Why do river deltas grow outward into the sea?
Quick Answer
Rivers slow down as they reach still water, dropping sand, silt, and clay. Over time, the deposited sediment builds new land at the river mouth.
The Full Story
A delta forms where a river loses speed entering a lake, sea, or ocean. As the current slows, it can no longer carry as much sediment, so the particles settle out and accumulate. Repeated flooding and shifting channels spread the sediment into a fan or triangle shape. Deltas can be highly fertile because they receive nutrient-rich deposits, but they are also vulnerable to flooding and sea-level rise. Famous examples include the Nile Delta and the Mississippi Delta.
Key Facts
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