Stream Deposition

A stream's sediment load is typically deposited, eroded, and redeposited many times in a stream channel, especially during climatic variations such as flooding. Sediments are deposited throughout the length of the stream as bars or floodplain deposits. At the mouth of the stream, the sediments are usually deposited in alluvial fans or deltas, which represent a lower‐energy, more “permanent” depositional environment that is less susceptible to changes in the stream flow.

Bars. Bars form in the middle of the channel or along the banks of a stream at points where the velocity decreases, resulting in the deposition of some of the sediment load. Bars are ridges generally made up of gravel‐ or sand‐sized particles. A subsequent flood event will erode bars, transport the sediments, and redeposit the material as a new bar farther downstream.

Floodplains. Floodplains are level strips of land on the sides of a channel that consist of fine‐grained silt and clay deposited during episodes of flooding. Higher ridges of sand and silt called natural levees are deposited near the edge of the channel. As the water spreads outward from the channel, it loses energy and carries less sediment. The poorly drained and marshy areas behind the levees are called backswamps.

Alluvial fans. Alluvial fans are similar to deltas and are large fanlike accumulations of sediment that form where streams emerge from rugged terrain onto a broad, flatter surface. Stream velocities fall quickly, and the fan is built by continual braided stream activity. Large fans show graded patterns in which the coarsest‐grained materials are deposited at the canyon mouth and the finer‐grained materials spread outward in a fan shape.

Deltas. Sediment deposited at the mouth of a stream usually forms a thick, roughly wedge‐shaped accumulation called a delta, the widest part of which is farthest from the stream mouth. Distributaries are dendritic, shifting channels that spread out across the delta from the main river channel and disperse the sediment load. Sediments on the delta's forward slopes are constantly shaped by water and wind action and redeposited by lake or ocean currents.

Topset beds are nearly horizontal layers of sediment deposited by the distributaries as they flow away from the mouth and toward the delta front. They overlie the sandy foreset beds that compose the main body of the delta, which dip downward from 5 to 25 degrees. These represent the gradual accumulations of sediment deposited over the forward slopes of the delta as it builds progressively outward into the receiving body of water. The bottomset beds are the finest silt and clay particles that are carried out into the deeper water or slide down the delta front into the deeper water.

Braided streams. A braided stream is one in which the water has lost its main channel and flows through a wandering network of rivulets around sandbars. Braided streams typically have wide channels. The braiding generally results from a flood‐deposited midchannel bar that splits the flow. The water is diverted to the sides and erodes stream banks, widening the channel. Streams will usually be braided if they have high bed loads and easily erodible banks. The distributaries in a delta are also braided.

Meanders and oxbow lakes. The course of a stream bed can be continuously affected by erosion on the outside of a curve and deposition on the inside. This process will transform a gentle curve into a hairpinlike meander. Meanders continuously change location as they swing back and forth across a valley or migrate downstream over time. An oxbow lake is formed when a meander begins to close on itself and the stream breaks through and bypasses the meander (Figure ). The cut‐off meander is dammed by sedimentary deposits in the new channel—resulting in a body of water that is shaped roughly like a U (the shape of an oxbow, a piece of wood used to harness an ox). Oxbow lakes mark the location of former stream channels.

Figure 1

A Meandering Stream and Oxbow Lake