Designing a river fieldwork day with limited time
A Form 4 geography class of 32 students will do a one-day river study from 08:00 to 15:30. The 1:50,000 map shows a river falling from 1,250 m at a forest reserve boundary to 980 m at a sawmill, a straight-line distance of 7.5 km. Three possible sites are marked: A, 1.5 km from the road near the forest; B, at a bridge beside a village; and C, 500 m downstream of the sawmill. The bus takes 35 minutes each way to the bridge. Students must measure channel width, depth, velocity and bedload at three cross-sections, and test whether velocity increases downstream. Only two flow meters and four 30 m tapes are available. The river is ankle-deep at A, waist-deep at B after rain, and discoloured at C. A thunderstorm is forecast for 14:00.
1. Select sampling sites and a timetable that let the class test the hypothesis fairly. Justify your choices.
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Use A, B and C as a downstream sequence, but adapt the timetable to safety and time. The gradient is (1,250 − 980) / 7.5 = 36 m/km, so the river is steep and velocities may be high after rain. A gives an upstream, less disturbed control near the forest; B gives a bridge section with village influences; C gives a downstream reach below the sawmill. That sequence allows a test of the hypothesis, but C may be contaminated by sawmill waste and B may be deep after rain. The class should sample three cross-sections at each site, spaced across the channel, using a systematic method: measure width with the tape, depth at equal intervals, and velocity at 0.6 depth with the two meters. Rotate groups so all students collect data. Timetable: travel 08:00–08:35, safety briefing, site A 09:00–10:30, site B 11:00–12:15, lunch and site C 13:00–14:00, leave by 14:15 before the storm. This gives comparable data without overrunning.
2. Identify the main safety and ethical issues and explain how the plan should respond.
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Safety comes first. After rain, B may be waist-deep and rising; students should not wade above knee height, and a rope and adult supervisor should be used. The bridge is a road hazard, so work off the carriageway and wear high-visibility vests. At C, sawmill effluent and discoloured water raise skin and ingestion risks; avoid contact and wash hands. The thunderstorm forecast means leaving the river by 14:00 and avoiding open ground and isolated trees. Ethical issues include asking permission from the village and sawmill owner, explaining the purpose, avoiding damage to crops, and not publishing photos that identify individuals. Sampling should not disturb fish spawning areas or banks. A consent note and a risk assessment should be completed before departure. If water levels rise, abandon B and use A and C, but record the change so the comparison is honest.
3. Describe how the class should present and analyse the data, including one limitation.
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Present each site as a summary table and graph. Plot mean velocity against distance downstream (0, 3.5 and 7.0 km from A), with error bars or ranges, and plot cross-section profiles drawn from depth measurements. A bar chart of bedload size or a scatter of velocity against depth can show process. Calculate mean velocity for each cross-section, discharge (width × mean depth × mean velocity), and gradient between sites. Analyse whether velocity increases downstream despite a possible decrease in gradient; if it does not, discuss roughness, bridge constriction, or bedload. One limitation is that one day gives a snapshot after rain, not a seasonal average; another is that C is affected by sawmill discharge. Use the limitation to qualify the conclusion rather than ignoring it.