Sedimentology

Sediment Sorter

Rivers are sorting machines. Change the flow, release a mixed load of sediment, and watch gravel, sand, silt and clay take very different journeys.

Cartoon sediment grains racing downstream, with larger gravel stopping sooner and finer grains travelling farther.

Flume controls

Set the river.

Flow speed 0.80 m/s

Faster water can keep larger grains moving.

Channel slope 1.0%

A steeper slope gives the flow more energy.

Sediment mix

Gravel 20% Coarse sand 20% Fine sand 20% Silt 20% Clay 20%

The mix is normalized automatically so each run releases about 720 grains.

Ready.

Set the river and sediment mix, then start a run. Sediment enters steadily from the source for 8 seconds.

Grains Gravel Coarse sand Fine sand Silt Clay

Erosion erosion surface / contact

Flow energy in this model In this teaching flume, flow begins to spread downstream, so local velocity and transport energy gradually fall before dropping further in the quiet-water basin.

Sediment source Flow spreads + slows Quiet-water basin

Upstream
Highest local transport energy

Spreading reach
Flow slows and transport competence falls

Quiet basin
Lowest energy; finest sediment settles last

Transport meter Near-source conditions

Gravel

Settling

Coarse sand

Moving

Fine sand

Moving

Silt

Suspended

Clay

Suspended

How sediment moves Transport mode depends on grain size and flow energy.

Bedload / traction

Larger grains roll or slide along the river bed. Gravel commonly travels this way.

Saltation

Sand grains repeatedly lift from the bed and bounce downstream before touching down again.

Suspension

Fine particles are carried within the water column. Silt and clay can remain suspended for much longer.

Deposition

Deposition is not a transport mode. It happens when the flow can no longer keep a grain moving.

This teaching model focuses on bedload and suspended sediment. Transport responds to local flow energy and grain properties; dissolved material is not represented.

Sorting strength Run experiment How clearly coarse and fine grains separate.

Upstream deposit No deposit yet Dominant settled grain near the source.

Fine sediment to basin 0% Fine sand, silt and clay reaching the quiet-water zone.

Downstream pattern Waiting Whether grain size fines downstream overall.

Depositional sequence Build the sedimentary record from repeated periods of steady sediment input.

0 layers

No layers yet. Run the flume, change the flow, then run it again.

Each completed run adds a thin, laterally continuous lamina. Grain classes overlap rather than forming separate piles, including a broad gravel-to-coarse-sand transition. Their final positions emerge from settling rate, entrainment threshold and the downstream change in local flow energy.

What just happened?

Moving water sorts sediment.

Flowing water can erode, transport and deposit sediment. In this simplified flume, all grains behave like similarly dense rock fragments, so grain size is the main difference. Larger grains need more energetic flow to stay in motion, while very fine particles can remain suspended for much longer.

As flow spreads toward the downstream basin, local transport energy falls. Coarser grains settle faster and need stronger flow to remain mobile, so they tend to deposit earlier. Finer grains stay suspended longer and travel farther before settling. Repeat the experiment under different flow conditions and those deposits can stack into a visible sequence of layers. A later energetic flow can scour the exposed bed and create an erosion surface. If deposition resumes, younger sediment lies above that surface, preserving the erosional contact inside the sedimentary sequence.

Why does sediment fine downstream here? In this teaching flume, flow spreads toward the basin, so local velocity and transport energy gradually decrease. Coarser grains also settle faster and require stronger flow to stay mobile. Fine grains remain suspended longer and are carried farther before the weakening flow can no longer transport them.

Does every real river weaken downstream? No. This energy gradient is a feature of this simplified flume. In natural rivers, discharge can increase downstream, slope often decreases, and velocity and stream power vary from reach to reach.

Real rivers are messier. Grain shape, density, turbulence, bed roughness, cohesion and changing discharge all matter. The layer thicknesses and erosion thresholds here are deliberately simplified so students can see sorting, deposition, reworking and erosion clearly rather than reproduce a real river quantitatively.

Challenges

Can you make the river behave?

1. Sort the sediment Keep gravel near the source while clay reaches the basin.

2. Move the gravel Increase the river's energy until gravel is transported.

3. Build the delta Carry fine sediment into the quiet-water basin and build a visible fine-grained deposit.

4. Build a sequence Make at least three layers using different flow speeds, slopes or sediment mixes. Can you read the changing conditions back from the stack?

5. Create an erosional surface Build two or more layers, then raise the river energy enough to scour them. Let younger sediment bury part of the dashed erosion surface and preserve it as a contact in the sequence.

Part of the Dig Earth Studio Lab

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