Alluvium
Alluvium is
soil or
sediments deposited by a river or other running water. The structure formed is called an
alluvial deposit. Flowing water associated with glaciers may also deposit alluvium, but deposits directly from ice are not alluvium (see
glacial till). The word comes from Latin
alluvius, from
alluere, "to wash against."
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
deltas of large rivers that large, geologically-significant alluvial deposits are found.
Alluvium is typically made up of a variety of materials. There are fine particles of
silt and
clay. There are also larger particles of
sand and
gravel, and these are also typically present in a wide range of sizes.
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.
Alluvial deposits often contain valuable ores such as
gold and
platinum and a wide variety of
gemstones.
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 to grow crops since at least the
4th millennium BC without artificial fertilization.
*
Alluvial plain*
Alluvial fan*
Alluvial desert*
Eluvium*
Fluvial
