Here's why:
* Rivers carry sediment: Rivers flow from higher elevations to lower elevations, eroding soil and rock along the way. This material, called sediment, is carried downstream by the flowing water.
* Slowing down at the mouth: When a river reaches the ocean, lake, or another body of water, its flow slows down significantly. This slowing causes the sediment to settle out of the water.
* Accumulation over time: Over time, this sediment builds up at the river's mouth, forming a landform called a delta.
Deltas are important ecosystems that provide habitats for many species of plants and animals, and they also play a crucial role in coastal protection.
