River Erosion Sandbox
Tilt a stream table, turn up the water and choose the sediment. Watch the river cut its channel, carry sand downstream and drop it on bends and in a delta, then read why on the Hjulström diagram.
- Pick a sediment, then set the slope and the water flow.
- Press Run to watch the channel cut down and a delta grow, or drag the time slider.
- Check the Hjulström diagram below: is your sediment eroded, transported or deposited?
The stream table from above
Water flows from left to rightSide view down the channel
Heights stretched ×2Fine sand: grains about 0.2 mm across, like beach sand.
- Water speed
- 37 cm/sat the start; now 27 cm/s
- Sediment eroded
- 480 cm³cut from the channel
- Deepest cut
- 2.3 cmbelow the starting surface
- Delta length
- 20 cmpast the old shoreline
4% slope, 3 liters per minute, fine sand. After 10 minutes: the water started at 37 cm/s and now flows about 27 cm/s. It has eroded about 480 cm³ of fine sand and cut the channel down 2.3 cm at its deepest. About 180 cm³ has been dropped on land. A delta reaches 20 cm past the old shoreline.
Erode, carry or drop? The Hjulström diagram
Point at the diagram to read off what happens to any grain size at any speed.
What each sediment does at 27 cm/s
The water’s speed in the channel now. Faster water erodes; slower water drops what it carries.
- TransportedClay (0.002 mm)Picked up above about 110 cm/s; settles below about 0.0004 cm/s.
- TransportedSilt (0.02 mm)Picked up above about 46 cm/s; settles below about 0.04 cm/s.
- ErodedFine sand (0.2 mm)Picked up above about 22 cm/s; settles below about 2.3 cm/s.
- TransportedCoarse sand (1 mm)Picked up above about 30 cm/s; settles below about 13 cm/s.
- TransportedGravel (3 mm)Picked up above about 53 cm/s; settles below about 25 cm/s.
How a river erodes, carries and drops sediment
Running water is one of the main forces that shape Earth’s surface. It wears valleys into mountains and builds new land at the coast. Everything it does comes down to three processes, and which one happens depends on how fast the water flows and how big the grains are.
- Erosion. Fast water drags on the grains of the stream bed. When the drag is strong enough, it lifts or rolls them away, and the channel gets deeper and wider. In the sandbox this happens where the water flows faster than the grain’s erosion speed.
- Transport. Once grains are moving, they keep going even if the water slows a little. Big grains roll and bounce along the bed; fine grains float in the water and make it look muddy. A river can only carry so much: the faster it flows, the more it can carry.
- Deposition. Where the water slows down, it can’t carry everything, so it drops the extra: the biggest grains first. Rivers slow on the inside of bends, where the land flattens out, and most of all where they flow into a lake or the sea. That is where you find sandbars, floodplains and deltas.
Reading the Hjulström diagram
In 1935 the Swedish geographer Filip Hjulström drew a famous diagram from river measurements. It shows grain size along the bottom and water speed up the side, both on log scales where each step is ten times bigger.
- Above the top curve, grains are eroded: picked up from the bed.
- Between the curves, grains that are already moving are transported, but grains lying on the bed stay put.
- Below the lower curve, moving grains settle out: they are deposited.
The surprise is the shape of the top curve. It dips lowest for sand: fine sand is picked up at about 22 cm/s. Gravel needs faster water because it is heavier. But silt and clay, which are far smaller and lighter than sand, also need faster water, because their tiny flakes stick together into a smooth layer. Once clay is floating, though, it sinks so slowly that almost any current keeps it moving, which is why muddy rivers stay muddy.
| Sediment | Typical grain size | Picked up above about | Settles below about | What it is like |
|---|---|---|---|---|
| Clay | 0.002 mm | 110 cm/s | 0.0004 cm/s | Tiny flakes that stick together |
| Silt | 0.02 mm | 46 cm/s | 0.04 cm/s | Feels like flour |
| Fine sand | 0.2 mm | 22 cm/s | 2.3 cm/s | Like beach sand |
| Coarse sand | 1 mm | 30 cm/s | 13 cm/s | Gritty, like sugar crystals |
| Gravel | 3 mm | 53 cm/s | 25 cm/s | Small pebbles |
Grain-size names follow the Wentworth scale: clay is smaller than 1/256 mm, silt up to 1/16 mm, sand up to 2 mm, and gravel bigger than that.
Why slope and water flow matter
Water runs faster down a steeper slope, and a bigger flow runs faster too, because deeper water is held back less by the bed. Engineers work out river speeds with Manning’s equation, and the sandbox uses it for a channel 3 cm wide. In this tray, doubling the water flow makes the water about 32% faster, and doubling the slope makes it about 23% faster. Faster water erodes more and can carry more, so steep mountain streams cut deep valleys while slow lowland rivers drop sediment on their floodplains.
Rivers also change their own slopes. A river that carries away more than it receives cuts down, and its slope gets gentler, until the water is just too slow to pick up more grains. Rivers can’t cut much below the level of the sea they flow into, which geologists call their base level. A river that is given more sediment than it can carry does the opposite: it drops the extra and builds up, steepening its slope, the way an alluvial fan grows where a mountain stream reaches flat ground. Try both in the sandbox: clear water on a steep slope, then lots of sediment fed in on a gentle one.
Worked example: the sandbox’s opening setup
The sandbox opens with fine sand (0.2 mm), a 4% slope, 3 liters of water a minute and no extra sediment fed in.
1. Will the water erode?
Manning’s equation gives a starting speed of about 37 cm/s. On the Hjulström diagram, fine sand is picked up above about 22 cm/s, so the water erodes.
2. After 10 minutes
The river has eroded about 480 cm³ of sand, about 2 cupfuls, and cut down 2.3 cm at its deepest, near the inlet where clear water first hits the sand. About 310 cm³ was dropped in the sea, building a delta 20 cm long, and about 180 cm³ was dropped in the channel just before the sea, where the water starts to slow.
3. The river slows itself down
As the channel cuts down, its slope gets gentler and the water slows: to about 27 cm/s after 10 minutes. By about 20 minutes it has slowed to just under the 22 cm/s needed to lift fine sand, and the channel stops cutting down. The river has reached a balance with its slope.
Change one thing at a time in the sandbox, such as the slope, and compare the numbers. That is how scientists run a fair test.
Common misconceptions
- “The smallest grains are always the easiest to wash away.” Not once they have settled. Clay and silt stick together, so they need faster water than sand to be picked up. They are only easy to carry once they are already floating.
- “Rivers only erode.” Every river erodes, transports and deposits. It erodes where it is fast and deposits where it slows, often in the same bend.
- “Erosion and weathering are the same thing.” Weathering breaks rock into pieces where it is. Erosion moves the pieces somewhere else.
- “Faster water always builds a bigger delta.” Only if the river carries grains that settle near the coast. Fine silt and clay can stay in the water and drift far out to sea instead.
- “A river keeps cutting down forever.” As it cuts down, its slope gets gentler and the water slows. It can’t cut much below the level of the sea or lake it flows into.
What this model simplifies
The sandbox is a model of a stream table, and a stream table is itself a small model of a river valley: a real river takes years to thousands of years to do what the tray does in minutes, and the animation runs faster still. The water’s speed comes from Manning’s equation for a channel that stays 3 cm wide, with one roughness for every sediment. The model works along the middle of the channel: the bends are drawn to show where water is faster and slower (about 25% either way), but depths and volumes are worked out down the middle, with banks that always slump at the same angle. Each run uses one size of sediment, not a mix, with no plants, rain or rock except the bottom of the tray. The erosion and settling speeds are approximate readings of Hjulström’s diagram for rivers about 1 m deep; in the shallow water of a real stream table, grains start moving at somewhat lower speeds, and real thresholds vary a lot. How much sediment the water can carry follows a simple rule chosen so the tray moves a few hundred cubic centimeters of sand in ten minutes, as real stream tables do. Volumes are loose, wet sediment, rounded to two figures.
Erosion and deposition activity
A one-page printable worksheet for grades 6–10, about 15 minutes: students run four stream-table setups, record the water speed, the sediment eroded, the deepest cut and the delta length, then answer six questions, including two that read the Hjulström diagram. It connects to NGSS MS-ESS2-2 and HS-ESS2-5, and works as a measuring-erosion lesson (4-ESS2-1) with younger students.
Share the sandbox with your class in one click. Students need no accounts or logins.
Share to Google ClassroomRiver erosion questions
What is the difference between erosion, transport and deposition?
Erosion is when moving water picks up sediment and carries it away from where it was. Transport is the sediment being carried along: rolled, bounced or floating in the water. Deposition is when the water slows down and drops it. One river does all three: it erodes where it is fast, carries sediment downstream, and deposits it where it slows, such as on the inside of bends and where it meets a lake or the sea.
What does the Hjulström curve show?
The Hjulström curve (or Hjulström diagram) shows how fast water must flow to pick up grains of each size, and how slow it must get before moving grains settle out. Sand is the easiest to pick up, at about 20 cm/s. Gravel needs faster water because it is heavy; silt and clay also need faster water, because their tiny grains stick together. Filip Hjulström published it in 1935; the curves are approximate, because real thresholds depend on water depth and how firmly the grains are packed.
Why is sand easier to erode than clay?
Clay grains are thousands of times smaller than sand grains, but they are flat flakes that stick to each other and settle into a smooth, firm layer. Water slides over that layer instead of grabbing single grains, so it takes much faster water to tear clay up. Sand grains are loose and stick out into the flow, so a gentle current can roll them along.
How does a river build a delta?
Where a river flows into a lake or the sea, it spreads out into still water and slows down. Slower water can carry less sediment, so the river drops it: the coarsest grains first, close to the mouth, and finer ones farther out. Layer by layer the deposit builds up to the water surface and pushes the shoreline outward. The Mississippi and the Nile have built huge deltas this way.
Why do rivers have point bars and cut banks?
As water goes around a bend it flows faster on the outside and slower on the inside. The fast water on the outside erodes the bank, making a steep cut bank. The slow water on the inside can’t carry as much, so it drops sand and gravel there, building a gently sloping point bar. Over time this makes bends grow and move, which is why rivers meander.
What is a stream table?
A stream table is a tilted tray of sand or soil with water running down it. Geologists and teachers use stream tables to watch erosion and deposition happen in minutes instead of years. You can build one with a plastic storage box, a hose or a jug of water, and a bucket under a drain hole. This sandbox lets you try many settings quickly, and the same experiments work in a real one.
Can I use the River Erosion Sandbox in class?
Yes. It is free with no sign-up and runs on school Chromebooks. Use the printable activity to compare four setups and read the Hjulström diagram, then let students try Challenge mode, eight questions answered with the sandbox. Teachers can find the answer key on the For teachers page.