Teacher answer key · Grades 6–10 · about 15 minutes

Answer key: Erosion and Deposition Activity

For teachers. This page is hidden from search engines and not linked from student pages, but it is not password-protected, so please don’t share the link with students. Student worksheet: Erosion and Deposition Activity.

Goal: Use a model stream table to find out how slope, water flow and grain size control erosion and deposition.

Connects to NGSS MS-ESS2-2 (Construct an explanation, based on evidence, of how geoscience processes have changed Earth’s surface at varying time and spatial scales.); HS-ESS2-5 (Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.)

Completed results table

Values after 10 minutes with fine sand and no sediment fed in, as the sandbox shows them (volumes rounded to two figures). Readings a slider step away are fine.

Setup	Speed at start (cm/s)	Eroded (cm³)	Deepest cut (cm)	Delta length (cm)
A: 2% slope, 2 L/min	25	130	0.8	12
B: 6% slope, 2 L/min	35	330	2.2	14
C: 2% slope, 6 L/min	39	500	1.9	28
D: 6% slope, 6 L/min	55	1,600	4.5	36

About 2.4 cm. On a gentle slope the water can’t carry that much sand, so it drops the extra near the inlet and builds a fan, which steepens the slope.

Question 4 at 40 cm/s: red dots are eroded, teal dots only transported.

Setup D after 10 minutes, from the side (heights stretched ×2): pink was cut away; gold was laid down near the sea and in the delta.

Answers

Which setup eroded the most sand, and which eroded the least? Use the water speeds to explain why.
Most: D, about 1,600 cm³, with the fastest water (55 cm/s at the start). Least: A, about 130 cm³ at 25 cm/s. Faster water picks up more sediment and can carry more of it.
Compare B and C: did more slope or more water make the water faster? Which built the longer delta?
More water: C started at 39 cm/s and B at 35 cm/s. C also built the longer delta, 28 cm against 14 cm, because more fast water carried more sand down to the sea.
Where did the sand cut from the channel end up? Why there?
Mostly in the delta: in C, about 460 of the 500 cm³ eroded was laid down in the sea. The river spreads out and slows down as it enters the still water, so it can’t carry its sand any farther and drops it. Some sand also settles on the inside of bends, where the water is slower.
At 40 cm/s on the Hjulström diagram, which sediments are eroded and which are only transported?
Eroded: fine sand (about 22 cm/s) and coarse sand (about 30 cm/s). Only transported: clay (about 110 cm/s), silt (about 46 cm/s) and gravel (about 53 cm/s); they keep moving if they are already in the water, but the water is too slow to pick them up from the bed.
Clay grains are smaller and lighter than sand. Why does clay need faster water to be picked up?
Clay flakes stick together and settle into a smooth, firm layer that the water slides over, so lifting them takes about 110 cm/s, while fine sand moves at about 22 cm/s. Once clay is floating, it sinks so slowly that almost any current keeps it moving.
Predict, then test: will gravel build a longer or a shorter delta than fine sand in setup D? Why?
Shorter: 10 cm instead of 36 cm. Gravel needs about 53 cm/s to be picked up, so D’s 55 cm/s water barely moves it, and it sinks fast (about 25 cm/s), so it drops right at the river mouth.

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