Characteristics of rivers and drainage basins
Every river collects water from an area of land around it.
That area is its drainage basin.
Rain that falls anywhere inside the basin will, sooner or later, end up in that river unless it evaporates first.
Learning the names of the parts of a basin and of a river channel is the first step in this topic, because every later question uses them.
The parts of a drainage basin
The edge of a drainage basin is the watershed.
It is usually a line of high ground, such as a ridge or a line of hilltops.
Rain falling on one side of the watershed drains into one river; rain falling on the other side drains into a different river.
The source is where the river begins.
It may be a spring, a marshy area on a hillside, a lake or the melting end of a glacier.
The mouth is where the river ends, flowing into the sea, a lake or a larger river.
A tributary is a smaller river or stream that flows into a larger one.
The point where two rivers meet is a confluence.
A large basin contains many tributaries, so it is made of many smaller basins nested inside it.
The river channel
The channel is the trough the water flows in: its floor is the bed and its sides are the banks.
You will measure a channel in fieldwork and read measurements from tables and graphs in the exam.
- Width: the distance across the water surface, from one bank to the other.
- Depth: the vertical distance from the water surface down to the bed. It changes across the channel, so you take several readings and work out the mean depth.
- Wetted perimeter: the total length of bed and banks that the water touches in one cross-section. Lay a chain or tape along the bed from one water's edge to the other to measure it. It is always longer than the width.
- Cross-sectional area: the area of the water in a slice across the channel. Estimate it as width × mean depth, in m².
The figure plots depth readings from a stream 4.0 m wide.
Depth is drawn as a negative number so that the bed appears below the water surface, the way the channel looks.
Velocity and discharge
Speed of flow/velocity is the distance the water travels in a given time, measured in metres per second (m/s).
In fieldwork you can time a float over a measured distance and divide distance by time, or use a flow meter.
Water flows slowest next to the bed and banks, where friction holds it back, and fastest near the middle of the channel just below the surface.
Discharge is the volume of water passing a point in the river each second.
It is measured in cubic metres per second (m³/s), often called cumecs.
You calculate it from the cross-sectional area and the velocity:
Discharge is the most useful single measure of a river's size.
It rises after rain and falls in dry weather, and it normally increases downstream as tributaries add water.
In fieldwork, work in a safe, shallow stream with a partner.
Stretch a tape across the water for the width, and take depth readings with a metre rule at equal intervals along the tape.
The long profile
The long profile is a side view of a river from source to mouth: a graph of height against distance.
Most long profiles are concave.
They are steep near the source and become gentler towards the mouth, where the river is close to sea level.
Geographers divide this into the upper course (steep, near the source), the middle course and the lower course (gentle, near the mouth).
Finding these features on a map
Exam papers often show drainage basin features on a map extract.
Rivers flow from higher contours to lower ones.
Where a river crosses a contour, the contour bends into a V, and the V points upstream.
The watershed follows the highest ground between two rivers, so look for hilltops and spot heights between the valleys.
Every map has a scale.
At 1:25 000, 1 cm on the map stands for 25 000 cm on the ground, which is 250 m, so 4 cm on the map is 1 km.
At 1:50 000, 1 cm stands for 500 m, so 2 cm is 1 km.
On both, one grid square is 1 km across.
The map below is used for the check questions here and the gradient work in the next section.
- Drainage basin: the area of land drained by a river and its tributaries.
- Watershed: the boundary of a drainage basin, usually a ridge of high land, which separates it from neighbouring basins.
- Confluence: the point where two rivers or streams join.
- Wetted perimeter: the length of the bed and banks in contact with the water in a cross-section of the channel.
- Discharge: the volume of water flowing past a point in a river per second, in m³/s (cumecs).
A student measures the stream in the cross-section figure.
The width is 4.0 m.
Depths across the channel are 0.20, 0.35, 0.50, 0.45 and 0.25 m.
A float takes 25 seconds to travel 10 m.
Calculate the discharge.
1. Mean depth: add the depths and divide by the number of readings. 0.20 + 0.35 + 0.50 + 0.45 + 0.25 = 1.75 m, and 1.75 ÷ 5 = 0.35 m.
2. Cross-sectional area = width × mean depth = 4.0 × 0.35 = 1.4 m².
3. Velocity = distance ÷ time = 10 ÷ 25 = 0.4 m/s.
4. Discharge = cross-sectional area × velocity = 1.4 × 0.4 = 0.56 m³/s.
Answer: 0.56 m³/s (cumecs).
Further downstream on the same stream the width is 9.0 m, the mean depth is 0.80 m and the velocity is 0.75 m/s.
Calculate the discharge here, and how many times greater it is than at the first site.
1. Cross-sectional area = 9.0 × 0.80 = 7.2 m².
2. Discharge = 7.2 × 0.75 = 5.4 m³/s.
3. Compare by dividing: 5.4 ÷ 0.56 = 9.64.
Answer: 5.4 m³/s, about 9.6 times the discharge at the first site.
Both the area and the velocity have increased downstream.
Calculators are allowed in every 0460 paper, but write each line of working: the formula, the numbers you put in, then the answer with its unit.
Discharge is always in m³/s; an answer in m² or m/s shows you stopped one step early.
- Using the deepest reading in place of the mean depth. Area uses the mean of all the depth readings.
- Mixing centimetres and metres. Convert depths in cm to m (divide by 100) before multiplying.
- Treating the watershed as a body of water. It is a boundary on high ground, not a river or lake.
- Thinking the wetted perimeter is the width. The width is measured across the surface; the wetted perimeter follows the bed and both banks, so it is longer.
Check your understanding
1. Using the map, give (a) the four-figure grid reference of the square that contains the confluence at point B and (b) the six-figure grid reference of the bridge where the road crosses the main river. [2]
2. Using the map, state the direction in which the main river flows and give one piece of map evidence for your answer. [2]
3. A river is 3.0 m wide with a mean depth of 0.25 m, and its velocity is 0.6 m/s. Calculate its discharge. Show your working. [2]
4. Explain why water flows more slowly next to the bed and banks of a river than in the middle of the channel. [2]
5. Define the term watershed. Point C on the map is on the watershed between the main river and its tributary: using the map, explain how you can tell. [3]
Answers
1. (a) 2453 [1]; (b) 262514 [1].
2. South-east [1]. Evidence, one of: the river flows from point A on the 160 m contour towards point D on the 80 m contour; the contours make Vs that point north-west, which is upstream [1].
3. Area = 3.0 × 0.25 = 0.75 m² [1]; discharge = 0.75 × 0.6 = 0.45 m³/s [1].
4. Water touching the bed and banks rubs against them, so friction slows it down [1]; water in the middle of the channel is furthest from the bed and banks, so it loses least energy to friction [1].
5. The boundary of a drainage basin, usually along high ground, separating it from the next basin [1]. C is on higher ground, between the 160 m and 180 m contours, than the valleys on either side of it [1]; the land falls away from C south-west towards the main river and east towards the tributary, so rain falling on each side drains into a different river [1].