River

What is Geomorphology?The word “geomorphology" comes from the Greek roots "geo,“ “morph,” and “logos,” meaning “earth,” “form,” and “study,” respectively. Therefore, geomorphology is literally “the study of earth forms.”

Geomorphologists are concerned primarily with earth’s surficial features, including their origin, history, composition, and impact on human activity.

Photo by NASA

Rivers

• Source – The place where a river begins.• Course – The route the river takes to the

sea• Tributary – A small river that joins a larger

river.• Confluence – The point where the

tributary joins the river.

Common River Terms

• Mouth – The point where the river enters the sea.

• Estuary – The part of the mouth that is tidal.

• Basin – The area of land drained by a river.

• Watershed – The high ground separating one river basin from another.

RIVER PROCESSES:

Three river processes:

1. Transportation

2. Deposition

3. Erosion

RIVER TRANSPORTATION

The load is transported by 4 ways:

Solution – material is dissolved in the water and is carried along by it. Salt carried in solution is the reason why seawater is salty. NOT VISIBLE

Suspension – fine material is carried along by the river. If the river slows down this material can be dropped on the bed of the river – this is how rivers & canals “silt up”. VISIBLE

Saltation – stones bouncing along the river bed – easily seen (bed load)

Traction – Heaviest stones dragged along river bed (bed load)

Controlling factors of river transportation:

1.Density of the sediments2.Size of the sediment3.Velocity of water

River depositionDeposition: when velocity begins to fall, it has less energy and no

longer had competence and capacity to carry all its load so largest

particles, materials begins to be deposited.

When occur?

1. Low discharge during period of low precipitation

2. Less velocity when river enter sea or lake.

3. Shallow water occurs on inside of a meander.

4. The load suddenly increase (debris from landslide)

5. River overflow its bank so velocity outside channel is reduced. (resulting in floodplain)

Hydraulic ActionThe force of moving water, aided by the process of cavitation

(popping of air bubbles in the river, creating tiny explosions)Abrasion (a.k.a. Corrasion)

The river’s load (stones and sand/grit carried by the water) wears away the bed and sides – river “sandpaper”

AttritionRocks hitting each other and being worn away

SolutionChemicals in the river water wearing away the bed and banks

River erosion.Erosion: wearing away of river bed and bank. There are four main process of erosion:

Bank CavingFormed by hydraulic action eroding the river banksLeads to a small temporary “cave” in the bank – this usually

collapses after a short period

Abrasion/Corrasion

Stones acting as “Sandpaper” – wearing down the river bed (bottom) and banks (sides)

Attrition

Stones hitting off each other and getting smallerIs the main reason why stones from the sea are rounded

SolutionChemical weathering by river water

Controlling factors of river erosion:1.Nature of the rocks2.Nature of water3.Velocity of water4.Presence of joints5.Land form6.Attitude of the rocks

Stages of a river

Youthful StageThe youthful river has a small amount of water

but it travels very quickly down the steep mountain slope.

Most of its energy is used to erode (wear away) the landscape.

This erosion (wearing away) occurs in 4 ways.

Landforms of the Youthful Stage

• All of the erosion by the river creates features or landforms on the landscape.

Three features created in the youthful stage are,

1. V-shaped Valleys

2. Interlocking Spurs

3. Waterfalls

V-Shaped Valleys

• V-shaped valleys get their name from their shape. The river erodes the landscape and creates a valley in the shape of a V.

• This occurs because of Vertical Erosion. The river cuts down into the river bed, making it deeper. It creates a narrow deep valley. Mechanical weathering and mass movement create the V shape. See Diagram.

V-Shaped Valleys

Interlocking Spurs

• As the rivers flows it meets areas of hard rock. It cannot erode these so it flows around them.

• This creates a zigzag course. See Diagram.

Waterfalls

• Waterfalls are formed when rivers flow over areas of hard and soft rock.

• The river erodes the soft rock but cannot erode the hard rock. This creates a step which the water starts to fall over.

• The falling water erodes deeper into the bed. The rivers load creates a Plunge Pool as it falls. See Diagram.

Waterfalls

The Mature Stage• We now know that during the youthful

stage the river is mainly eroding the landscape.

• Erosion breaks off particles of rock and soil from the bed and banks of the river. They are carried along by the river and are known as the Load.

• The load is carried from upland to lowland areas. This process is called River Transportation.

Landforms of the Mature Stage

In the Mature Stage the river begins to slow down,

and so it begins to deposit some of its load. It

creates the following features.

1. Wide river valley

2. Meanders

3. Flood plain

Landforms of the Mature Stage

Wider River Valley

In the mature stage the river moves from

side to side and the valley becomes wide

and flat. Weathering and Mass Movement

continue to wear away at the sides of the

valley. See diagram.

Landforms of the Mature Stage

Meanders

Meanders are bends or curves along the river. They are formed by erosion anddeposition. Erosion occurs on one side of the river while deposition occurs on the opposite side. This continues, making the bends sharper. See diagrams.

Meanders

Meander parameters:

Meander Length :It is the axial length of one meander i.c the tangential distance between the corresponding points of a meander.

Meander Belt: It is the distance between the outer edges of clockwise and anti-clockwise loops of the meander.

Meander ratio: It is the ratio of meander belt to meander length. Crossings or cross-overs: The short straights reaches of the river connecting two consecutive

clockwise and anti-clockwise loops, are called crossings or cross-overs.

Sinuosity is the ratio between channel length and displacement (straight line distance) between two points in the river’s course.

Landforms of the Mature Stage

Flood Plain

A flood plain is the flat area of land on either sideof the river. After heavy rain the river sometimesfloods. The water spreads out over the land oneither side of the river. When the river retreats itleaves behind a thin layer of alluvium. After manyfloods a thick layer of alluvium is created. This isvery fertile soil.

The Old Stage

• In the Old Stage the river is carrying lots of sand and silt. It is now flowing over flat land and so it is moving slowly. Therefore it begins to drop off its load. This is called deposition. Like erosion in the Youthful Stage, deposition also creates many features or landforms.

Landforms of the Old Stage

The following features or landforms are

created by river deposition in the Old Stage.

1. Ox-Bow Lakes

2. Levees

3. Delta

Landforms of the Old Stage

Ox-Bow Lakes

An ox-bow lake is a horseshoeshaped lake found beside ariver. Ox-bow lakes are formedwhen continued erosion anddeposition create verypronounced meanders.

Eventually the river cuts throughthe neck of the meander.Deposition then occurs whichleaves the ox-bow lakeseparated from the river.

How an Ox-Bow Lake is Made

Landforms of the Old Stage

Levees

Levees are raised banks of deposited materialfound along the banks of the river. When the riverfloods and spreads out over the floodplain, theheaviest material is deposited close to the river.Over time and after many periods of flooding thisdeposited material forms levees along the banks ofthe river.

Levees – Raised Banks

Landforms of the Old Stage

Delta

A Delta is a triangular shaped piece of landwhich is formed at the mouth of the river. Asthe river enters the sea it drops off all theremaining material it is carrying. Thismaterial builds up to form new land. The river isforced to break up into smaller channels calleddistributaries.