A river from source to mouth
A river is fresh water flowing in a channel from high land to low land.
- Source: where the river starts: a spring, a melting glacier, a lake or a marsh in the hills.
- Course: the path of the river. In the hills it is fast and cuts deep valleys. On the plains it is slow and wide and bends in loops (meanders).
- Mouth: where the river ends, in a sea, a lake or a bigger river. Some mouths build a delta of mud with many branches; others are a wide funnel called an estuary.
Facing downstream, the bank on your left is the left bank and the one on your right is the right bank.
River system, basin and divide
A tributary is a river that flows into a bigger river. The main river plus all its tributaries make a river system.
The whole area of land whose water ends up in one river is its river basin (also called the drainage basin or catchment). The high ground that separates two basins is the divide (watershed). Rain falling on one side of the divide goes to one river, rain on the other side goes to another.
River regime
A river's regime is how its flow changes through the year. Rivers fed by snow and glaciers are high in spring and early summer. Rivers fed by monsoon rain are high in the rainy season. Very high water can cause floods; very low water harms boats, fish and farms.
Why rivers matter
Rivers give drinking water, water for farms and factories, hydroelectric power, transport routes and homes for fish and birds. River valleys and wetlands are rich in wildlife, so many are protected.
Lakes and how they form
A lake is a body of still water in a hollow of the land. Lakes are named by how the hollow formed:
- Glacial lakes: ice sheets and glaciers scraped hollows, or left ridges of rock that block water. Common in northern Europe and high mountains.
- Tectonic (rift) lakes: the Earth's crust cracked and a block sank. Often long, narrow and very deep, like Lake Baikal.
- Volcanic (crater) lakes: water fills the crater of an old volcano.
- Oxbow lakes: a river loop is cut off from the river. U-shaped.
- Coastal lakes (lagoons): a sand bar cuts off part of the sea, like Chilika in India.
- Karst lakes: rain dissolves limestone and makes hollows.
- Reservoirs: human-made lakes behind a dam.
Lakes store water, calm floods, and give fish and recreation. Small, shallow lakes slowly fill with mud and plants and can become marshes.
Groundwater
Some rain soaks into the ground. It fills the tiny spaces between grains of soil and in cracked rock. This is groundwater. Rock that holds and lets through a lot of water (sand, gravel) is an aquifer.
The top of the soaked layer is the water table. It rises after rain and drops in dry times or when too many wells pump water out.
Where the water table meets the surface, water flows out as a spring. Springs feed rivers, so a river can keep flowing even when it has not rained for weeks. Some groundwater is warm or full of minerals (thermal and mineral springs).
Enclosed seas and pollution
Some seas, like the Baltic Sea and the Black Sea, are almost closed: they connect to the ocean only by a narrow strait. Many big rivers flow in, so the water is less salty (brackish) near the surface. Water changes very slowly, so whatever rivers bring in stays a long time.
Why such seas get degraded
- Fertiliser and sewage carry nitrogen and phosphorus. Algae grow fast (an algal bloom). When they die and rot, bacteria use up the oxygen. Large dead zones with almost no oxygen appear near the sea floor. This is called eutrophication.
- Oil spills, plastics and industrial waste.
- Overfishing and ships bringing in alien species.
Solutions: treating sewage, using less fertiliser near rivers, protecting wetlands that filter water, and countries in one basin working together.
Try it
- Crumple a sheet of paper, open it a little and put it on a tray. Spray water on it like rain. Watch the drops gather in the folds and join into "rivers". The ridges are divides!
- Fill a glass with sand, then pour water in slowly. Where is the water table? Push a straw down as a "well" and suck: the level drops.
- In the 3D free-play step, set rain very low and then very high. What happens to the river and the lake?
Key formulas and definitions
- Source → course → mouth
- Main river + tributaries = river system
- River basin = all land drained by one river; divide = high ground between basins
- Lake types: glacial, tectonic, volcanic, oxbow, coastal (lagoon), karst, reservoir
- Fertiliser → algal bloom → rotting → low oxygen (eutrophication)
Worked examples
1. Rain falls just on one side of a mountain ridge. Which river does it reach?
The river whose basin is on that side. The ridge is the divide between two basins.
2. A long, very deep lake lies in a valley between steep fault cliffs. What type is it?
A tectonic (rift) lake, formed where a block of the crust sank.
3. A U-shaped lake sits beside a slow river on a flat plain. How did it form?
It is an oxbow lake: the river cut through the neck of a meander and left the old loop behind.
4. Why can a river keep flowing after weeks without rain?
Groundwater keeps feeding it through springs and the river bed, and snow or glaciers may melt into it.
5. Why are almost-closed seas more likely to have dead zones than open oceans?
Their water is exchanged slowly, so nutrients from rivers build up, algae bloom, and rotting algae use up the oxygen.
Common mistakes
- Mixing up source and mouth. The source is where the river starts (high); the mouth is where it ends (low).
- Thinking a basin is only the river channel. A basin is the whole area of land that drains into the river.
- Calling every lake a reservoir. A reservoir is a lake made by people behind a dam.
- Thinking groundwater is an underground river. Mostly it fills tiny gaps in soil and rock.