Definition: Water flowing downstream carries along particles from geological structures in its path. These particles are materials like clay, silt, gravel, and sand. The deposit the stream leaves behind is called alluvium. When the stream is flowing downhill it carries most of the sediment along with it, but when it reaches a plain, it deposits it. This sediment tends to make the flat area, or plain, fertile. An alluvial plain is also called a floodplain.
