Grades 3–5 · Earth & Space
How can a river build new land at its mouth?
A river can carry sand, silt and other loose material toward a lake or sea. This material is called sediment. Near the river's mouth, some sediment settles instead of continuing farther into the larger body of water.
In the diagramA river can add sediment faster than it is removedLake or seaSediment depositTop view • shoreline and channels are symbolic8 arrive − 3 removed → 5 addedInvented sediment units over one interval.
The river spreads into that water, and its flow can slow. Larger grains often settle sooner than very fine ones. Repeated deposits can build up the bottom and eventually make land. This growing deposit at a river mouth is called a delta.
New sediment does not always stay. Waves, tides and other currents can carry it away. A delta grows when the river supplies more sediment than those processes remove. In our invented interval, eight sediment units arrive and three are removed, leaving five units added to the deposit.
As deposits build, water may find new paths around them. A river can split into several channels near its mouth. Deltas have many shapes; they are not all neat triangles.
About this explanation
Starting knowledge: Understand a river flowing into a larger body of water and compare amounts added and removed.
Sources
- National Park Service — Delta Landforms
- NOAA — What is an estuary? Delta System