A stream is always doing one of three things to the grains in it. Which one depends on two things: how fast the water moves and how big the grain is.

Moving grains travel two ways, set by size. Fine clay and silt ride high in the water as suspended wash load and barely settle. Coarse pebbles, cobbles, and boulders stay low, rolling and bouncing along the bottom as bed load. Sand crosses over: bed load in a gentle current, lifted into suspension once the water runs fast. Turn on Inspect and tap a grain to read which mode it is in.

Try this

Set a fast current, drop pebbles, and watch them wash downstream. Then press Storm: the flood lifts everything, and as the water drains the grains rain back down heaviest first, sorting into a graded bed. Turn on Pour to keep feeding the selected sediment in.

Every grain size and every speed at once. Up is faster water; right is bigger grains. The dashed line is the speed you set. The orange curve is the speed needed to tear a resting grain loose; the blue curve is the slowest speed that still keeps it moving. Tap a class to mark where it sits.

What the current does to each grain size at 100 cm/s. Tap a row to highlight it on the map.

Why clay is stubborn

Sand near half a millimetre erodes at the lowest velocity of any grain, roughly 25 to 30 cm/s. Clay particles are tiny but stick together, so dislodging them needs a current as fast as the one that moves cobbles. Once lifted, clay stays suspended at almost any velocity, which is how it travels all the way to the sea.

A classroom interactive for the NY Regents Earth Science unit on stream erosion and deposition. The settling curve and particle classes follow the ESRT page 6 chart, and the erosion curve follows the standard Hjulström relationship.

Grain sizes in the channel are schematic, not to scale, since real diameters span from clay near 0.0001 cm to boulders over 25 cm.