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.
Slow the river down and its carrying power falls. Gravel tends to stop first, followed by sand. Silt and clay can travel much farther before settling.
Real rivers are messier.
Grain shape, density, turbulence, bed roughness, cohesion and changing discharge all matter.
This experiment highlights the basic sorting pattern rather than reproducing every process.