Three Runs on the Stream Table
Students built a 1-metre stream table on a 15° slope and packed it with a mixture of sand and clay. They ran 2 litres of water per minute for 6 minutes and weighed the sediment caught in a tray at the bottom: 40 g. Without repacking the surface, they ran the same flow again: 12 g. A third identical run gave 11 g. The teacher says the first result "proves streams erode faster on steep slopes." The class also noticed that after the first run a narrow channel ran down the middle of the table, and that the runoff water was muddiest in the first minute and cleared steadily after that.
1. Is the teacher's conclusion supported by the three sediment masses? Explain what the drop from 40 g to 12 g to 11 g actually shows.
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No. Slope never changed - it was 15° in all three runs - so slope cannot explain a 40 → 12 g drop. What changed is the table's surface.
The first run met loose fine sand and clay, which washed away easily. By run 2, that loose material was gone and a channel had formed, so water followed a smooth path and carried less. Runs 2 and 3 agree closely (12 g and 11 g) because the surface had reached a steadier state.
The 40 g figure measures an unprepared surface, not a steep slope. The teacher's claim needs runs at several slopes, each starting from the same freshly packed surface, or it is comparing a first run with later runs and calling that a slope effect.
2. Design a procedure that would actually test whether steeper slopes erode more, and describe the pattern of results that would support the claim.
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Procedure:
- Repack the table to the same depth and press it with the same weight before every run, so the starting surface is identical each time.
- Keep flow (2 L/min), duration (6 minutes), and sediment-tray position constant; change only the slope - for example 5°, 15°, and 25°.
- Do each slope twice, in mixed order (15°, 5°, 25°, then 25°, 15°, 5°) so the table's history does not line up with slope.
- Weigh and dry the sediment after each run.
Supporting pattern: sediment mass rises steadily with slope - roughly 8 g, 25 g, 45 g at 5°, 15°, 25° - and repeats at the same slope land close together. That would show the surface, not the run order, was driving the numbers.
3. Explain why the runoff was muddiest in the first minute of each run and cleared afterwards, and say what this means for comparing runs.
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The loosest, finest particles sit on top and are lifted first, so the first minute carries the most sediment. Once those are gone, heavier grains are left behind and partly protect the surface below - the bed "armours" itself - so the water clears.
What this means: the muddiness curve, not just the total mass, records how prepared the surface was. A run that starts on a fresh surface will always look more erosive, whatever the slope. So comparisons only work if every run starts from the same freshly packed bed, and if the class records sediment over time (e.g., each minute) rather than only the final total. Totals alone hide whether a difference came from slope or from a leftover channel.