Primary Fieldwork Methods for River Landscapes

Measuring River Discharge (Quantitative Methods)

Every time a river bursts its banks, scientists rely on precise measurements of its flow to predict the scale of the flood. River discharge is the volume of water passing through a specific point of a river channel in a given period of time.

Discharge is calculated using quantitative data and the following formula:

$$Q = A \times V$$

Where:

• $Q$ = Discharge, measured in cubic metres per second ($m^3/s$), also known as cumecs.
• $A$ = Cross-sectional area ($m^2$), which is the total area of the "slice" of water from bank to bank.
• $V$ = River velocity ($m/s$), which is the speed of the water flow.

Step-by-Step: Calculating Cross-Sectional Area

To find the cross-sectional area, 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.

• Measuring Width: The wetted width is recorded using a tape measure stretched across the river from where the water meets the bank on one side to the other. The tape must be held taut and at a 90-degree angle to the flow to prevent sagging and inaccurate over-reading.
• Measuring Depth: A metre rule is placed into the water at regular intervals along the tape measure. This is known as systematic sampling (e.g., measuring every 50 cm) and ensures a representative profile of the river bed.
• Accuracy check: The metre rule must be placed sideways (narrow edge facing the flow) to prevent water from "piling up" against it, which would artificially increase the depth reading.
• Calculating Area: All individual depth readings are added together and divided by the number of readings to find the mean depth. The cross-sectional area is then calculated by multiplying the wetted width by the mean depth.

Step-by-Step: Measuring River Velocity

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.

• Flow Meter Method: An impeller (a rotating fan) is placed at least 3cm below the water surface to avoid surface turbulence. Readings are taken at three equal distances across the width (left, centre, right) and repeated to calculate a mean. This method is highly accurate because it can measure velocity at various depths.
• Float Method: A set distance (usually 10 metres) is measured along the bank. A float (like an orange) is released at the start, and a stopwatch is used to time its journey. Velocity is calculated as distance divided by time.
• Evaluation: While the float method is useful if the water is too shallow for an impeller, it is less reliable because it only measures surface velocity and the float can be trapped in eddies or blown by the wind.

Worked Example: Calculating Discharge

A fieldwork group measures a river with a wetted width 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 river discharge.

Step 1: Calculate the mean depth.

• $\text{Mean Depth} = (0.1 + 0.2 + 0.4 + 0.3) / 4 = 0.25\text{ m}$

Step 2: Calculate the cross-sectional area ($A$).

• $A = \text{Width} \times \text{Mean Depth}$
• $A = 4.0\text{ m} \times 0.25\text{ m} = 1.0\text{ m}^2$

Step 3: Calculate the discharge ($Q$).

• $Q = A \times V$
• $Q = 1.0\text{ m}^2 \times 0.5\text{ m/s} = 0.5\text{ m}^3\text{/s}$ (cumecs)

Recording River Landforms (Qualitative Methods)

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 qualitative data 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), Annotation, Scale, Information (date/weather), and Sketch.

• Geolocated Photography: This involves taking a digital photo that is tagged with GPS coordinates (latitude and longitude).

• Geolocation provides metadata that allows the images to be uploaded and layered onto digital maps using a Geographic Information System (GIS).

• While photography is much faster and completely objective (unbiased) compared to sketching, it cannot filter out visual obstructions.

Annotating vs. Labelling

A crucial skill in recording visual data is knowing the difference between a label and an annotation. An annotated photograph or sketch goes beyond simply naming a feature.

• Label: "Plunge pool."
• Annotation: "A deep plunge pool formed by the immense hydraulic action and abrasion of the falling water at the base of the waterfall."
• Annotations must explicitly link the visible feature to the geographical processes that formed it.