Which Landform Represents a Flowing Moving Stream of Water That Feeds Into an Ocean Coastline

Identify and describe channel types and sedimentary loads formed and carried by rivers and streams.

In this section, you will learn the different types of streams. You will also learn the different types of stream loads.

What You'll Learn to Do

• Understand the different types of rivers and streams, as well as processes associated with them.
• Understand the processes of erosion and sediment transport and deposition

Types of Streams and Rivers

Streams have a major role in geology. Streams sculpt and shape the earth's surface by eroding, transporting, and depositing sediment. By eroding sediment from uplifted areas and creating landforms made of deposited sediment in lower areas, streams shape the earth's surface more than glaciers do, more than waves on a beach do, and far more than wind does.

What Are Streams?

A stream is flow of water, driven by gravity, in a natural channel, on land. A small brook in a meadow and the Amazon River are both streams. It is interesting to watch water on a recently bulldozed construction site with a slope. At first the water saturates the ground and begins to flow downhill across the surface of the slope in a thin sheet. Soon, the water excavates small channels, known as rills, in the dirt. Rills coalesce to form larger channels. A network of streams, including tributaries, has formed. If not prevented, the channels may continue to deepen and erode soil from the construction site.

Over longer intervals of time the same processes we've imagined on the construction site have built systems of streams and stream valleys on the surface of the earth. Most valleys on earth are the product of streams. Streams erode dirt and rocks, transport the sediment, and redeposit it in new locations, shaping the earth's surface into a system of stream valleys.

Streams flow downhill due to the force of gravity. The higher the hill, the more gravitational energy there is to drive the stream. Where the slopes are steepest and the hills the highest, the streams will be the most energetic and the rate of erosion will be fastest.

Drainage Area

The drainage area of a stream encompasses all the land from which surface runoff flows into that stream. A stream drainage area is also called a watershed. Boundaries between stream drainage areas are called drainage divides. What stream drainage do you live in?

Stream Order

It is common for one stream to flow into another. The smaller of the two streams is a tributary of the larger stream. A stream with no tributaries is a first order stream. A stream with only first-order tributaries is a second order stream. A stream that has any second-order tributaries and none higher is a third-order stream, and so on. The Mississippi River is a tenth order stream, one of the highest order streams on earth. As more and more tributaries join together a larger stream network is formed and the master stream, the highest order stream in the system has a discharge that is the sum of all the tributary discharges. When flooding occurs, higher order streams take longer to build up to flood stage than lower order streams and longer for the flood to subside.

Drainage Patterns

A stream system that includes multiple tributaries exhibits a distinct drainage pattern as seen on a map. The drainage pattern depends on the rock types and geologic structures underlying the stream system. Some types of rock are harder and more resistant to erosion than others. If the geology underlying a stream system is fairly uniform—rocks equally resistant to erosion in all directions—a dendritic drainage pattern will develop, as shown in figure 1. A dendritic drainage pattern is the most common type.

Figure 1

If a region is underlain by layered formations of rock that have been folded, and the layers have different degrees of resistance to erosion, the stream valleys will tend to follow the layers of less resistant rock, and the layers of harder rock will become ridges. This results in a trellis drainage pattern, as show in figure 2.

Figure 2.

In some places the geology consists of a single type of rock that is resistant to erosion but the rock contains sets of parallel joints where it erodes more easily. The sets of joints typically intersect each other at high angles. As stream valleys develop in the joint system a rectangular drainage pattern develops, as shown in figure 3. The stream valleys will bend sharply where they switch from following one joint set to another.

Figure 3.

Streams will radiate in all directions from the center of a broad, high-elevation area, such as a composite cone. This is known as a radial drainage pattern.

Figure 4.

Graded Profile

Because streams erode more and remove more sediment where the stream gradient is higher, and deposit more sediment where the stream gradient is lower, a stream will develop a graded profile as shown. The graded profile shows how the elevation of the stream changes along the length of the stream, from its beginning at the highest elevation to its base level where it ends at the lowest elevation it reaches.

Figure 5.

A graded profile starts with a steep slope at the beginning of the stream and tapers to a gentle slope at the base level of the stream. Imagine a hiker following a stream up to its beginning and tracking her progress on a topographic map. She starts out where the stream valley is low and wide and the gradient is gentle so the hiking is easy. As the day goes on, the going gets steeper as she approaches the stream source. Checking the topographic map, she sees that the contour lines are much closer together than they were earlier in the day and farther down the valley. As she approaches the small lake on the side of the mountain where the stream begins, the slope is so steep that nearly loses her footing. She has experienced for herself a typical stream profile that steepens from near the base-level to the origin of the stream.

Lakes and waterfalls are temporary features in a stream drainage. If a lake forms the stream feeding it will slow down and deposit sediment until the lake has been filled with sediment. Where a waterfall forms the energy of the stream going over the waterfall is high will erode the base of the waterfall, causing the waterfall to retreat upstream until a graded profile is established.

Stream Discharge

The amount of water flowing through a stream and speed at which it is moving is expressed as the discharge of the stream. Discharge is measured by multiplying the cross-sectional area of a stream by the average speed of water through that cross-section. Multiplying the area of the cross-section by the average speed of the water results in units of volume/time. For example, if a stream has a cross-sectional area of 150 square feet (ft²) and is moving at an average speed of 10 feet per second (ft/s), multiplying the area and the speed results in a discharge of 1,500 cubic feet per second (ft³/s).

Sediment Load

The water flowing in streams erodes, transports, and deposits sediment. Most rocks and minerals are much denser than water. Sufficient energy in the stream is required to dislodge rocks from the earth and move them. The faster a stream flows, the more energy it has and the larger pieces of sediment it can transport. The competence of a stream refers to the maximum size of the pieces of sediment it can move. Faster-moving water has greater competence and can move larger pieces of sediment.

The capacity of a stream is the total amount of sediment it can move. Capacity depends on how fast the stream is moving and its total discharge. As a stream slows down, its competence and capacity are reduced. The stream starts to deposit sediment, beginning with the largest pieces. Several factors cause a stream to slow down, including the stream channel widening, the stream overflowing its banks and spreading into a floodplain, the stream gradient (downhill slope) diminishing, and the stream emptying into a larger, slower moving body of water.

Streams transport clastic sediment in two ways depending on the size of the particles. The coarser sediment is called bed load and consists of particles too large to be suspended in the moving current of water for an extended length of time. The bed load particles, the largest particles of sediment transported by a stream, spend most of their time on the bottom of the stream channel, rolling, sliding, or bouncing downstream in fits and starts. The finer sediment is called suspended load and consists of particles small enough to be suspended in the moving current of water. Suspended load moves at about the same rate as the flowing water. In a muddy river, the mud is suspended load

Floodplains

Streams build floodplains through a combination of erosion and deposition at lower gradient stretches of stream valleys. Although a floodplain has a general downhill slope consistent with the overall stream gradient, a floodplain is relatively flat.

Floodplains are filled with sediments spread by the stream. These sediments are known as alluvium. Because alluvium is loose material that is easy for the stream to erode and redeposit, the location of a stream channel in a floodplain changes frequently.

Meanders

A stream running down a slope, even the gentle slope of a floodplain, will seldom follow a straight path for very long. Depending on the distribution of sediments and turbulence of the stream, one side of the channel may erode more easily than the other. The stream will migrate toward the area undergoing erosion, developing a curve in that direction. Once the stream channel has begun to curve, the energy of the water is concentrated on the outside of the curve.

The diagram shows a stretch of stream channel with a significant bend, also known as meander. The blue line shows how erosive energy is concentrated along the outside of each bend in the stream. As erosion occurs on the outside bank of a meander, deposition occurs on the inside bank where the water slows and drops sediment.

Figure 6.

The diagram below shows two well-developed meanders that have formed in a stream. Along each meander, the outer stream bank that is being cut into by erosion is called a cut bank. The inner bank, which has grown by accretion of deposited sediment, is called a point bar.

Figure 7.

In a stream, meanders enlarge and migrate downstream because the stream continually erodes its cut banks and grows its point bars. The diagram below shows the enlargement and downstream migration of a meander in a stream channel. As the meander is enlarged, its neck gets narrower. Eventually, the stream may cut through the neck of the meander, either as a result of gradual erosion and channel migration, or abruptly during high water and flooding. Once the stream has cut through the neck of the meander, the openings get filled with sediment dropped by water that slows down as it enters from the main stream. The sediment deposits will separate the cut off meander from the river channel and turn it into an oxbow lake. As the years go by, the oxbow lake will eventually be completely filled in with sediment because it is a low spot on the floodplain where any water that enters, such as during flooding, will come to a standstill and deposit its sediment load.

Figure 8.

Entrenched Meanders

Typical meandering stream channels flow through broad flood plains full of alluvial sediment. However, in some situations meanders may cut directly into bedrock. A meander that has cut into bedrock is known as an incised or entrenched meander. In contrast to meanders in alluvium that erode and migrate rapidly or get cut off at the neck abruptly, entrenched meanders are relatively fixed. This is because entrenched meanders are walled in by bedrock on both sides and have little floodplain to easily erode and redeposit.

Entrenched meanders form as a result of tectonic uplift of the stream drainage area. The uplift increases the gravity-driven energy of the stream causing it to incise rapidly down through the flood plain alluvium into the bedrock beneath. Entrenched meanders are striking landscape features because they are unusual and they provide strong evidence of tectonic activity in a region. Classic examples of entrenched meanders include the Goosenecks of the San Juan River, which are incised in the Colorado Plateau east of the Grand Canyon, and a stretch of the Yakima River with entrenched meanders, which are incised in a recently uplifted ridge of basalt in the Columbia Plateau of eastern Washington state.

Braided Streams

Rather than a single channel, some streams have multiple channels that weave in and out of each other forming what is known as a braided stream. Braided streams are associated with excessive amounts of sediment entering a stream system. Valleys draining alpine glaciers are common settings for braided streams. The glaciers deposit more sediment into the meltwater stream system than a stream of that discharge has capacity to transport in a single channel system. Braided stream systems are indicators that there is an additional source of sediment in the system besides the stream itself. Sources of excess sediment that lead to braided streams include glaciers, eruptions of pyroclastic material by volcanoes and landslides.

Flooding and Flood Frequency

Streams flood. Flooding is a normal part of stream behavior. The rate at which streams erode, transport, and deposit sediments greatly increases during flooding. A flood occurs when the water depth in a stream exceeds the depth of the stream channel and spreads beyond the stream channel onto the surrounding land. When a stream completely fills its channel it is said to be at bankfull stage. When a stream surpasses bankfull stage it is said to be in flood. When the flood spreads widely enough to cause property damage, a stream is said to be at flood stage.

Different streams have different flood behaviors. Some rivers tend to have an annual flood associated with a rainy season or snowmelt season in the higher part of the drainage area. Some streams only flood sporadically, often with years between floods. In the United States all the larger streams have their flow measured at gaging stations that are installed and monitored by the US Geological Survey. Records of stream flow must be kept for at least 30 years to be statistically valid. The likelihood of a flood of a specified magnitude occurring on a stream in a given year can be calculated from the stream flow data. The magnitude of a flood that has a 1 out of 100 chance of happening in a given year—also known as the 100-year flood—is calculated for a stream from it record of stream flow. The 100-year flood magnitude, and maps that show what would be covered by a 100-year flood, are key elements in granting permits for land use and building construction in floodplains.

Low order streams may flood rapidly during or immediately after heavy rainfall. Such rapid floods are called flash floods. Flash floods are often unpredictable. If a river drains through a canyon from a high elevation area, people may be walking along a stream in the canyon in the sunshine and not realize that a thunderstorm is occurring at the stream's source. They may be surprised by a flash flood sweeping through the canyon. A slow building flood on a higher order stream can usually be predicted because major streams are monitored and it takes longer for water from heavy rainfall or rapidly melting snow to move through the higher order stream system. Floods on the highest order streams, such as the Mississippi River, may take several weeks to crest and subside.

Changes to the land in the drainage area of a stream, such as the addition of buildings and roads, can change how a stream floods. Buildings and pavement that cover the ground prevent infiltration and cause increased surface run off. Increased stream runoff means smaller amount of rain will cause the stream to reach flood stage than before the alterations to the land. The stream will flood more frequently. For the same amount and rate of rainfall that caused flooding prior to building, the stream will reach flood stage quicker and the flood will be deeper. The diagram below shows the flood response of a stream to a given amount of rainfall before and after the stream drainage area was urbanized by removal of forest and addition of buildings and roads. After urbanization, the stream reaches its peak flood level quicker and rises to a higher level than before urbanization. Unless countermeasures are engineered and installed, an urbanized area will flood more frequently and severely than the same area when it was forested.

Figure 9.

Alluvial Fans

Figure 10.

At the location where a stream reaches its base level, it slows down and deposits nearly all of the sediment it is carrying. A stream that comes down a canyon and enters a flat valley or plain builds a fan shaped deposit of sediment known as an alluvial fan. Alluvial fans are built mostly during flash floods. Alluvial fans are easy to recognize in arid areas but they form in wetter climates as well.

The diagram in figure 11 shows in cross-section how an alluvial fan develops over time as sediment is eroded from higher elevation and deposited on the adjacent lower elevation plain.

Figure 11.

Deltas

Deltas are important landforms to civilization. They provide fertile soils, flat land, and water for agriculture, as well as river channels for transportation.

A delta is a landform composed of sediment deposited where a stream enters a larger, slower moving body of water, such as an ocean, a lake, or a larger river. The term delta comes from the triangular shape of the Greek letter delta (Δ). Ancient Greek geographers recognized the triangular shape of the land created by the Nile River where it emptied into the Mediterranean Sea and gave the name delta to that landform. The Nile River delta is one of several types of deltas that are defined by the predominate processes that shape them. The Nile delta is a wave-dominated delta. Waves of the Mediterranean Sea have pushed and distributed sediment along the coast, flattening the seaward side of delta.

The Mississippi River delta is an example of a stream dominated delta. Deposition of sediments has built the delta into the Gulf of Mexico faster than waves or tides could redistribute the sediment. As commonly occurs in a delta, the Mississippi River splits in the downstream direction into several branches that discharge across the delta into the Gulf of Mexico. These branches are known as distributaries. The mouth of each distributary has built part of the delta farther out into the Gulf of Mexico forming what is known as a bird's foot delta, another name for a stream dominated delta based on the way it looks on a map.

The Ganges River delta is a tide dominated delta formed from sediment eroded from the Himalaya Mountains, the largest mountain range in the world. The mouth of the Ganges River is at the northern end of the Bay of Bengal, a large embayment of the Indian Ocean. The shape of this large bay has a magnifying effect on the tides. The combination of strong tides and the consistently high discharge from a river caring a large sediment load create a branching pattern of distributaries, in effect a braided stream system across the delta.

Deltas are lowlands that lie barely above sea level and are at high risk of being submerged under water. There are several ways in which deltas can be inundated by rising water. Floods coming down the river can cover a delta. Marine deltas can be subject to storm surges when extreme winds raise sea level along the coast and push ocean water inland. In the last several decades a new risk of submergence has arisen for marine deltas. Many marine deltas are undergoing gradual submergence as global sea level rises.

Drainage Basins

Figure 12. Cawston Creek near Keremeos, B.C. The blue line shows the extent of the drainage basin. The dashed red line is the drainage basin of one of its tributaries. [SE]

A stream is a body of flowing surface water of any size, ranging from a tiny trickle to a mighty river. The area from which the water flows to form a stream is known as its drainage basin. All of the precipitation (rain or snow) that falls within a drainage basin eventually flows into its stream, unless some of that water is able to cross into an adjacent drainage basin via groundwater flow. An example of a drainage basin is shown in Figure 12.

Cawston Creek is a typical small drainage basin (approximately 25 km²) within a very steep glaciated valley. As shown in Figure 13, the upper and middle parts of the creek have steep gradients (averaging about 200 m/km but ranging from 100 to 350 m/km), and the lower part, within the valley of the Similkameen River, is relatively flat (<5 m/km). The shape of the valley has been controlled first by tectonic uplift (related to plate convergence), then by pre-glacial stream erosion and mass wasting, then by several episodes of glacial erosion, and finally by post-glacial stream erosion. The lowest elevation of Cawston Creek (275 m at the Similkameen River) is its base level. Cawston Creek cannot erode below that level unless the Similkameen River erodes deeper into its flood plain (the area that is inundated during a flood).

Figure 13. Profile of the main stem of Cawston Creek near Keremeos, B.C. The maximum elevation of the drainage basin is about 1,840 m, near Mount Kobau. The base level is 275 m, at the Similkameen River. As shown, the gradient of the stream can be determined by dividing the change in elevation between any two points (rise) by the distance between those two points (run). [SE]

Metro Vancouver's water supply comes from three large drainage basins on the north shore of Burrard Inlet. The boundary between two drainage basins is the height of land between them. A drop of water falling on the boundary between the Capilano and Seymour drainage basins (a.k.a., watersheds), for example, could flow into either one of them.

The pattern of tributaries within a drainage basin depends largely on the type of rock beneath, and on structures within that rock (folds, fractures, faults, etc.). The three main types of drainage patterns are illustrated in Figure 14. Dendritic patterns, which are by far the most common, develop in areas where the rock (or unconsolidated material) beneath the stream has no particular fabric or structure and can be eroded equally easily in all directions. Examples would be granite, gneiss, volcanic rock, and sedimentary rock that has not been folded. Most areas of British Columbia have dendritic patterns, as do most areas of the prairies and the Canadian Shield. Trellis drainage patterns typically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Rocky Mountains of B.C. and Alberta are a good example of this, and many of the drainage systems within the Rockies have trellis patterns. Rectangularpatterns develop in areas that have very little topography and a system of bedding planes, fractures, or faults that form a rectangular network. Rectangular drainage patterns are rare in Canada.

Figure 14. Typical dendritic, trellis, and rectangular stream drainage patterns. [SE]

In many parts of Canada, especially relatively flat areas with thick glacial sediments, and throughout much of Canadian Shield in eastern and central Canada, drainage patterns are chaotic, or what is known as deranged(Figure 15, left). Lakes and wetlands are common in this type of environment. A fourth type of drainage pattern, which is not specific to a drainage basin, is known as radial (Figure 15, right). Radial patterns form around isolated mountains (such as volcanoes) or hills, and the individual streams typically have dendritic drainage patterns.

Figure 15. Left: a typical deranged pattern; right: a typical radial drainage pattern developed around a mountain or hill. [SE]

Over geological time, a stream will erode its drainage basin into a smooth profile similar to that shown in Figure 16. If we compare this with an ungraded stream like Cawston Creek (Figure 12), we can see that graded streams are steepest in their headwaters and their gradient gradually decreases toward their mouths. Ungraded streams have steep sections at various points, and typically have rapids and waterfalls at numerous locations along their lengths.

Figure 16. The topographic profile of a typical graded stream. [SE]

Figure 17. An example of a change in the base level of a small stream that flows into the Similkameen river near Keremeos. The previous base level was near the top of the sandy bank. The current base level is the river. [SE]

A graded stream can become ungraded if there is renewed tectonic uplift, or if there is a change in the base level, either because of tectonic uplift or some other reason. As stated earlier, the base level of Cawston Creek is defined by the level of the Similkameen River, but this can change, and has done so in the past. Figure 17 shows the valley of the Similkameen River in the Keremeos area. The river channel is just beyond the row of trees. The green field in the distance is underlain by material eroded from the hills behind and deposited by a small creek (not Cawston Creek) adjacent to the Similkameen River when its level was higher than it is now. Sometime in the past several centuries, the Similkameen River eroded down through these deposits (forming the steep bank on the other side of the river), and the base level of the small creek was lowered by about 10 m. Over the next few centuries, this creek will seek to become graded again by eroding down through its own alluvial fan.

Another example of a change in base level can be seen along the Juan de Fuca Trail on southwestern Vancouver Island. As shown in Figure 18, many of the small streams along this part of the coast flow into the ocean as waterfalls. It is evident that the land in this area has risen by about 5 m in the past few thousand years, probably in response to deglaciation. The streams that used to flow directly into the ocean now have a lot of down-cutting to do to become regraded.

Figure 18. Two streams with a lowered base level on the Juan de Fuca Trail, southwestern Vancouver Island. [SE]

The ocean is the ultimate base level, but lakes and other rivers act as base levels for many smaller streams. We can create an artificial base level on a stream by constructing a dam.

Exercise:The Effect of a Dam on Base Level

Figure 19. Revelstoke Dam and Revelstoke Lake on the Columbia River at Revelstoke, BC [SE]

When a dam is built on a stream, a reservoir (artificial lake) forms behind the dam, and this temporarily (for many decades at least) creates a new base level for the part of the stream above the reservoir.

• How does the formation of a reservoir affect the stream where it enters the reservoir, and what happens to the sediment it was carrying?
• The water leaving the dam has no sediment in it. How does this affect the stream below the dam?

Take a moment to think about your answers to these questions. You can record them in the space below, if you'd like:

Figure 20. Terraces on the Fraser River at High Bar. [Marie Betcher photo, used with permission]

Sediments accumulate within the flood plain of a stream, and then, if the base level changes, or if there is less sediment to deposit, the stream may cut down through those existing sediments to form terraces. A terrace on the Similkameen River is shown in Figure 17 and some on the Fraser River are shown in Figure 20. The Fraser River photo shows at least two levels of terraces.

In the late nineteenth century, American geologist William Davis proposed that streams and the surrounding terrain develop in a cycle of erosion (Figure 21). Following tectonic uplift, streams erode quickly, developing deep V-shaped valleys that tend to follow relatively straight paths. Gradients are high, and profiles are ungraded. Rapids and waterfalls are common. During the mature stage, streams erode wider valleys and start to deposit thick sediment layers. Gradients are slowly reduced and grading increases. In old age, streams are surrounded by rolling hills, and they occupy wide sediment-filled valleys. Meandering patterns are common.

Figure 21. A depiction of the Davis cycle of erosion: a: initial stage, b: youthful stage, c: mature stage, and d: old age. [SE]

Davis's work was done long before the idea of plate tectonics, and he was not familiar with the impacts of glacial erosion on streams and their environments. While some parts of his theory are out of date, it is still a useful way to understand streams and their evolution.

Geologic Processes and Flowing Water

Figure 22. As streams flow towards the ocean, they carry weathered materials.

Streams—any running water from a rivulet to a raging river—complete the hydrologic cycle by returning precipitation that falls on land to the oceans (figure 22). Some of this water moves over the surface and some moves through the ground as groundwater. Flowing water does the work of both erosion and deposition.

Erosion and Deposition by Streams

Erosion by Streams

Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water. Sediments are carried as:

• Dissolved load: Dissolved load is composed of ions in solution. These ions are usually carried in the water all the way to the ocean.

  Figure 23. Rivers carry sand, silt and clay as suspended load. During flood stage, the suspended load greatly increases as stream velocity increases.

• Suspended load: Sediments carried as solids as the stream flows are suspended load. The size of particles that can be carried is determined by the stream's velocity (figure 23). Faster streams can carry larger particles. Streams that carry larger particles have greater competence. Streams with a steep gradient (slope) have a faster velocity and greater competence.
• Bed load: Particles that are too large to be carried as suspended load are bumped and pushed along the stream bed as bed load. Bed load sediments do not move continuously. This intermittent movement is called saltation. Streams with high velocities and steep gradients do a great deal of down cutting into the stream bed, which is primarily accomplished by movement of particles that make up the bed load.
• Here is a video of bedload transport.

Stages of Streams

As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a stream'sheadwaters, often high in the mountains, gradients are steep (figure 24). The stream moves fast and does lots of work eroding the stream bed.

Figure 24. This stream begins as snow melt from the mountains.

As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (figure 25).

Figure 25. (a) At a meander, a stream actively erodes its outer banks and deposits material along the inside curves. This causes these meanders to migrate laterally over time. (b) This stream has deposited larger materials such as gravel and pebbles along the inside curve of a meander. (c) This image is a topographic map. The San Juan River eroded the land surface as the Colorado Plateau uplifted. The river's meanders were preserved as a feature called incised meanders.

As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat level area surrounding the stream channel (figure 26).

Figure 26. The Vistula River in Poland flows onto its floodplain.

Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do.

Stream Deposition

As a stream gets closer to base level, its gradient lowers and it deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. Placer mineral deposits, described in the Earth's Minerals chapter, are often deposited there. A stream's floodplain is much broader and shallower than the stream's channel. When a stream flows onto its floodplain, its velocity slows and it deposits much of its load. These sediments are rich in nutrients and make excellent farmland (figure 27).

Figure 27. The Mississippi floodplain is heavily farmed. Flooding can wipe out farms and towns, but the stream also deposits nutrient-rich sediments that enrich the floodplain.

A stream at flood stage carries lots of sediments. When its gradient decreases, the stream overflows its banks and broadens its channel. The decrease in gradient causes the stream to deposit its sediments, the largest first. These large sediments build a higher area around the edges of the stream channel, creating natural levees (figure 28).

Figure 28. After many floods, a stream builds natural levees along its banks.

When a river enters standing water, its velocity slows to a stop. The stream moves back and forth across the region and drops its sediments in a wide triangular-shaped deposit called a delta (figure 29).

Figure 29. (a) The Nile River delta has a classic triangular shape, like the capital Greek letter delta. (b) Sediment in the Yellow River delta. The main stream channel splits into many smaller distributaries.

If a stream falls down a steep slope onto a broad flat valley, an alluvial fandevelops (figure 30). Alluvial fans generally form in arid regions.

Figure 30. An alluvial fan in Iran. The mountains are in the lower right corner of the photograph.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz doesnot count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

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