Recall that a stream continually picks up sediment in one part of its channel and deposits it downstream. These small-scale channel deposits are most often composed of sand and gravel and are commonly referred to as bars. Such features, however, are only temporary, and the material is picked up again and eventually carried to the ocean. In addition to sand and gravel bars, streams also create other depositional features that have somewhat longer life spans. These include deltas, natural levees, and alluvial fans.

Deltas

A delta forms where a sediment-laden stream enters the relatively still waters of a lake, an inland sea, or the ocean (Figure 1). As the stream’s forward motion slows, sediments are deposited by the dying current. As the delta grows outward, the stream’s gradient continually lessens. This circumstance eventually causes the channel to become choked with sediment deposited from the slowing water. As a consequence, the river seeks a shorter, higher-gradient route to base level, as illustrated in Figure 1. This illustration shows the main channel dividing into several smaller ones, called distributaries. Most deltas are characterized by these shifting channels that act in an opposite way to that of tributaries.

Rather than carry water into the main channel, distributaries carry water away from the main channel. After numerous shifts of the channel, a delta may grow into a roughly triangular shape like the Greek letter delta (Δ), for which it is named. Not all deltas exhibit this idealized shape, however. Differences in shoreline configuration and in the nature and strength of wave activity result in a diversity of shapes. Many large rivers have deltas extending over thousands of square kilometers. The delta of the Mississippi River is one example. It resulted from the accumulation of huge quantities of sediment derived from the vast region drained by the river and its tributaries (Figure 2). Today, New Orleans rests where there was ocean less than 5000 years ago. Figure 3 shows that portion of the Mississippi delta that has been built over the past 6000 years. As you can see, the delta is actually a series of seven coalescing subdeltas. Each formed when the river left its existing channel in favor of a shorter, more direct path to the Gulf of America (Gulf of Mexico). The individual subdeltas interfinger and partially cover one another, producing a very complex structure. The present subdelta, called a bird-foot delta because of the configuration of its distributaries, has been built by the Mississippi in the past  years.

The shape and size of a delta is dependent on the interaction between the river’s sediment supply, and the action of waves and tides. Deltas like that of the Mississippi River are dominated by sediment input. Conversely, the world’s largest river delta, the Ganges-Brahmaputra Delta of Bangladesh and India is dominated by strong tidal currents and is triangular and arc-shaped. And the wave-dominated São Francisco Delta of Brazil is rather small with well-developed beaches and only a few distributary channels.

Natural Levees

Some rivers occupy valleys with broad floodplains and build natural levees that parallel their channels on both banks (Figure 4). Natural levees are built by successive floods over many years. When a stream overflows its banks, its velocity immediately diminishes, leaving coarse sediment deposited in strips bordering the channel. As the water spreads out over the valley, a thin layer of fine sediment is deposited over the valley floor. This uneven distribution of material produces the very gentle slope of the natural levee.

The natural levees of the lower Mississippi rise about 6 meters (20 feet) above the floodplain. The area behind the levee is characteristically poorly drained because water cannot flow up the levee and into the river. Marshes called back swamps result. A tributary stream that cannot enter a river because levees block the way often has to flow parallel to the river until it can breach the levee. Such streams are called yazoo tributaries, after the Yazoo River, which parallels the Mississippi for more than 300 kilometers (about 190 miles).

Alluvial Fans

Alluvial fans typically develop where a high-gradient stream leaves a narrow valley in mountainous terrain and comes out suddenly onto a broad, flat plain or valley floor (Figure 5). Alluvial fans form in response to the abrupt drop in gradient combined with the change from a narrow channel of a mountain stream to less confined channels at the base of the mountains. The sudden drop in velocity causes the stream to dump its load of sediment quickly in a distinctive cone- or fan-shaped accumulation. As illustrated in Figure 6, the surface of the fan slopes outward in a broad arc from an apex at the mouth of the steep valley. Usually, coarser material is dropped near the apex of the fan, while finer material is carried toward the base of the deposit.

Between rainy periods in deserts, little or no water flows across an alluvial fan, which is evident in the many dry channels that cross its surface. Thus, fans in dry regions grow intermittently, receiving considerable water and sediment only during wet periods. Because steep canyons in dry regions are prime locations for debris flows, many alluvial fans have debris-flow deposits interbedded with the coarse alluvium.

• A delta may form where a river deposits sediment in another water body at its mouth. The partitioning of streamflow into multiple distributaries spreads sediment in different directions.

• Natural levees result from sediment deposited along the margins of a stream channel by many flooding events. Because the levees slope gently away from the channel, the adjacent floodplain is poorly drained, resulting in back swamps and yazoo tributaries flowing parallel to the main river.

• Alluvial fans are fan-shaped deposits of alluvium that form where steep mountain fronts drop down into adjacent valleys.

• alluvial fans: Fan-shaped deposits of sediment formed when a stream's slope is abruptly reduced.

• back swamps: Poorly drained areas on a floodplain that result when natural levees are present.

• bara: The common term for sand and gravel deposits in a stream channel.

• delta: An accumulation of sediment formed where a stream enters a lake or an ocean.

• distributaries: Sections of a stream that branch off and flow away from the main stream.

• natural levees: Elevated landforms that parallel some streams and act to confine their waters, except during flood stage.

• yazoo tributaries: Tributaries that flow parallel to the main stream because a natural levee is present.