Alluvial soils or alluvium (Latin alluvio, stranding ') are young alluvial soils of marine coastal, river and lake shores. Also glacier can be deposited alluvial soils.
Due to the flow of a watercourse occurs in the course of a river to a continuous process of transport and deposition of particles of rock and mud. Doing more particles are entrained than are deposited there again on fast-flowing parts of the river. Only at flow poorer places such as the deltas which are then redeposited and form the alluvial soils. Also in periodically flooded areas there will be a significant deposit.
The amount of solids transported and deposited by large rivers is enormous. The names of many rivers relating eg to the color of the material, which is transported from the river, which gives it its color. So does the name of the Chinese River Huang He translated the Yellow River and the Missouri River in the U.S. has the nickname Big Muddy (literally: great Muddy ). It is estimated that the Mississippi River transported in the United States 406 million tonnes of unconsolidated sediment, the Huang He even about 796 million tons and the Italian Po still around 67 million tonnes.
Although smaller rivers can cause such deposits, it is the alluvial soils in large river deltas that have a certain geological significance. These soils typically consist of a variety of different materials. The fines, also called silt, composed mainly of sand and clay. However, larger particles such as gravel and boulders are often present in a wide particle size range.
In addition, these alluvial soils may contain large amounts of ores, precious metals like gold and platinum, and precious stones. Such so-called placer deposits can be very productive.
By also usually high proportion of organic compounds alluvial soils are rich in nutrients in general. Thus for example, the annual deposition of sediments on the banks of the Nile to the fact that is cultivated in Egypt as early as the 4th millennium BC cereal without artificial fertilization.
Over time, many shallow lakes were fed flooded with alluvium, which very fertile land have emerged.