- Mixing up attrition (stones hitting each other) and abrasion (stones scraping the bed and banks).
- Saying deltas form at every river mouth. They need lots of silt and fairly calm sea; strong waves and tides wash silt away (an estuary forms instead).
- Reading lag time from start of rain to start of rising limb. It is from PEAK rainfall to PEAK discharge.