deposition

Alluvial fans are cone-shaped landforms that form at the base of slopes, near the mouth of a mountain canyon. Material is deposited by a river as it emerges from the gorge. There is a reduction in the velocity of the river, and it drops some of its load. The heavy, coarse material is deposited first, and the finest material is deposited later, further from the mountain face, helping to produce the conical slope.

In earth science, the dumping of the load carried by a river, glacier, or the sea. Deposition occurs when the river, glacier, or sea is no longer able to carry its load for some reason, for example a shallowing of gradient, decreasing speed, decreasing energy, decrease in the volume of water in the channel, or an increase in the friction between water and channel. Glacial deposition occurs when ice melts.

Many types of deposition are found along the course of a river.

Alluvial fans These are found in semi-arid areas where mountain streams enter a main valley or plain at the foot of the mountains. The sudden decrease in velocity causes the stream to deposit its load. Smaller fans are common in glaciated areas at the edge of major glacial troughs, particularly at the base of a hanging valley.

Riffles These are small ridges of material deposited where the river velocity is reduced midstream. If there are many riffles the river is said to be braided.

Levees and flood plain deposits These are formed, over a long period of time, in places where a river regularly bursts its banks. Water loses velocity quickly leading to the rapid deposition of coarse material near the river channel edge to form embankments, called levees. Finer material is carried further away and deposited on the flood plain.