The Stream Table Test
Two groups build stream tables from identical trays tilted at the same angle. Group 1 packs one tray with bare soil. Group 2 packs the other tray with soil and grows a thick mat of grass on top for two weeks. Each group pours 2 L of water from the same height at the top of its tray and collects runoff in a beaker. Group 1's runoff is muddy and leaves 45 g of sediment in the filter. Group 2's runoff is much clearer and leaves 8 g of sediment. Both trays lost some soil from the surface. A student says, "The grass stopped erosion completely." The teacher asks the class to use the data to evaluate that claim and to think about a real riverbank where trees have been cut down.
1. Use the data to evaluate the claim that the grass stopped erosion completely.
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The claim is too strong. Group 2 still left 8 g of sediment in the filter, and both trays lost some soil from the surface. If erosion had been stopped completely, the runoff would have carried no sediment. The data do show that grass greatly reduced erosion: 8 g is much less than 45 g. A careful conclusion is that grass reduced erosion by a large amount, not that it stopped it entirely. Scientists should describe the size of the effect rather than claim a complete stop when the measurement shows some erosion still happened.
2. Explain how the grass reduced the amount of soil washed away.
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Grass roots hold soil particles together, making them harder for moving water to pick up. The grass blades also slow the water as it flows downhill, so the water has less energy to carry soil. Some water soaks into the soil around the roots instead of running off. In Group 1, bare soil had nothing holding it, so the moving water carried away more particles and made the runoff muddy. The smaller sediment mass in Group 2 is evidence that roots and grass cover protected the soil.
3. Predict what would happen to a real riverbank if all its trees were cut down, and describe one way people could reduce that damage.
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Without trees, the riverbank would probably erode faster. Tree roots no longer hold the soil, and rainwater would run over bare ground instead of soaking in, so more soil would wash into the river. The water would likely become muddier, and the bank could collapse in places. One way to reduce the damage is to plant new trees or deep-rooted plants along the bank and keep a buffer of vegetation. Another is to place rocks or other protective material where the water flows fastest. The stream table predicts that protecting the soil surface reduces sediment in runoff.