Discuss landforms resulting from the deposition of sediment by streams.

Streams shape Earth’s surface not only by cutting into it, as we have seen, but also by transporting and depositing sediments downstream. Many important natural fluvial landforms are built by accumulations of stream-deposited sediments.

As long as stream capacity and stream competence are maintained, the stream load will continue to be transported. Where stream capacity or stream competence is reduced, material carried by the stream begins to settle out. Where a stream stops flowing at base level, it loses all kinetic energy, and all the material it carries, except its dissolved load, settles out.

Sediments are sorted by size as they are deposited by streams. The heaviest material in transport settles out first. The largest boulders are high in the drainage basin because they are too heavy to be carried very far. They eventually break into smaller and lighter fragments that will move more easily. Cobbles and pebbles will be moved farther downstream than boulders. Sand, silt, and clay settle out of the water still farther downstream.

To get a sense of how the sorting process works, imagine a white quartz boulder in a stream channel where all other rocks are dark. Because the boulder is white, we can easily see it and follow it from a high mountainous area and on downstream as it is broken into ever-smaller fragments over time. As the stream overflows its banks, these materials will be deposited along the course of the stream (Figure 16.24). Sediments deposited on a floodplain by a stream are referred to as alluvium.

If a stream’s capacity and competence become insufficient to carry all the sediments that enter the stream, the sediments will settle out in the stream channel and accumulate. The buildup of sediments in a streambed is called aggradation. Aggradation in the low-gradient sections of a stream forms a floodplain (Figure 16.25).

Sediment input into a stream channel increases where vegetation is removed by fire or human activity. Sediments from unpaved roads in steep terrain can wash into streams as well. During the Gold Rush in California in the middle 1800s, miners blasted the hillsides with water in search of gold. This hydraulic mining eroded the slopes and overwhelmed the streams with sediments (Figure 16.26).

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Aggradation also occurs where streams flow out of glaciers. Glaciers flow downhill and grind the bedrock beneath them into rock fragments (see Section 17.3). The glacier’s outlet stream can move this material only during peak summer flow periods. At most times, the stream occupies only a small portion of the streambed, flowing around elongated islands of gravel and sand. Such streams that form intertwining channels around sediments in the streambed are called braided streams (Figure 16.27).

Streams may also lose energy after they flow through a steep-walled, V-shaped canyon that abruptly opens up onto a flat valley floor. While in the canyon, the stream has a steep gradient and high velocity, but where the stream exits the canyon, its velocity slows, its capacity and compentency are reduced, and it drops its sediment into an alluvial fan. An alluvial fan is a gently sloping accumulation of sediment that is deposited at the base of a mountain by an ephemeral stream in arid regions (Figure 16.28).

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A meander is a looping bend in a stream channel that forms on a floodplain. Meanders develop when the fastest-flowing portion of the stream, called the thalweg, erodes one bank more than the other. That bank, called a cut bank, where erosion exceeds deposition, becomes the outside edge of the meander. On the opposite bank, where the flow is slower, deposition exceeds erosion, and accumulations of silt, sand, and gravel, called a point bar, form.

Stream meanders are spatially dynamic features. Through time, stream channels shift across the floodplain, forming new meanders and abandoning old ones (Figure 16.29). In places where a meander is cut off from the stream channel and the channel is abandoned, a water-filled oxbow lake forms.

When a stream overflows its banks and floods, the standing sheet of water on the floodplain has no energy to carry its suspended load of silts and clays. These sediments settle out of the floodwaters and form deposits of alluvium. The smallest particles (clay and silt) are carried farthest away from the stream channel and accumulate on the floodplain. The largest particles (sand and gravel) accumulate near the stream channel in raised barriers that parallel the stream channel, called natural levees (Figure 16.30).

Tributaries entering a floodplain sometimes flow parallel to the trunk stream because they cannot flow over the higher floodplain and natural levees bordering the trunk stream channel. As a result, permanently flooded areas called wetlands (also called swamps, backswamps, or bayous) and yazoo streams form (Figure 16.31).

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All streams stop flowing at base level. Water in all streams eventually finds the oceans, with the exception of those in internal drainage systems (see Section 16.1).

Along the way to the ocean, natural dams (such as beaver dams or landslide debris) may form a temporary base level (also called a local base level) as a pond or lake develops behind the obstruction. Over time, fluvial erosion will eventually cut through the obstruction, and the stream will flow freely again.

Streams are also dammed by people. Damming creates an artificial lake called a reservoir. Stream capacity and stream competence are zero in lakes and reservoirs, and as a result, all sediments settle out. At the end of this chapter, the Geographic Perspectives further explores the problem of sedimentation in reservoirs.

A delta is an accumulation of sediments that forms where a stream reaches base level. Because of the amount of material deposited in deltas, the stream channel often becomes blocked, forcing the stream into a new channel around the sediments, as also occurs in a braided stream. In a delta, the trunk stream can shift and form a smaller branching network of streams called distributary channels, or distributaries. As shown in Figure 16.32, tributaries occur at the headwaters of a drainage basin, and distributaries occur in the delta.

Stream deltas are so named because the triangular shape of some deltas resembles that of the Greek letter delta (Δ), but not all deltas take this shape (Figure 16.33). A delta’s shape depends mainly on the stream’s suspended load and on the influence of currents at the stream’s mouth. Arcuate deltas are triangular and often have a braided or branching pattern of distributaries. The distributaries in a bird’s foot delta resemble toes on a bird’s foot.

The largest deltas form where coastal ocean currents are at a minimum and major streams carry a heavy load of suspended sediments. Where coastal currents are strong, however, river mouths have small or no deltas because the currents carry sediments away. The Columbia River, on the border of Oregon and Washington, the Congo River, in western Africa, and the Amazon River are examples of major streams with no coastal deltas.

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In many deltas, the land elevation drops due to natural subsidence: the lowering of land elevation through the compaction of sediments. Deltas keep pace with natural subsidence as their streams continually deposit new sediments that build up the land. As long as sediments are renewed by fluvial deposition, there is an equilibrium between subsidence and sedimentation.

When artificial levees are built around a stream to control water flow, however, this equilibrium is disrupted. With levees in place, the river can no longer flood and form new meanders. The river’s sediments are carried through and past the delta to the seaward edge and are deposited offshore, rather than in the delta. The interior delta becomes sediment starved, and as it sinks, the sea gradually encroaches on the land. Picture This (below) highlights the effects of this process in New Orleans, which is built on the Mississippi River delta.

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