Option 7A (physical): River landscapes — investigating change in a river channel

Option 7A (Physical): River Landscapes — Investigating Change in a River Channel

Welcome to your fieldwork study notes for Topic 7: River Landscapes. In Paper 3 (Geographical Investigations: Fieldwork and UK Challenges), you will be asked questions about the river fieldwork you carried out. Don't worry if fieldwork seems daunting at first; the exam tests your understanding of the six standard enquiry stages using everyday geographical skills. By breaking the investigation down step by step, you will be fully prepared to answer any question from short calculations to extended 8-mark and 12-mark evaluations.

Note: For the overall enquiry framework, refer to the chapter "The fieldwork enquiry process: questions, methods, presentation, analysis, conclusions and evaluation". This chapter focuses specifically on investigating changes along a river channel.

Stage 1: Framing Enquiry Questions and Hypotheses

Every geographical investigation starts with a clear aim. For this option, the prescribed task is to investigate change in a river channel from upstream to downstream.

Geographical Theory: The Bradshaw Model

To ask sensible questions, geographers use an underlying theory known as the Bradshaw model. This model predicts how river characteristics change as a river flows from its source (upper course) to its mouth (lower course):

Characteristics that increase downstream:

• Channel width (due to lateral erosion and higher discharge)
• Channel depth (due to vertical erosion downstream)
• Mean velocity (less friction from the river bed and banks as the channel becomes smoother and larger)
• Total discharge (the volume of water passing a point per second, increasing as tributaries join)
• Load quantity (the total amount of sediment carried)

Characteristics that decrease downstream:

• Bedload particle size (sediment becomes smaller due to attrition and abrasion)
• Bedload particle angularity (sediment becomes rounder and smoother due to attrition)
• Channel bed roughness (large, jagged boulders in the upper course are worn down to silt and sand)
• Slope gradient (the river valley flattens out)

Formulating Testable Hypotheses

A hypothesis is a prediction you can test using numerical measurements. Here are classic fieldwork hypotheses for a river enquiry:

• Hypothesis 1: "River channel cross-sectional area increases with distance downstream."
• Hypothesis 2: "Average bedload size decreases and particle roundness increases with distance downstream."
• Hypothesis 3: "Average water velocity increases with distance downstream."

Health and Safety: Risk Assessment

Before stepping into the river, geographers must assess potential hazards and establish precautions:

• Hazard: Slippery rocks and uneven bed \(\implies\) Risk: Slips, falls, sprains or broken bones \(\implies\) Precaution: Wear sturdy, gripped footwear (such as wellingtons or walking boots) and take small, deliberate steps.
• Hazard: Fast-flowing or deep water \(\implies\) Risk: Swept off feet, drowning \(\implies\) Precaution: Measure water depth with a ranging pole before stepping in; do not enter water deeper than knee level; carry throw-lines.
• Hazard: Waterborne infections (e.g. Weil's disease/Leptospirosis) \(\implies\) Risk: Bacterial illness \(\implies\) Precaution: Cover open cuts with waterproof plasters; wash hands with antibacterial gel before eating; never drink river water.
• Hazard: Adverse weather (heavy rain or extreme cold) \(\implies\) Risk: Hypothermia or sudden flash flooding \(\implies\) Precaution: Monitor weather forecasts; wear waterproof clothing; check river levels upstream before entering.

Key Takeaway: River fieldwork explores whether river channel variables match theoretical models (like the Bradshaw model). Sound planning requires clear, testable hypotheses and a thorough risk assessment.

Stage 2: Methods and Data Collection

Fieldwork requires collecting different types of data: primary data (collected by you in the field), secondary data (collected by others, such as maps or historical flood records), quantitative data (numerical measurements), and qualitative data (descriptive observations).

Sampling Strategies

You cannot measure every single pebble or every single millimeter of a river, so you must select a sample. Three main sampling strategies are used:

1. Systematic sampling: Taking measurements at regular, equal intervals (e.g. measuring river depth every 0.5 metres across a transect, or choosing field sites exactly 2 km apart downstream).
• Strength: Easy to follow; removes personal bias.
• Limitation: Might miss key features if they fall between intervals.

2. Random sampling: Choosing sample points using a random number table or generator (e.g. selecting coordinates on a grid randomly).
• Strength: Completely unbiased.
• Limitation: Points might cluster by chance, missing important river sections.

3. Stratified sampling: Dividing the river into distinct sub-sets or zones (such as upper, middle, and lower course, or riffles and pools) and taking a set number of samples from each.
• Strength: Ensures all distinct sections of the river are represented.
• Limitation: Requires prior knowledge of the site to set up groups.

Step-by-Step Measurement Methods

1. Measuring Channel Width

• Equipment: Two ranging poles, tape measure.
• Method: Place one ranging pole at the water's edge on the left bank and another directly opposite on the right bank (wetted width). Pull the tape measure taut between the two poles just above the water surface.
• Ensuring accuracy: Keep the tape measure level and perpendicular (at \(90^\circ\)) to the direction of flow. Do not let the tape sag into the water.

2. Measuring Channel Depth

• Equipment: Metre ruler (or calibrated ranging pole).
• Method: Using the taut tape measure from the width measurement, record the water depth at regular systematic intervals across the channel (e.g. every 20 cm or 50 cm depending on river width).
• Ensuring accuracy: Turn the thin edge of the ruler into the current to reduce water surging up the face of the ruler; ensure the ruler rests lightly on the riverbed without sinking into soft mud.

3. Measuring Velocity

You can measure velocity using either a digital flowmeter or the float method:

• Method A: Digital Flowmeter (impeller)
Place the impeller into the water facing directly upstream at roughly \(60\%\) of the depth from the surface (where average velocity occurs). Read the digital speed directly from the meter.
• Method B: Float Method (Stopwatch & Float)
Measure a set distance along the bank (e.g. 10 metres) using a tape measure and mark start and finish lines. Place a buoyant object (e.g. an orange or small dog biscuit) gently upstream of the start line. Start the stopwatch as it crosses the start line and stop it when it crosses the finish line. Calculate velocity using the formula: \(\text{Velocity (m/s)} = \frac{\text{Distance (m)}}{\text{Time (s)}}\). Repeat this at least three times across the channel (left, centre, right) and calculate a mean.
• Float factor: Water moves faster at the surface due to less bed friction, so surface float speeds are often multiplied by a correction factor (around \(0.85\)) to estimate mean channel velocity.

4. Measuring Bedload Characteristics (Size and Roundness)

• Equipment: Metre ruler or callipers, Powers' Scale of Roundness chart.
• Method: At each systematic depth point across your transect, reach down with eyes closed (or looking away to avoid selection bias) and pick up the pebble resting at the tip of your boot.
• Measuring size: Use callipers or a ruler to measure the pebble's longest axis (the a-axis) in millimetres.
• Measuring roundness: Compare the pebble against a standard Powers' Scale of Roundness (graded from 1 = Very Angular to 6 = Well Rounded) and assign a score.

Qualitative and Secondary Data

• Qualitative data: Annotated field sketches of the channel and valley sides, and photographs recording channel features, bedload sorting, and bank vegetation.
• Secondary data: 1:25,000 or 1:50,000 Ordnance Survey (OS) maps to measure river gradient and distance from source; Environment Agency interactive flood maps to check flood risk zones and historical discharge data.

Key Takeaway: Accurate field data relies on choosing the correct equipment, maintaining standard techniques (e.g. edge-on ruler, taut tape measure), avoiding sampling bias, and combining primary fieldwork with secondary map data.

Stage 3: Data Processing and Presentation

Raw numbers recorded on a clipboard need to be processed using calculations and visualised through geographical presentation techniques.

Numerical and Statistical Calculations

1. Mean Depth and Cross-Sectional Area:
To calculate cross-sectional area (\(\text{CSA}\)), first find the mean depth of your transect points:
\(\text{Mean depth} = \frac{\text{Sum of all depth measurements}}{\text{Number of depth measurements}}\)
Then approximate the cross-sectional area:
\(\text{Cross-Sectional Area } (m^2) = \text{Channel Width } (m) \times \text{Mean Depth } (m)\)

2. River Discharge:
Discharge (\(Q\)) is the volume of water flowing past a point per second, measured in cubic metres per second (\(m^3/s\) or cumecs):
\(\text{Discharge } (Q) = \text{Cross-Sectional Area } (m^2) \times \text{Mean Velocity } (m/s)\)

3. Percentage Change:
If you compare site 1 (upstream) and site 2 (downstream), you may be asked to calculate the percentage increase or decrease in a variable:
\(\text{Percentage Change} = \frac{\text{New Value} - \text{Old Value}}{\text{Old Value}} \times 100\)
Example: If the width at Site 1 was \(2.5\text{ m}\) and at Site 2 was \(4.0\text{ m}\):
\(\text{Percentage Change} = \frac{4.0 - 2.5}{2.5} \times 100 = \frac{1.5}{2.5} \times 100 = 60\%\text{ increase}\).

4. Measures of Central Tendency and Spread:
• Mean: The average value.
• Median: The middle value when data is placed in numerical order.
• Mode: The most frequently occurring value (e.g. modal class of sediment roundness).
• Range: The difference between the highest and lowest values (\(\text{Highest} - \text{Lowest}\)).
• Inter-quartile Range (IQR): The range of the middle \(50\%\) of data (\(Q_3 - Q_1\)), which removes extreme outliers.

Presentation Techniques

Different types of data require appropriate presentation methods:

• Channel Cross-Sections: Drawn on graph paper with distance across the channel on the horizontal axis (\(x\)-axis) and depth below water level plotted downwards on the vertical axis (\(y\)-axis). Connecting these points creates a profile of the riverbed.
• Common mistake to avoid: Forgetting that depth goes downwards from the surface! Make sure your vertical axis starts at 0 at the top and goes deeper as you go down.

• Scatter Plots with a Line of Best Fit: Ideal for showing bivariate data (the relationship between two continuous variables). For example, plotting "Distance downstream (km)" on the \(x\)-axis against "Average pebble size (mm)" on the \(y\)-axis. A trend line (line of best fit) highlights positive or negative correlations.

• Bar Charts and Histograms: Excellent for showing discrete or grouped data, such as the frequency of pebbles falling into each Powers' Roundness category at different sites.

• GIS and Located Graphs: Superimposing mini cross-sections or bar charts directly onto an OS map or digital mapping layer at the exact coordinates where data was gathered. This clearly links channel changes to local topography and land use.

Key Takeaway: Cross-sectional area and discharge are calculated using width, depth, and velocity. Presenting bivariate data on scatter graphs with lines of best fit helps identify correlations.

Stage 4: Analysing and Explaining Data

Analysis means describing patterns shown in your graphs and using geographical knowledge to explain why those patterns occur.

Explaining Expected Trends (Links to Theory)

• Why does cross-sectional area increase downstream? Downstream, smaller tributaries join the main channel, increasing the total volume of water (discharge). The river has more kinetic energy to erode its banks laterally and its bed vertically, leading to a wider and deeper channel.
• Why does velocity increase downstream? Even though the slope/gradient is steeper in the upper course, the channel is shallow, rough, and choked with large boulders. This creates high friction, which slows the water down. In the middle and lower courses, the channel is wider, deeper, and smoother, meaning less water touches the wetted perimeter relative to total volume. This reduced hydraulic friction allows water to flow faster.
• Why does bedload size decrease and roundness increase? As rocks travel downstream, they continuously collide with each other and with the riverbed. This process of attrition chips away sharp edges (making them rounder) and breaks particles into smaller pieces. Chemical solution also dissolves soluble rock types.

Identifying and Explaining Anomalies

An anomaly is a data point that deviates from the overall trend or pattern (an outlier on your scatter plot).

• Example anomaly: A sudden drop in velocity at a downstream site, or unexpectedly large, angular boulders in the lower course.
• Possible explanations:
1. Human intervention: Artificial weirs, bridges, or channelisation (concrete banks) alter natural flow speeds and trap sediment.
2. Tributary inputs: A steep mountain tributary may have joined just above the survey site, depositing fresh, large, angular rocks into the main stream.
3. Local geology: The river may cross a hard band of rock that resists erosion, creating a narrow gorge or shallow rocky section that disrupts the downstream pattern.
4. Measurement error: A ruler placed behind a large boulder in an eddy or dead-water zone will record an uncharacteristically low velocity reading.

Key Takeaway: Use geographical terms (friction, wetted perimeter, attrition, discharge) to explain trends. Always actively identify and suggest reasons for outliers and anomalies.

Stage 5: Drawing Evidenced Conclusions

In this stage, you answer your original enquiry question and state whether your hypotheses are supported or rejected. High-level conclusions must always refer directly to specific numerical evidence from your data.

Weak Conclusion: "The river got wider and deeper downstream, so my hypothesis was right." (Lacks data, precision, and depth).

Strong, Evidenced Conclusion: "The data supports Hypothesis 1. The river channel cross-sectional area increased steadily downstream. At Site 1 (0.8 km from source), the channel width was 1.4 m and mean depth was 0.12 m, giving a cross-sectional area of \(0.17\text{ m}^2\). By Site 3 (4.2 km from source), width had increased to 3.8 m and mean depth to 0.28 m, producing a cross-sectional area of \(1.06\text{ m}^2\). This represents an increase of over \(500\%\), matching the predictions of the Bradshaw model."

Key Takeaway: Never make unsupported statements in your conclusion. Quote exact figures, units, site names, and calculated changes (e.g. percentage increase) to substantiate your judgement.

Stage 6: Evaluation

Evaluation is a critical look at your investigation. You examine how reliable your data is, identify weaknesses in your equipment and methods, and consider the validity of your conclusions.

Accuracy vs Reliability

• Accuracy: How close a measurement is to the true value (e.g. did the ruler measure true depth, or did it sink into mud?).
• Reliability: The consistency of your measurements (e.g. if you repeated the velocity test three times, did you get similar results?).

Common Methodological Limitations and Improvements

1. Float Method for Velocity:
• Limitation: Floats can get stuck in eddies, snagged on vegetation, or blown by surface wind; human reaction time errors with the stopwatch introduce variability.
• Improvement: Use an electronic digital flowmeter with an impeller, held at \(60\%\) depth to record true average velocity automatically.

2. Measuring Sediment (Pebble Selection):
• Limitation: Students tend to pick up larger, more visible pebbles rather than fine silt or sand (operator bias); small sample sizes (e.g. 10 pebbles per site) are unrepresentative.
• Improvement: Increase sample size to 30 or 50 pebbles per site; use a systematic sampling grid (such as a gravelometer or transect pin drop) rather than subjective hand selection.

3. Powers' Scale of Roundness:
• Limitation: Judging pebble roundness by eye against a 2D drawing is subjective; two students may classify the same pebble differently.
• Improvement: Have the same person assess all samples after standardising against reference stones, or use digital photographic analysis software.

4. Temporal and Weather Limitations:
• Limitation: Fieldwork only captures a "snapshot in time" on a single day. Rainfall the previous day could cause a temporary spike in discharge that does not reflect normal seasonal conditions.
• Improvement: Repeat measurements across different seasons and weather conditions (e.g. summer baseflow vs autumn post-storm conditions) or consult secondary Environment Agency river gauge data.

Key Takeaway: Evaluation is not just saying "we did a good job". You must identify specific sources of error, evaluate their impact on the validity of your conclusions, and propose practical, realistic improvements.

Quick Revision Checklist

Before your exam, check that you can:

• Define the Bradshaw model trends (what increases and what decreases downstream).
• Identify key river hazards and corresponding risk mitigation strategies.
• Explain how to measure width, depth, velocity, and bedload using standard field equipment.
• Differentiate between systematic, random, and stratified sampling.
• Calculate cross-sectional area, discharge (\(Q = A \times V\)), mean, median, range, and percentage change.
• Justify the choice of cross-section diagrams and scatter plots with lines of best fit.
• Give reasons for anomalies (e.g. human structures, tributary inputs, geology).
• Write an evidenced conclusion using data figures and percentage comparisons.
• Evaluate the accuracy, reliability, and limitations of methods like the float test or Powers' scale.