Every time a river bursts its banks, scientists rely on precise measurements of its flow to predict the scale of the flood. is the volume of water passing through a specific point of a river channel in a given period of time.
Discharge is calculated using and the following formula:
Where:
To find the , you must measure both the width and the depth of the river. Measurements must be taken in a straight reach of the river, as meanders create turbulent flow and uneven depths that reduce accuracy.
You can easily see the surface of a river rushing past, but the water near the bed moves much slower due to friction. Therefore, geographers use two main methods to measure velocity, each with different accuracy levels.
A fieldwork group measures a river with a of 4.0 m. They take four depth measurements: 0.1 m, 0.2 m, 0.4 m, and 0.3 m. Using a flow meter, they record a mean velocity of 0.5 m/s. Calculate the .
Step 1: Calculate the .
Step 2: Calculate the ().
Step 3: Calculate the discharge ().
Why does a photograph sometimes fail to capture the most important geographical features of a landscape? Because it captures everything, including irrelevant clutter. To document landforms like meanders, waterfalls, or floodplains, geographers rely on techniques that record visual characteristics.
Field Sketches: A sketch allows a geographer to be selective, removing background "noise" (like trees or people) to focus purely on the landforms.
The page is divided into a 3x3 grid to maintain scale. You should draw the skyline first, followed by the foreground, and finally the middle ground.
A high-quality sketch uses the OASIS mnemonic: Orientation (compass direction), , Scale, Information (date/weather), and Sketch.
Geolocated Photography: This involves taking a digital photo that is tagged with GPS coordinates (latitude and longitude).
provides metadata that allows the images to be uploaded and layered onto digital maps using a .
While photography is much faster and completely objective (unbiased) compared to sketching, it cannot filter out visual obstructions.
A crucial skill in recording visual data is knowing the difference between a label and an . An annotated photograph or sketch goes beyond simply naming a feature.
Students often confuse labelling with annotating. To gain marks for an annotation, you must explain HOW the feature formed (e.g., naming 'lateral erosion'), not just point to it and state what it is.
Always remember to convert your depth measurements from centimetres to metres before calculating the cross-sectional area, otherwise your final discharge units will be completely wrong.
In 8-mark fieldwork evaluation questions, examiners expect you to assess the reliability of your methods. A great point to make is that the float method lacks accuracy because it only measures surface velocity, which is artificially faster due to less friction with the river bed.
River discharge
The volume of water passing through a specific point of a river channel in a given period of time, calculated as Cross-sectional Area × Velocity.
Cumecs
The unit of measurement for river discharge, standing for cubic metres per second (m³/s).
Cross-sectional area
The total area of the slice of water within the river channel from bank to bank, calculated by multiplying width by mean depth.
Wetted width
The horizontal distance straight across the river from the water's edge on one bank to the water's edge on the opposite bank.
Systematic sampling
A sampling method where measurements or data are collected at regular, predefined intervals, such as taking a depth reading every 50 cm.
Mean depth
The average depth of a river channel, calculated by adding all individual depth readings together and dividing by the total number of readings.
River velocity
The speed at which water flows through the river channel, typically measured in metres per second (m/s).
Impeller
The rotating fan-like part of a flow meter that spins in the water to record the velocity of the river.
Quantitative data
Numerical information that can be measured and quantified, such as river depth, width, or velocity.
Qualitative data
Non-numerical information that describes the visual characteristics or subjective qualities of a geographical feature.
Annotation
A descriptive sentence added to a sketch or photograph that links a visible feature to the geographical processes that formed it.
Geolocation
The process of identifying and recording the real-world geographic location (like GPS coordinates) of an image or data point.
Geographic Information System (GIS)
Digital mapping software that allows geographers to layer, analyse, and display spatial data and geolocated photographs.
Put your knowledge into practice — try past paper questions for Geography A
River discharge
The volume of water passing through a specific point of a river channel in a given period of time, calculated as Cross-sectional Area × Velocity.
Cumecs
The unit of measurement for river discharge, standing for cubic metres per second (m³/s).
Cross-sectional area
The total area of the slice of water within the river channel from bank to bank, calculated by multiplying width by mean depth.
Wetted width
The horizontal distance straight across the river from the water's edge on one bank to the water's edge on the opposite bank.
Systematic sampling
A sampling method where measurements or data are collected at regular, predefined intervals, such as taking a depth reading every 50 cm.
Mean depth
The average depth of a river channel, calculated by adding all individual depth readings together and dividing by the total number of readings.
River velocity
The speed at which water flows through the river channel, typically measured in metres per second (m/s).
Impeller
The rotating fan-like part of a flow meter that spins in the water to record the velocity of the river.
Quantitative data
Numerical information that can be measured and quantified, such as river depth, width, or velocity.
Qualitative data
Non-numerical information that describes the visual characteristics or subjective qualities of a geographical feature.
Annotation
A descriptive sentence added to a sketch or photograph that links a visible feature to the geographical processes that formed it.
Geolocation
The process of identifying and recording the real-world geographic location (like GPS coordinates) of an image or data point.
Geographic Information System (GIS)
Digital mapping software that allows geographers to layer, analyse, and display spatial data and geolocated photographs.