Sediment on the Move: Rivers, Coasts and Where It Settles
Moving water carries grains until it slows, and the size it can carry falls sharply as it does, which is why deposits are sorted. Following a grain from a hillside to a delta explains most river and coastal landforms without needing a separate rule for each one.
What a learner can do afterwards
- Explain why a slowing current deposits coarse grains before fine ones
- Account for a named river or coastal landform as erosion and deposition in one system
- Predict what happens downstream when sediment supply is cut off by a dam or a sea wall
1 · Read
Fast water grips heavy grains, and slowing water lets go. Pebbles and sand roll along in a quick current, but where the flow eases, the load drops with the heaviest grains first. That sorting is why a riverbed changes on its way to the sea: gravel gives way to sand, and sand gives way to mud. Watch any slope after a storm and you will see the grades laid out by speed.
Erosion and deposition are one connected system. What a river tears away upstream becomes what it builds with downstream. Where a river meets the sea it slows hard and dumps its load grain by grain, growing a delta of splitting channels and mud-trapping marshes. Wind works the same way as a river of air: it lifts sand from bare ground and drops it where it slows, so dunes march downwind gust by gust.
Cut the supply and the system bites back. A dam traps the gravel that used to rebuild land below, so the river below erodes instead and the delta stops growing and washes away. A wall shielding one bank can starve beaches further along the coast. Good river plans treat the whole basin as one budget of sediment, balancing safety, farming, and wildlife together.
To predict any stretch, follow one grain in your head. It breaks loose on a hillside, rolls in fast water, drops as the current eases, rests in delta mud, and reaches the sea. Then ask what changed: less supply upstream means erosion wins downstream, every time.
Slowing water sorts and drops its load, and one connected system links erosion to deltas.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.