⛰ Everyday picture: the bowl
Think of a bowl with a hole in the bottom. The rim is the watershed, the water pouring in from all sides is the drainage basin, and the plughole is the mouth.
Rivers and Drainage Basins » Rivers and Drainage Basins: Key Features
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 1.1.1
- The key features of a river and its drainage basin: source, mouth, tributary, confluence, channel and watershed
- How to describe a river's channel: width, depth, velocity, wetted perimeter and discharge
- How to calculate discharge in cumecs
- What the long profile shows and how to name the three courses
One raindrop's journey
Imagine a single raindrop landing high on a mountain. It trickles into a tiny stream, joins a bigger stream, flows down a valley, passes under bridges and through towns, and finally reaches the sea. Every river on Earth works like this: it collects water from a large area of land and carries it downhill.
This lesson gives you the names for every part of that journey. You will use these words in almost every river question in the exam, so learn them well.
The drainage basin and its features
All rivers sit inside a drainage basin. Picture a giant bowl: rain falls anywhere inside the bowl and runs down towards the river at the bottom. The edge of the bowl is the high ground that separates one basin from the next.
Key terms:
- Drainage basin: the area of land drained by a river and its tributaries.
- Watershed: the edge of a drainage basin, the line of high ground (such as a ridge) that separates it from the next basin. Rain falling on one side flows into one river, and rain on the other side flows into a different one.
- Source: the place where a river begins. This is often a spring or a small stream high in the hills.
- Mouth: the place where a river ends, where it flows into a sea, a lake or another river.
- Tributary: a smaller river or stream that flows into a larger river.
- Confluence: the point where two rivers meet and join together.
- Channel: the path the river flows along, made up of the bed (bottom) and the banks (sides).
🌿 Everyday picture: the tree
Look at a tree from above. The twigs are streams, the branches are tributaries, the trunk is the main river and the place where two branches join is a confluence.
Describing a river channel
Geographers measure the channel to describe a river. There are five main characteristics. All five are measured at a point along the river, so the numbers are different in different places.
| Characteristic | What it means | Unit |
|---|---|---|
| Width | Distance from one bank to the other across the surface of the water | metres (m) |
| Depth | Distance from the water surface down to the river bed | metres (m) |
| Velocity (speed of flow) | How fast the water is moving | metres per second (m/s) |
| Wetted perimeter | The length of the bed and banks that are in contact with the water | metres (m) |
| Discharge | The volume of water passing a point every second | cubic metres per second (cumecs) |
Wetted perimeter is the one students find hardest. Imagine dipping your finger along the bed and up both banks, but only as far as the water level. The length your finger gets wet is the wetted perimeter. The more bed and bank the water touches, the bigger the wetted perimeter.
A cumec is one cubic metre of water passing a point every second. A river with a discharge of 6 cumecs moves six cubic metres of water past a point every second.
Calculating discharge
Learn this
Discharge (cumecs) = cross-sectional area (m2) x velocity (m/s)
The cross-sectional area is the area of water you would see if you sliced across the channel. For a simple channel, work it out by multiplying width by average depth.
Worked example (made-up figures)
A river is 6 m wide and has an average depth of 2 m. The water flows at 0.5 m/s.
1. Cross-sectional area = width x depth = 6 x 2 = 12 m2
2. Discharge = area x velocity = 12 x 0.5 = 6 cumecs
Always include the unit. If this channel were rectangular, its wetted perimeter would be 2 + 6 + 2 = 10 m (one bank, the bed, the other bank).
The long profile
The long profile of a river is a line showing how the height of the river's bed changes from its source to its mouth. It is drawn as a graph, with distance from the source along the bottom and height above sea level up the side.
The line is not straight. It is steep near the source and gets gentler as it goes, finishing almost flat at the mouth. This gives a smooth, concave curve, like the inside of a slide. To describe it, a river is split into three sections called courses:
- Upper course: near the source, high up in the hills. The long profile is steep here.
- Middle course: the middle section, where the slope is gentler.
- Lower course: near the mouth, where the land is low and the slope is very gentle.
If you are asked to sketch a long profile in the exam, draw and label the axes (distance from source in kilometres, height above sea level in metres), draw a curve falling steeply then gently, and label the source, the mouth and the three courses. How the channel itself changes along these courses is taught in the lesson on how rivers change downstream.
Common mistakes
- Mixing up the watershed and the drainage basin. The watershed is only the edge. The basin is the whole area inside it.
- Saying the mouth is just "the end of the river" with no detail. Say that it is where the river meets a sea, a lake or another river.
- Using width instead of depth in the discharge sum, or forgetting to multiply by velocity.
- Thinking wetted perimeter is the same as width. It includes the sides and bed, so it is always longer than the width.
Exam-style question
Fig. 1.1 shows measurements taken at one point on a river (made-up data).
| Measurement | Value |
|---|---|
| Width | 8 m |
| Average depth | 1.5 m |
| Velocity | 0.4 m/s |
(a) Define the term tributary. [1 mark]
(b) Calculate the discharge of the river at this point. Show your working and give the unit. [3 marks]
(c) Describe how the height of a river changes from its source to its mouth. [4 marks]
Model answer
(a) A smaller river or stream that flows into a larger river. (1)
(b) Cross-sectional area = 8 x 1.5 = 12 m2 (1). Discharge = area x velocity = 12 x 0.4 (1) = 4.8 cumecs (1).
(c) The river is highest at its source in the hills (1) and is lowest at its mouth (1). The fall is steep in the upper course (1) and becomes gentler through the middle and lower courses (1), so the long profile is a smooth curve.
Exam tip
For a calculation, write the formula, then the numbers, then the answer with its unit. You can earn marks for the method even if the final answer is wrong.
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