A river is continually picking up and dropping solid particles of rock and dirt from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium.
Even small streams make alluvial deposits, but it is in the flood plains and deltass of large rivers that large, gelogically-significant alluvial deposits are found.
The amount of solid matter carried by a large river is enormous. The names of many rivers derive from the color that the transported matter gives the water. For example, the Huang He in China is, literally translated, Yellow river and the Missouri River in the United States is also called Big Muddy. It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Huang He 796 million tons, and the Po River in Italy 67 million tons.
Throughout history many shallow lakes have been filled in with alluvium, leaving fertile plains. Alluvial soils are almost always very fertile. The alluvial mud annually deposited by the Nile has enabled the Egyptians have been able to grow crops since at least the 4th millennium BC without artificial fertilization.