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Geomorphology
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This gives a general understanding of wetlands and fluvial systems. Their formation and effects.
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Seminar Lecture 1 - What is dryland fluvial geomorphology?What are drylands?
Defined by aridity index
Average annual precipitation for the potential evapotranspiration
Dryland regions have AI < 0.65
o The way the rainfall is delivered – variable rainfall patterns and seasonal in
South Africa
o El Nino oscillation (cycles of droughts and floods)
Why do fluvial environments in drylands matters?
41% landmass is dryland – large population.
o People tend to live around rivers (people dependent)
o Ecosystems are highly developed and hotspots of biodiversity.
Large amount of pressure therefore threatened.
Sediments important (availability)
o Not a lot of vegetation and therefore high sediment (rivers seem brown)
o Sediment supply exceeds transport capacity (transport limited systems)
Transport capacity
o Flow is variable and have periods of low flow.
o Impact on ecosystem diversity – biota and nutrient cycling dependent on
variability.
South African Context
Interior of Africa has high elevation which is related to the collision zone and
associated with orogenic uplift.
o Implications for geomorphology, the way sediment is moved, the climate.
African super swell
o Lester King: mapped on a series of surfaces and landscapes are slowly
lowered over time.
If was the case, then how get high lying areas.
Parallel retreat.
Isostatic rebound.
In gowandaland 180m
Patrige and Maud: drainage patterns
o Uplift events = uplift was more in the east and then west (5m and 20m)
Baby 4 scenarios:
o Africa formed before breakup and therefore make Triassic age
o Topography associated with rifting – and slowly retreating
o Late creteaous and mantle plum and the hot material lifting up land and
therefore with the African rift.
o Late Cenozoic (patrige and maud)
Went around south Africa – took cores and dated the cores and looked for periods of
massive rates of erosion (70-110 m)
o East coast erosion was for longer
African surfaces in two phase (long period of time for landscape to be exposed)
o 1st phase: 93-66 m
o 2nd phase: 30-25m
,How affect south Africa:
Development of circum-polar current (western offshores and massive amounts of
global cooling in the south)
o Development of subtropical high pressure – descending and arid)
o Increased seasonality of south Africa.
Closure of the equatorial current
Descending air which makes rainfall very difficult
Aridification of South Africa
Warm moist air from the east and doesn’t get far because of the mountains.
o Leeward side = dry air
Dettinger and Diaz: SA get rain and then move in rivers (low lag)
o But if snow then there is a lag because has to melt first
o SA seasonal – east summer months while the southern cape is winter
months.
Strongly seasonal – all of rain and flow in a few months and therefore
rivers have periods with no flow.
o Mean runoff efficiency – what portion of rain flows in river (less than 25% of
rainfall converted into flow)
Mainly evaporated.
Wetlands – don’t have a lot of water and eroding therefore wetlands are associated
with drainage lines (group water and concentrating the resource)
o Evaporation (salts therefore concentration of salts and nutrient cycling affects)
Wetlands in South Africa
Locally positive water balances and some by faulting
Wetlands have sediment accumulation and know eroding so how long going to be
there for
Tooth and McCarthy (2007)
1. More frequent and longer period of desertification
2. Channels decrease in size as downstream – mostly evaporated, gravels and
vegetation and not more tributaries.
3. High levels of chemical sedimentation – evaporation and solutes in water and
therefore accumulate because of high evaporation.
4. Longer timescales of development that may extend far back into the Pleistocene – 18
thousand years (glacial ice age – Europe and other places was carved off).
5. More frequent fires that reduces the potential for thick organic accumulations and
promote aeolian activity (not really present anymore)
a. Fires affect the sediment supply to wetlands.
Influences of climate on geomorphic processes
Compared patterns and processes of two small catchments.
Occur on almost identical karoo supergroup geology.
Karoo (Kamberg) – high variability of rainfall.
, o Floodout valleys (gullies and then alluvial fans) = same in KZN even though
different climate.
o Karoo – channel that was discontinues and then channel reform in the lower
sections.
o KZN – meandering with floodplains
Headwater similar but the floodplains were different because of the climate.
Karoo V shaped valley and KZN flat channel
o KZN erode side to side (laterally eroding)
o Karoo gullies down and fills (cut and fill)
Larkin
If climate to river, then can predict how going to be in the future.
Australia – unconfined valleys
Decrease rainfall – type 2-5 in the paper (type 5 is the most arid and type 1 is least
arid and looked at the fluvial style)
Fluvial style: planform (what it looks like from above) and the cross section and how
this relates to process.
Process-form feedback and climate change
Imposed boundary conditions – don’t change quickly (geology)
o Macroscale geomorphology
Flux boundary conditions – flow (flooding) and fire
Seminar 2 Lecture: Understanding basics of rivers and wetlands
If know what types of wetlands, then know type of ecosystem services.
Most wetlands in SA occur in drainage lines – water locally collecting.
o Sediment supply is important (interflow, rainfall and surface flow)
o Some disconnected from drainage lines – pans.
Drainage line to wetland = force sediment accumulation (switch from erosion to
deposition).
What is a wetland?
Transitional terrestrial and aquatic, water table at or near surface + periodically
covered with water.
o Wide variety of environment (top mountains, some only fed by ground water)
o Different vegetation – trees to grasses.
o Different in terms of nutrient and productivity – SA productive in nutrient
cycling.
Intermitted to permanently flooded (fluctuating water level and this has impact on
nutrients)
o Nutrients affect productivity = vegetation create oxygen which can add y=to
recycling processes.
Depth is difference between lake and wetland.
o Wetland = vegetation has to be emergent.
Phosphate rapid recycling when seasonality that causes the flux.
Water has to be at the top 50 cm of the soil – root zone that has to be flooded.
o Support hydrophytic vegetation – anaerobic conditions (no oxygen in the soil)
o Most plants need oxygen on roots to grow.
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