Primary Fieldwork Methods for River Landscapes

Two ways to investigate a river landscape

River fieldwork uses primary data: evidence collected first-hand at the investigation site. Numerical measurements, such as water depth and flow speed, are quantitative data. Photographs and field sketches provide qualitative data, recording what the landscape looks like.

These methods complement one another. Measurements allow you to calculate how much water passes through a channel, while a sketch or photograph records the landforms surrounding it. Using consistent methods at different sites helps you investigate change along a river channel.

Discharge combines channel size and flow speed

River discharge is the volume of water flowing past a point each second. Its unit is cubic metres per second, written $\text{m}^3/\text{s}$. It is not simply the speed of the water: a large channel can carry more water than a small channel even if their flow speeds are the same.

A practical estimate of discharge combines the cross-sectional area occupied by water with its mean velocity:

$$\text{discharge} = \text{cross-sectional area} \times \text{mean velocity}$$

The measurements needed are therefore width, depth and velocity. A tape measure, metre rule, recording sheet and either a flow meter or a float and stopwatch allow these to be collected.

Measuring the water-filled cross-section

Stretch a tape measure across the river, from the point where the water meets one bank to the equivalent point on the opposite bank. This measures the wetted width, rather than the distance between the tops of the banks. Keep the tape taut and above the water so that it does not sag or interfere with the flow. Record the width in metres.

Measure depth at regular intervals across this width. At each position, place a metre rule vertically on the river bed and read the distance from the bed to the water surface. Hold the rule with its broad, flat face towards the banks so that it obstructs the flow as little as possible. Record each depth and its position across the channel.

One depth reading cannot represent a channel that varies from shallow to deep. Regularly spaced readings capture more of that variation and allow a mean depth to be calculated:

$$\text{mean depth} = \frac{\text{sum of depth readings}}{\text{number of readings}}$$

Then estimate the water-filled cross-sectional area:

$$\text{cross-sectional area (m}^2\text{)} = \text{width (m)} \times \text{mean depth (m)}$$

This is an estimate of an uneven channel, not a claim that the river has a perfectly rectangular cross-section.

Measuring velocity

Velocity can vary across the channel. Take measurements towards the left bank, in the centre and towards the right bank, rather than relying on one position.

With a flow meter, take readings at three evenly spaced positions across the width. Place the meter at least 3 cm below the surface and take three readings at each position. Calculate a mean from the recorded velocities. Repeated readings reduce the influence of a single unusually high or low reading.

A float provides an alternative method. Measure a known distance along the river, such as 10 m. Release the float at the upstream start point and use a stopwatch to time its journey to the downstream end point. Repeat three times at each of the three channel positions and record the results.

For a fixed travel distance, calculate a mean travel time and use:

$$\text{velocity (m/s)} = \frac{\text{distance travelled (m)}}{\text{time taken (s)}}$$

The float estimates surface velocity. It does not measure movement throughout the water column, so its result may not represent the mean velocity of the whole cross-section. Repetition helps with variable timings, but does not remove this limitation.

Turning measurements into a discharge estimate

Consider an example calculation. Suppose the wetted width is 4 m, mean depth is 0.25 m, and a float travels 10 m in a mean time of 20 s.

The estimated cross-sectional area is $4 \times 0.25 = 1\text{ m}^2$. The float velocity is $10 \div 20 = 0.5\text{ m/s}$. Multiplying these gives an estimated discharge of .

This means approximately half a cubic metre of water passes the measurement point each second. Because the calculation uses estimated area and surface velocity, the answer is also an estimate.

Recording landforms with a field sketch or photograph

A field sketch is a selective geographical drawing, not an artwork. Choose a viewpoint that shows the relevant channel and landforms. Record the site, date and viewing direction, draw the main outlines, and add labels and annotations linked to the features observed. A photograph can record the same view and be annotated afterwards.

For example, where a bend is visible, label the curved channel as a meander. If a gently sloping deposit is visible on the inside of the bend, identify the slip-off slope and annotate it: ‘material deposited where flow is slower’. This connects the observed landform with a river process. Keep the distinction clear between what is visible and your explanation of how it formed.

Photographs preserve detail, while sketches make it easier to select the most relevant features. Both have limitations: a viewpoint may hide parts of the landscape, and a sketch depends on the observer’s choices. Neither replaces measurements of width, depth or velocity. Together, qualitative records and quantitative measurements give a fuller account of the river landscape.