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IEB geography fluvial morphology
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In depth of the fluvial section in ieb geography
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FLUVIAL PROCESSESTHE DRAINAGE BASIN
• Land based part of the hydrological cycle
• The area drained by a river system [main river and tributaries]
• Catchment area – area of land which forms the drainage basin [where
rain is caught in order to feed the rivers and groundwater]
DRAINAGE SYSTEMS
• Water in the river may come directly from rainfall or snowmelt, or
indirectly from surface run-off [water flowing downslope over the land]
and may come from groundwater
Supply of water to a drainage basin
• Saturated zone - when all air spaces in the earth’s crust is filled with water
underground
• Water table – upper limit of saturated zone
• Aerated zone – area above the water table that water can infiltrate
• Direct run off/channel flow/ discharge – river water confined to a river
channel
• Indirect run-off – water in the river that has come from a spring or
surface run-off/base flow
GR12 GEOGRAPHY FLUVIAL NOTES Hannah Koff
, • Base flow [groundwater] – groundwater that feeds the river
• Source – start of a river [mountains]
• Mouth – end of a river [ocean]
- In land: drainage basin
• Tributary – small river that joins the main stream
• Confluence – where 2 tributaries/rivers meet
• Watershed – a high lying area separaUng drainage basins
[hills/mountains]
• Interfluve – separates 2 tributaries [small watershed]
• River system – a river with its tributaries
• Lag Ume - Ume between peak rainfall and peak discharge [how long it
takes water to flood the river aWer a storm]
• Discharge – amount of water in the river
PROCESSES IN A RIVER
EROSION: break down and removal of [wears down the river bed and sides]
• Hydraulic acUon: force of water
• Abrasion: rocks suspended in river [hit banks]
• A[riUon: rocks suspended in the river hit against each other [smooth into
pebbles]
• Corrosion: chemicals and minerals in the water act on the bed [chemical
rxn]
• SoluUon: the finest sediments in the water [exfoliate] - silt
• Pot-hole scoring: circular currents in the water
DirecUon of erosion:
• VerUcal – channel gets deeper
• Lateral – widens the channel
• Headward – lengthens channel [backwards]
Erosion landforms: waterfalls, gorges, V-shaped valleys, potholes, open flood
plains, incised meanders
DEPOSITION:
• When a river loses energy, or its load capacity is exceeded, the heavy load
is deposited first
GR12 GEOGRAPHY FLUVIAL NOTES Hannah Koff
, • The ferUle silt deposited – alluvium
DeposiUon landforms: alluvial dans, braided streams [river splits into channels] ,
meanders, oxbow lakes, levees, swamps [marsh and vlei]
TRANSPORTATION: the material carried in the river is called the ‘load’
• TracUon – big boulders are pulled along the river bed
• SaltaUon – smaller rocks and stones bounce along bed
• Suspension – silt carried in the flow of water
• SoluUon – very fine, almost dissolved sediment is carried in the flow of
water
• FloaUng – very light material and sediment floats on the surface of the
water
TYPES OF RIVERS
1. Permanent rivers
- Flow all year round where there is high rainfall and the water table lies
above the river bed
- Found on the eastern side of RSA and in the southern cape
- Perennial
2. Periodic rivers
- Only flow during the wet season when the water table is closer to the
surface of the earth
- Dry season: dry up as no water is fed into the river by base flow
- Central South Africa and eastern cape [The Drakensberg Mountains’
rain shadow]
- Non-perennial
3. Episodic rivers
- Seldom flow as they are in low rainfall areas and the water table is
always far below the earth’s surface
- River flows aWer unusually heavy rainfall
- Western part of RSA
4. ExoUc rivers
- Found in dry areas but flow all year round as they are fed by
tributaries found in higher rainfall areas
- Western secUon of the orange river
GR12 GEOGRAPHY FLUVIAL NOTES Hannah Koff
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