The Muddy Runoff Problem
A fifth-grade class is studying a small stream that runs behind their school. After a heavy rain, the water turns muddy brown, and the students notice that the mud settles out in the calm pool at the bottom of the hill but stays cloudy in the fast-moving section above it. The class measures how long a jar of stream water takes to clear: the water from the calm pool clears in about 20 minutes, while the water from the fast section is still cloudy after an hour. Upstream, a new housing development has removed most of the trees and grass along the bank. The students want to explain why the stream gets muddy and where the mud comes from.
1. Use the evidence in the setup to explain why the stream water is muddy after rain and why the two sections of the stream behave differently.
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The mud comes from soil washed off the bare ground in the housing development, where trees and grass no longer hold the soil in place. Rain loosens the soil and carries it into the stream.
The two sections differ because of water speed. In the fast section, moving water keeps soil particles suspended, so the water stays cloudy for a long time. In the calm pool, the water slows down, so the heavy soil particles settle to the bottom and the water clears faster (20 minutes vs. still cloudy after an hour).
So the muddiness has two causes: (1) a source of loose soil from the bare bank, and (2) fast-moving water that keeps that soil suspended. Both are needed to explain the observations.
2. A student suggests that planting grass and shrubs along the bare bank will make the stream clearer. Use the setup to predict what would happen and explain the reasoning.
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Planting grass and shrubs should reduce muddiness over time. The roots hold soil in place, so less soil washes into the stream during rain. With less soil entering the water, there are fewer particles to keep the water cloudy, even in the fast section.
This is a reasonable prediction because the evidence already links the bare bank to the mud. Removing the cause (loose, unprotected soil) should reduce the effect (muddy water). The prediction is about the source of the mud, not about changing the stream's speed, so the calm pool and fast section would still behave differently, but both should become clearer than before.