Earth surface process

University of Malaya

SGES 1202 - Lecture

Rivers / Streams (running water)

• Hydrological cycle – Review• Drainage basins - divides• River systems & steep gradients (meandering, braided rivers) - erosion & deposition balance • Shaping stream valleys – landform change• Depositional landforms – deltas, levees, alluvial fans

Dr. Masatoshi Sone 1

Review: Surface Runoffs (Overland Flow)

• Splash Erosion : Dispersal of fine particles due to impact of rain drops

• Unconcentrated (Sheet) Wash : Thin layer of water moving over slopes during rainfall.

• Concentrated Wash : Flow of water on slope during rainfall in small & narrow channels (Rills) or broad & deep channels (Gullys).

• Overland flow can detach & transport fine grained sediments (silt & clay sizes) on slopes.

• Most overland flow reaches the foots of slopes in valleys where streams are found.

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Note: On the oceans evaporation exceeds precipitation, whereas on the lands precipitation exceeds evaporation. Thus, Surface Runoff is an excess (36K km³) from land. It is added to the ocean, then the cycles are blanced.

Saline

groundwater

Groundwater

flow

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A summary of the principal surface environments

Today, Streams and rivers, etc.

STREAMS & RIVERS

1. Bodies of water that flow from highland to lowland areas along well defined channels located in valleys;

2. only difference being that streams carry small volumes of water.

3. Most flow to the sea or oceans, except for those in inter-montane basins that often flow into lakes.

4. Found in all areas of the world, though having more active roles as agents of erosion & deposition in areas with humid climates, particularly in the humid tropics.

Drainage basin: An area influenced by a river & its tributary streams. Limited by drainage divide.

1st order streams from

headwaters

Drainage basin analysis

Drainage basin: An area influenced by a river & its tributary streams. Limited by drainage divide.

“Two drainage basins” separated by the divide

How to define/delimit/outline a drainage basin?

Identify dividing points of opposite 1st order streams and trace them – It is the dividing range of two basins.

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Geological map of Peninsular Malaysia

Q.

Where are

drainage dividing

ranges for major

river basins in the

peninsula.

•

Pahang River Drainage Basin

The longest river in Peninsular Malaysia?

Two river systems (Sg. Jelaiand Sg. Tembeling sub-basins) join at Kuala Tembeling to become Sg. Pahang (Pahang River).

Kuala – river junction in Malay

The Main Range granite massif forms the divide between the Pahang R Basin (East) and the Perak R Basin (West).

Sg. Pahang (flowing down from Kuala Tembeling

towards Jerantut then Temerloh)

Sg. Jelai

Sg. Tembeling

Kuala Tembeling Jetty (entrance to Taman Negara)

South

Kuala Tembeling

LONGITUDINAL PROFILE

• A longitudinal profile is a

cross-section along the

length of a stream (red line

in the figure).

• Note the concave-

upward curve of the profile

• A steeper gradient

upstream from the

headwaters and a gentler

gradient downstream.

Changes from Upstream to Downstream

STREAM CHANNEL

• WIDTH (W) : Width of channel between river banks –measured along the surface.

• DEPTH (D) : Difference in height between surface of water and channel floor or bed. Depth usually variable along cross-section between river banks.

• CROSS-SECTIONAL AREA (A) : Area of cross-section between river banks - measured perpendicular to the banks.

• WETTED PERIMETER (P) : That part of the cross-section that is in direct contact with the water.

Stream Discharge

Discharge (m³ / sec) = channel width (meters) x channel depth (meters) x velocity (m/sec)

The discharge of a stream is the volume of water flowing past a certain point in a given unit of time.Determined by multiplying a stream’s cross-sectional area by its velocity.

1. Amazon River 212,400 m³/sec2. Congo River 39,650 3. Yangtze 21,800? Sg. Pahang max 4~5,000 m³/sec

# The Amazon R is responsible for about 20% of all the water reaching the ocean via rivers.

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River types

• Four types of river channel recognised

• Braided and meandering are most common in geological record

Nichols (1999)

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Braided river

Pre-Cambrian to the recent

Meandering riveronly from Silurian to the recent

Because of vegetation

Common in rain-forest tropics

Note: Vegetation cover & flat relief

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The Baram River – a Meandering River System

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The Baram River

A photo taken from the airplane of Miri - Kota Kinabalu line.

A typical meandering river form

Numerous oxbow lakes

Mt. Mulu near top left corner

Aerial Photo by Masa

Gunung MuluIndonesia

Miri

Brunei

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Formation of a meandering river

Oxbow

lake

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Wikipedia

The Bramahputra

A long river system changing from braided to meandering types, flowing from the Himalayas through Assam and Bangladesh to the Bengal Bay.

LONGITUDINAL PROFILE- Variations in gradient & velocity- Changing from Straight to braided to meandering- Graded profile – Equilibrium between erosion & deposition

Erosion predominant

Deposition predominant

Time

River Evolution – Valley Widening to form a floodplain

INCREASED DISCHARGE ⥤ FLOODSCoarse sediments deposited close to channel (levees),

while finer sediment deposited further away (Back-swamp deposits)

Flood Plain = Fertile Soil

Now flooded. Will repeat this later.

Development of natural levees over flooding episodes

Development of natural levees

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The Baram Delta

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Stages of development in an ideal delta system development

A. Radially decreasing current velocities from jet mouth, deposit concentric arcs of sand, silt and clay.

B. Delta protrudes, forcing a channel through marginal levees.

C. Channel mouth chokes, levee ruptures and a new delta builds out from the crevasse.

May eventually “overlap” one after another (see Mississippi R.)

Selley, 2000

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Marshak

The overlap distribution of the post-Pleistocene lobes forming the modern Mississippi delta

Since some 7,500 years ago.

Many lobes over time

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Different types of deltas

The Nile – Triangle shaped delta

The Niger in W Africa – Arc-shaped delta

The Mississippi – A bird’s foot delta

SUMMARY

• Streams drain land areas & transport sediment from highland to lowland areas.

• In highland areas, down-cutting of channels (linear erosion) is dominant, whilst in lowland areas, lateral movement of river channels is dominant.

• Variations in velocity due to environmental factors (especially rainfall), give rise to different sites of erosion or deposition (i.e. different sizes of particles, etc).

• At river mouths, deposition predominates, giving rise to deltas.