Fluvial Process And Related Land Forms Presentation Transcript
FLUVIAL PROCESSES AND LANDFORMS Unit six The Dynamics of Earth’s environment presentation by: S.T GIS postgraduate program (BDU) Email: firstname.lastname@example.org
Running water present in almost all environments, at least occasionally but other agents are limited to small parts of the continents (action of wind, waves and currents) or glacial ice.
Nearly every part of the Earth has seen, at sometime in it’s geologic past, the imprint of fluvial processes.
In this paper we will discussed about the various types of erosion, features of drainage basin and types of drainage pattern geomorphic works of rivers: erosion, transportation and deposition, the development of river valley, the stages of river and various land forms associated with these courses.
Explain briefly the various forms of surface and cannel erosion
Describe the features of a drainage basin.
Explain how drainage patterns within a drainage basin form.
Explain the factors that control the processes of river erosion,
transportation, and deposition.
Describe channel patterns and explain its role in stream flow.
Describe the processes and feature formed from fluvial processes.
Compare and contrast the several types of deltas.
By the end of this unit you will be able to:
WATER AS AN AGENT OF DEGRADATION
Of the of other agents of erosion, Running water (moving water), aides by mass wasting, is the most important geologic agent in eroding transporting and depositing sediment.
Almost every landscape on the earth shows the result of stream erosion or deposition.
Water erosion is a two-part process involving the detachment and transport of weathered materials.
Erosion from water typically occurs in the following ways.
1. Surface Erosion
Water moving on the surface without being confined to a channel. The two forms are : splash and sheet erosion .
A. Splash Erosion
First step in the erosion process
Raindrops typically fall with a velocity of 20-30 feet per second.
Disaggregating and displacement of wet surface soil creating small craters in bare soil.
Energy of these impacts is sufficient to displace soil particles as high as two feet vertically
splash erosion Cont…
B. Sheet Erosion
Gradual removal of a uniform layer of soil from the surface
Entrainment of loose particles in overland flow
Overland flow on smooth slopes is laminar (layered), so particles can only be displaced but not suspended
Difficult to detect its effect until it develops into rill erosion
The potential for sheet erosion is dependent on the soil type, velocity, vegetation, gradient and volume of flow over the surface.
Long slopes, steep slopes, and slopes that carry higher volumes of runoff are more susceptible to sheet erosion
2. Channel Erosion
Water is confined to long trough-like depressions called channel
A. Rill Erosion
The first stage of channel erosion and intermediate process between sheet and gully erosion
Are many closely spaced, innumerable and small shallow channels eroded by threads of turbulent flow
B. Gully Erosion
Larger versions of rills
Most gullies extend up slope as a result of headwall migration.
It is the collapse and slumping of the sidewalls, which usually contributes the greatest proportion of soil loss.
C. Stream Erosion
Eventually gullies develop into streams.
Streams/Rivers contain more water and have more capacity to do the work of erosion, transportation and deposition.
Stream channel erosion consists of both streambed and stream bank erosion.
In this paper the words stream and river are used interchangeably.
· Forms erosion on an exposed slope
River as a System
Drainage/ River/ Basins
River is anybody of fresh water flowing from an upland source to a large lake or to the sea, fed by such sources as springs and tributary streams.
A river starts on hillsides as small channels, or rills.
The rills combine to make larger channels or tributaries that eventually come together, forming distinct streams.
The largest channels formed by this convergence of tributaries are rivers, and they can carry large quantities of fresh water and sediment across continents.
The world’s longest rivers Cont’d River name Continent Total length Nile Africa 6695 Km Amazon South America 6400 Km Yangtze Asia 6300 Km Mississippi-Missouri-Red Rock North America 5970 Km Yeisey-Angara Asia 5550 Km Yellow River Asia 5464 Km Ob’-Irtysh Asia 5410 Km Rio Parana-ril grande South America 4500 Km Amur-shilka Asia 4416 Km Lena Asia 4400 Km Congo Africa 4374 Km Mackenzie-Peace-Finlay North America 4241 Km Mekong Asia 4200 Km Niger Africa 4180 Km
Rivers have different sources.
They may start from the melt waters of glaciers, e.g. Rhone (France), lakes, e.g. Nile (Africa), springs, e.g. Thames (England) from regions of steady rainfall, e.g. Zaire (Africa).
Likewise, they may end at in the sea, e.g. Amazon (Atlantic), the Niger (Gulf of Guinea) and the Indus, (Arabian Sea). Sometimes the mouth could be in a lake, e.g. Volga (Caspian), or in a swamp, e.g. Chari River (Lake Chad).
Moves from its sources to its mouth captures several tributaries.
Sources of Stream Flow Cont’d
Geologists have devoted much time and effort to the study of drainage systems and their evolution. The reasons for their interests are:
Firstly, there is the obvious point that a drainage system is a major feature of the physical landscape.
Secondly, evolutionary studies of drainage systems may afford valuable information about the denudational history of an area. For instance, it is often useful to attempt a reconstruction of the initial form of a river system in order to gain evidences of the nature and mode of origin of the land surface on which that system began its existence.
A drainage basin is a portion of the Earth's surface that contains a main stream and its tributaries.
Drainage divide- a ridge or strip of high ground dividing one drainage basin from another.
It determines into which basin precipitation flows. The upland between tributaries is called interfluves.
Mississippi river basin Cont’d
North America drainage system Continental divide Cont’d
Streams within the drainage basin are either perennial or intermittent in flow.
Perennial streams -permanent streams that flow all year.
Intermittent stream — a stream that carries water only part of the year.
Tributary streams- small streams that enter into the main stream. Some streams are classified as exotic streams , originate in a humid region but flow through an arid region. E.g.. Nile and Colorado Rivers
Superposed Streams Development of water gap Folded Units Cont’d
Stream ordering system
First order streams The smallest streams in a drainage network have no tributary streams.
Second order Two first order streams unite to form a second order stream.
Third order -only have second and first order streams as tributaries, etc.
The main stream is always the highest order in the basin
Drainage density is the total length of streams per unit area.
Drainage Density = total river length
There are three different drainage densities. These are:
- Low drainage densities
- Medium Drainage densities
- High Drainage densities
Drainage Patterns - arrangement of streams in a watershed
Often controlled by geology structures and surface materials
Reflects the particular plan or design which individual river courses collectively form.
The most common patterns dendritic, parallel, trellis, rectangular, radial, centripetal and deranged or contorted.
Dendritic Pattern _ Is the most common form and looks like the branching pattern of tree roots.
Develops in regions underlain by homogeneous material horizontal sediments or uniformly resistant rocks
Parallel Pattern _ develops in regions of parallel, elongate landforms like outcropping resistant rock bands.
Sometimes indicates the presence of a major fault that cuts across an area of steeply folded bedrock.
Trellis drainage pattern _ develops in folded topography like that found in the Appalachian Mountains
Tributary streams enter the main channel at sharp angles as they run down sides of parallel ridges called anticlines.
Rectangular pattern found in regions that have undergone faulting.
Movement of the surface due to faulting offsets the direction of the stream; As a result the tributary streams make shape bends &enter the main stream at high angles.
Radial pattern -develops around a central elevated Point
Conically shaped features like volcanoes &domes
Centripetal pattern: the opposite of the radial pattern
Streams flow toward a central depression.
This pattern is typical in the western and southwestern portions of the United States where basins exhibit interior drainage.
During wetter portions of the year, these streams feed ephemeral lakes, which evaporate away during dry periods.
Deranged drainage : areas recently disturbed by events like glacial activity or volcanic deposition; many lake and wetland, short streams with few tributaries.
The patterns described above are accordant or correlated with the structure and relief over which they flow.
Those streams that are discordant with the rocks over which they flow are either antecedent or superimposed.
Antecedent streams flowed across bedrock structures prior to uplift.
E.g. Columbia River that cuts across the Cascade Mountains.
superimposed streams often not controlled by underlying structure because river course established according to structure of overlying strata.
Is the configuration of the individual stream channel as viewed from the air.
Controlled by a linear zone of weakness in the underlying rock, like a fault or joint system.
Straight channel does not mean a straight flow.
Velocity is highest in the zone overlying the deepest part of the stream. In these areas, sediment is transported readily resulting in pools.
Sediments are deposited to form bars where the velocity of the stream is low.
2 . Meandering pattern
Usually on flat land with unhurried flow with enough force to erode banks
Geoscientists use the sinuosity ratio to determine whether a channel is straight or meandering.
Sinuosity ratio is the distance between two points on the stream measured along the channel divided by the straight line distance between the two points.
If the sinuosity ratio is 1.5 or greater the channel is considered to be a meandering one.
Lateral (sideways) erosion of the river channel results in the river forming a winding pattern.
Erosion on outside banks, deposition on inside banks, causing rapid and sometimes abrupt changes in the channel.
A network of small channel separated by small and often
temporary islands and bars
A stream that consists of a multiplicity of interwoven and
interconnected shallow channels separated by low islands of sand, gravel, and other loose debris.
Usually occurs with a very flat stream channel with a heavy load of alluvium and a period of low discharge.
Islands may become resistant to erosion if they become
inhabited by vegetation
Braided Streams Cont’d
Characteristics of channel patterns
Rivers as Agents of Degradation
Rivers are one of the greatest sculpturing agents at work in humid regions. accounts for 85-90% of total sediment transport to the ocean basins (glaciers account for only 7%).
From the source to mouth of a river three processes are taking place.
River erosion River transport River deposition
There are 4 different ways water erodes the river away
Attrition is the collision of rock fragments in the water against one another.
-The rock particles are broken into smaller pieces and become smoother the longer the process continues.
See how these rocks are becoming smoother at the edges
The results of attrition
Abrasion ( or Corrasion ,) is the grinding of rock fragments carried by the river against the bed and banks of the river.
-This action causes the channel to widen and deepen.
-This grinding is most powerful in flood time when large fragments of rock are carried along in the river bed.
An additional process is also at work caused by the force of the water itself known as Hydraulic action.
In this rocks are dragged away from the bed and banks by the force of the running water.
Corrosion is the process by which river water reacts chemically with soluble minerals in the rocks and dissolves them.
although this is often called solution – confusing as solution is also a word used in transportation .
We often say things like water pipes corrode – the chemicals in the water seem to eat them away!
Factors of river Erosion
Stream velocity – the distance water travels in a stream per unit time.
Velocity is largely controlled by the stream gradient , channel shape , and channel roughness .
-Expressed in ft./sec.
-The stream reaches its maximum velocity near the middle of the channel .
Velocity is the key factor in a stream’s ability to erode, transport, and deposit
- High velocity – results in erosion and transportation
- Low velocity – causes sediment deposition
When the stream goes around a curve, the region of maximum velocity is displaced by inertia towards the outside of the curve
With in a stream channel, three types of flow can be observed
Laminar flow -Under very low velocities water flows through a stream as smooth sheets running parallel to the bed In this type of flow the direction of water in the stream is not altered in its direction
Only the finest particles kind be detached, so laminar flow is basically non erosive.
Turbulent flow - under higher flow velocities, resistance within the flow and that caused by the bed and sides of the channel (channel topography) cause the flow to break down into separate currents.
- More erosive than laminar flow and help suspend material in the stream.
- Is the "normal" type of flow in most streams
Helical flow - spiral flow in a stream, Caused by channel shape.
Gradient – the downhill slope of the stream bed.
The difference in elevation between two points on a stream divided by the distance between them
The stream gradient is one of the factors that controls a stream’s velocity.
The steeper the slope, the faster the velocity of flow and the more the energy of the stream will be to erode materials
Stream Gradient Cont’d
Channel Shape and Roughness
The shape of the channel also
controls stream velocity.
In wide, shallow channel water
flow slowly and in narrow channel
it flows rapidly
The stream in ‘B’ flows slower than
‘ A’ because the wide, shallow channel
in ‘ B’ has more surface for the moving
water to drag against – higher friction .
The roughness of the channel also
Roughness creates more friction and
Roughness of the channel
Cont’d Channel shape
In streams in humid climates, discharge increases downstream for two reasons:
as river water evaporates into the air
and soaks into the dry ground
In an arid climate, a river’s discharge can decrease in a downstream direction
1. Water flows out of the ground into the river through the streambed. 2. Small tributary streams flow into a larger stream along its length, adding water to the stream as it travels. Discharge - is the volume of water that flows past a given point in a unit of time.
A river carries or transports eroded materials such as mud, sand, boulders and dissolved materials on its journey.
These materials are known as its load .
Materials or loads transported by the river can be sub divided in to bed Load, suspended load and dissolved load
Bed load : heavy sediment particles that travel on the streambed. Such materials include sand and gravel
These particles may move individually along the bottom, or they may travel in groups.
They will deposited when ever the local flow conditions will not entrain them.
Suspended load : are fine or light enough to remain lifted indefinitely above the bottom by the water turbulence. e.g. silts and clays .
Dissolved load : soluble products of chemical weathering process can make up a substantial dissolved load in a stream.
Most streams contain numerous ions in solution. Such as bicarbonate, calcium, potassium, sodium, chloride, and sulfate.
Generally, most material is transported in the suspended load; the least, in bed load
The load is carried along by four processes.
Traction is where boulders or stones are rolled or sliding along the stream bed by the force of the water
Saltation :t he speed of the water lifts the fragments( tiny pebbles ) of the river bottom, and they bounce along and leapfrog over each other in a process called saltation (from a Latin word meaning jump).
- Saltation, therefore is a combination of traction and suspension.
Suspension is when very fine particles (suspension) such as clay, silt and fine sand are carried along in the river.
- The small particles are held up by the water as they float because they are very, very small!
Cont’d An example of suspension
Transportation in solution is when dissolved materials containing minerals like calcium and sodium are carried in the water and are invisible to the naked eye.
- No picture here because you cannot see them!
Time in transport will round transported grains &decrease size
A river drops its load when the speed or volume of the river decreases or competence is reduced .
Competence - the size of the largest particle that can be transported by a stream.
Capacity - the maximum load that a stream could transport under given conditions; its potential.
-The heavier material is deposited first and the finer material carried further.
-Rivers reduce their speed when they enter flat land, enters a lake or the sea or reach an arid area or with change in gradient or as channel widens or changes direction.
-The volume can be reduced during a dry season or when the river passes over more porous landscapes e.g. sand or limestone.
Some of the depositional land forms include levees ,flood plain, delta, alluvial funs , back swamp, point bars and etc.
-The details of these features will be discussed in the fallowing section.
-materials deposited by river are known as Alluvium .
-Most alluvium is deposited into quiet water (ocean or lake).
River valley development and associated land forms
Streams develop their valley through the fallowing ways;
1.Down cutting – the process of deepening a valley by erosion of the streambed.
If a stream removes rock from its bed, it can cut a narrow slot canyon down through the rock.
Such canyons are not common because down cutting is usually is usually accompanied by mass wasting and sheet erosion of the valley walls .
The limit of down cutting is known as base level ; it is a theoretical limit for erosion of the earth’s surface
2.Lateral erosion: a stream can de deepening its channel by cutting while part of its energy is also widening the valley by lateral erosion.
The erosion of a stream’s bank and valley walls as the stream swings from side to side across its valley floor.
3.Head ward erosion :a stream can also lengthen its valley by h ead ward erosion .
The slow uphill growth of a valley above its original source through gulling, mass wasting, and sheet erosion.
Stream capture (Stream piracy) - an event where a portion of the flow of one stream is diverted into that of another by natural processes.
A. Graded stream
Has regulated its various stream parameters (depth, width, slope, velocity, etc.) to obtain the most efficient conditions for flow and sediment transport.
A graded stream is capable of maintaining a steady-state condition.
Characteristics of a graded stream
Slope of the longitudinal profile is concave upward, steeping upstream.
No falls or basins exist within the channel profile.
No net erosion or deposition occurs along its channel.
The stream is capable of handling all sediment introduced to it from its tributaries.
B. Non-graded stream
Energy within the system is not evenly distributed along the profile; contains falls and basins.
Falls result in a concentration of energy, which promotes erosion.
Basins result in a decrease in energy, which promotes deposition.
Young Stage -Has steep-sided and narrow V-shaped valley created by vertical erosion -characterized by waterfalls and rapids. -Water is often quite clear because the river is not carrying much load in suspension. -Velocity is high. -Down cutting and head ward erosion are the major processes of river erosion. and head ward erosion is dominant
Valley profile in the upper course
V shape valley
Long Profile o f a typical stream
Is a depiction of the down slope gradient of stream.
A gorge is a steep, V-shaped valley formed by the vertical (down ward) erosion of the land by the river.
The Indus Gorge in Kashmir is the deepest in the world (about 5180 m.).
If the region is arid where there is little weathering and mass wasting on the valley sides, the gorge will be impressively deep. If such a gorge is deep, long and U-shaped, we call it canyon.
In the case of a canyon the valley is deepened more than they are widened.
Slot-canyon: Young stream, eroding down and no mass wasting
Water fall : a steep drop in the course of a river.
It has a high head of water and a characteristic Plunge pool at the base.
Waterfalls often form when a band of resistant rock lies over softer, less resistant rocks.
Knick point- a sharp irregularity in a stream-channel profile, commonly caused by abrupt changes in bedrock resistance and at which Waterfalls starts.
Plunge pool : also known as a waterfall lake
Is a pool, lake, or pond that is small in diameter, but deep.
Plunge pools can remain after the waterfall has ceased to exist or the stream has been diverted.
Cont’d - Cap Rock - layer of hard resistant rock forming the 'step' over which the 'falls' occur in a waterfall.
Water fall Tis Isat” (Blue Nile) Waterfall Cont’d
Are depressions that are eroded in to the land rock of the streambed by abrasive action of the sediment load.
When a stream is full, the swirling water can cause sand and pebbles to scour out smooth, cylindrical or bowl- shaped depression in hard rock.
Tend to form in spots where the rock is a little weaker than the surrounding rock.
They may contain certain sand or an assortment of beautifully rounded pebbles.
spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course.
Interlocking spurs form where the river is forced to swing from side to side around these
Interlocking Spurs River channel Cont’d
Many tributaries have fed the river upstream.
The volume of water increases and lateral erosion is taking place
River channel has become much wider and deeper
The surrounding valley has also become wider and flatter with a more extensive floodplain.
Bends or loops in the river
One of the most distinctive features of the river in the middle course is its increased sinuosity.
Unlike the relatively straight channel of the upper course, in the middle course there are many meander (bends) in the river.
As water flows into a meander it takes on a helical or spiral flow that determines where erosion and deposition is concentrated.
Centrifugal force draws water toward the outside bank causing erosion and creates a bend . Sediment eroded from the outside bank is deposited on the inside bank
Slip off Slope - a small beach found on the inside of a meander bend where deposition has occurred in the low energy zone.
River Cliff - a small cliff formed on the outside of a meander bend due to erosion in this high-energy zone. when the water in an ox-bow lake dries
Neck & Cutoff
Neck Is the upland between tow consecutive (opposing) meanders of a stream.
Cutoff or an Oxbow : occurs when the neck between river meanders is totally eroded away and the meanders join to shorten the length of the channel.
A gently meandering river flows through an area of relatively flat terrain.
Water flows at different speeds as it goes around bends in a meandering river. On the outside banks of corners, the river water moves the fastest, causing lateral erosion and undercutting
Meanwhile, on the inside banks of corners in the river, the water flows more slowly, leading to sediment settling out of the water & building up on the inside banks.
4. Gradually, the inside banks are filled in with accumulated deposits, and the outside bends extend further and further, forming a wide loop in the river.
5. The loop continues to bend further and further, until a thin strip of land called a neck is created at the beginning and the end of the meander
6. Eventually, the narrow neck is cut through by either gradual erosion or during a time of flooding. When this happens, a new straighter channel is created, diverting the flow of the river from the loop into the new channel.
7. Deposition finally seals the cut-off from the river channel, leaving a crescent-shaped land form known as oxbow lake.
Meander scar - feature left behind when the water in an ox-bow lake dries up
Meander neck becomes smaller
new course of the river
When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.
Final stage of a river is reached when the river is flowing on more or less flat surface.
The speed so low that no net erosion occurs any more.
Erosion on the slopes is balanced by deposition on the floodplains.
Peneplain -a flat and relatively featureless landscape with minimal relief; considered to be the end product of the geomorphic cycle. Peneplain means “almost a plain. The process is known as peneplanation
From now on the rivers just sweep over the floodplain and rework the sediment, creating various floodplain features. Isolated remnants of resistant bedrock may rise over the Peneplain surface, the so-called monad nocks or inselbergs .
Yazoo stream _ a tributary unable to enter the main stream because of natural levees along the main stream; instead flows downstream to the back swamp zone and runs parallel to the main stream until it finds an entrance.
Old or mature stage landforms may be uplifted due to climate change or tectonic activity
Entrenched meanders are the result.
Entrenched meander :a winding, sinuous stream valley with abrupt sides.
In a fashion, the landscape reverts to an earlier stage in its evolution (e.g. from intermediate/mature to the initial/young stage
Because the river is moving on a flat surface, deposits sediments at the bed often act as barriers to free flow of water making it to form meanders.
The meander, in this case, does not, however, occupy the whole valley bottom as in the case of the middle course
Is a depositional feature of streams.
Point bars are found in abundance in mature or meandering streams. Point bars are composed of sediments that are well stored and typically reflects the overall capacity of the stream.
Point bar deposits on the inner meander bend where there is low energy
Deposition and Erosion of River Bars Flood: Increased discharge and erosion! Normal Flow Post-flood; new bars deposited
A relatively flat area that borders a stream, which is periodically inundated with water during high flow periods.
The resulting decrease in velocity causes sediment carried by the stream, such as gravel, sand, silt, and clay, to deposit as alluvium on the floodplain.
Large particles are deposited first, and much of this material is laid down alongside both banks.
Bluff : a relatively steep slope at the outer edge of a floodplain, marking the outer limit of lateral erosion and undercutting.
Flood plain Cont’d Bluff
is a narrow ridge of alluvium deposited at the side of the channel. During high discharge periods when the stream floods, coarse sediment settles out near the stream channel and grades to finer material further away.
Located some distance away from the stream channel on the floodplain.
When water spills over onto the floodplain, the heaviest material drops out first and finest material is carried a greater distance.
The fine-grained alluvium holds much water and drains rather slowly creating wetland areas.
important "sponges" that retain water that might cause severe flooding downstream.
Land forms produced by river Cont’d
Are elevated portions of a floodplain created when the stream down cuts and creates a new floodplain at a lower elevation.
Occur at same elevations on opposite valley sides; produced by intermittent down cutting with changes in discharge , load or base level.
Fragment of former floodplain “accidentally” preserved (e.g. by a rock buttress) as a meandering stream slowly degrades it
A landform at the mouth of a river produced. by the sudden dissipation of stream’s velocity and the resulting deposition of the stream’s load.
Distributaries: branching stream channel that crosses a delta
For the formation of a delta the following necessary conditions should be fulfilled.
-low velocity and
-loads must be deposited faster than it can be removed by the action of tides& currents
The shape of a delta depends on following factors
the rate of sediment supply,
wave action and coastal currents reworking the deposited sediment,
and the rate at which the alluvial deposits subside.
We have four basic types of delta. These are:
- Bird foot
Fan or inverted cone-shaped delta .
Has many active, short distributaries taking coarse and fine sediment to their mouths.
In areas of significant wave activity sediment will be redistributed laterally along the delta front to forma smooth arcuate shoreline with many sand bars, beach ridges and/or barrier islands.
Examples : Nile, the Ganges, the Indus, the Irrawaddy, the Mekong and the Hwang – Ho.
2.Bird’s foot Delta
Where delta formation is river-dominated and less subject to tidal or wave action, a delta may take on a multi-lobed shape which resembles a bird’s foot.
Tend to have one or a very few major distributaries near their mouths.
It consists of very fine martial, called silt. There is a broad, shallow shelf that deepens abruptly, so the trend is to grow long and thin like a bird's toe.
-Mississippi Delta and
Bird’s foot Delta
3. Cuspate Delta
Formed by a river that usually has one distributaries' emptying into a flat coastline with wave action hitting it head-on.
This tends to push the sediment back on both sides of the mouth, with a "tooth" growing out onto the shelf.
-Tiber River of Italy
-The delta of Niger River
Cuspate Delta Tiber River delta of Italy
4. Estuarine Delta
Developed when a river that empties into a long, narrow estuary that eventually becomes filled with sediment (inside the coastline).
Examples of this type of delta include ;
-The delta of Seine River of France
-The deltas of the River Vistula (in Poland) and
-The Ob river (USSR)
Are fan-shaped alluvial deposits generally found when a mountain stream runs on to a flatter surface at the front of a mountain system.
Mountain streams carrying a heavy stream load loses their kinetic energy as they flow out on to the flat plain depositing alluvium.
Alluvial fans are quite common in arid regions where water is lost to evaporation and infiltration into coarse surface material when the stream exits the mountain front.
Alluvial Fans - Brooder Peninsula, Baffin Island, Canada.
Bajada _ forms when several individual alluvial fans merge into one broadly sloping surface.