Fluvial Geomorphology Flashcards
Fluvial geomorphology
- study of running water and its influence in modifying our land surface
- characteristics of rivers as a function of landscape position
- how river accomplish ‘work’
- how rivers respond to change
Geomorphic work
- running water exerts a force = landscape offers resistance
- fluvial processes dominate in most parts of the world, where precipitation exceeds evaporation
- except in cold regions (ice dominated), and dry regions (wind = most erosive agent)
Channel
the morphological feature within which flow is constrained
Floodplain
the flat area surrounding the channel which is inundated with water when the river floods
Bankfull
The maximum water level that is achieved before water s disperse onto the floodplain
Catchment/drainage basin
the area of land where all surface water will run to a common low point in the landscape
Discharge
volume of flow per unit of time
= velocity(average depthchannel width)
Perennial rivers
- flow all year round
- are maintained by subsurface/groundwater flow
- typical of humid/sub-humid regions (groundwater maintained by high or constant rainfall)
- persistent rainfall across the year
- form is shaped by high frequency, low-magnitude events
Ephemeral rivers
- flow intermittently respond to surface runoff events
- respond to surface runoff events
- semi-arid regions (rainfall now high enough to maintain effective subsurface flow)- flow responds to low-frequency, high magnitude events
how climate influences river processes
- amount of water entering the ground controls height of the water table
- channel flow occurs where land surface intersects the water table
- in arid/semi-arid water table is so low that it frequently isn’t intersected by landscape
- consequently no ground water (baseflow) driven river activity
How geology influences river processes
- bedrock permeability has major influence on river flow patterns
- neighbouring catchments in the thames basin
- identical climatic conditions
- different flow regime
Regional drainage networks
Consequent drainage - produced from original uplift - streams follow slope of land over which originally formed Subsequent drainage - - developed after initial incision - soft strata, faults
Old terminology
assumes ordering of events which may be incorrect
Modern terminology
invokes structural control on drainage development
- strike, dip and anti-dip streams
base level in fluvial systems
- the lowest point at which a stream can erode to
- ultimate base level = sea level
- falling sea level steepens gradient = incision
- rising sea level reduces energy = submerged river valleys
regional drainage patterns
Dendritic - horizontal, uniform strata/sediment, no structural control
Parallel - strong structural control
Rectangle - primary and secondary drainage directions, alternative soft-hard rock/sediment
Radial - volcanic cones, uplift domes
Centrifugal - high areas on inside of meander bends
Centripetal - central low point, basin floor e.g. caldera subsidence basin
Deranged Drainage
develops on newly exposed land
- following glacial retreat
- no structural/bedrock control
- irregular stream courses
- short tributaries- lakes
- bogs
Fluvial erosion and transport
streams are powerful erosional agents
Stream power
the power available to transport sediment
= water densitygravitational accelerationstream discharge*channel slope
Critical power
the power required to transport the available sediment load
stream load
Green River, Colorado River confluence
- higher suspended sediment load form the green river
- greater stream power, potential for erosion and transport
Hjustrom curve
illustrates the water velocity at which particles can be entrained and transport. Consider; cohesion, shape, density and particle size
- applies to alluvial channels only
Corrosion
chemical weathering of bed and bank material
Corrasion/abrasion
wearing away by impact or grinding or particles
Cavitation
high flow velocities at base of waterfalls/rapids = shockwaves from exploding bubbles produced by pressure changes
Evorsion
force of water smashed bedrock (no solid material involved)
Erosional processes
Streams may erode their channels
- downward, net removal of sand and gravels
- Laterally, undercutting and bank collapse
depositional processes
- channel deposits
- channel margin deposits
- overbank floodplain deposits
- valley margin deposits
Incision
the process downcutting/erosion of a river into its bed (lower of channel height)
Aggradation
the large scale deposition of sediment causing a vertical increase in floodplain height
Channel deposits
- resting bed load, temporary
- lag deposits, course, heavy deposits
- channel fills, in abandoned or aggrading channel segments
Channel margin deposits
Form by lateral accretion and area preserved by channel shifting
- point bars; inside meander loop, slowing water, deposition
vertical acceretion
- fine grained material
- natural levees and back swamps
splays
small accumulations of flood debris
valley margin deposits
- colluvium: slope wash and sol creep
- mass movement deposits; debris avalanches and landslides
Fluvial erosion landforms
- rills and gullies
- bedrock channels
- alluvial channels (straight, meandering, braided, anastromosing, anabranching)
- valleys
rills, gullies and valleys
- increase in scale and energy with time
- rills <30cm wide, 60cm deep
- gullies, continuous or discontinuous
Straight channels
- uncommon and restricted to V-shaped valleys
- channel migration limited by structural controls
meandering channel
- grade 1 of alluvial channel form
- caused by instabilities in turbulent water on an erodible bank
- sinuosity of 1.5 (channel length/valley length)
Braided channels
- high energy, steep gradients, high sediment supply
- forms when the stream cannot hold its sediment load and dumps it in the middle of the channel
- channel bars are collected piles of sediment
- channel bars force the river to flow around them which separates river into braids
- typical width of channels is similar to that of bars
Anastomosing channels
- multiple interconnected channels
- floodplain is stabilised
- typical width of channels is smaller than that of the bars
stream ordering
- 2 schemes (Strahler and Shreve)
- drainage density is the balance between stream erosion and resistance of the surface
- influenced by; climate, lithology and vegetation
balance models
how rivers respond to changed in sediment load, base level change and climate
- climate change may lead to alterations in all factors
in glacial conditions
- relative valley slope increases
- channel forming discharge increases
- lack of vegetation
- high precip (Seasonality)
- glacial melt water
current rivers
- relatively low energy
- vegetated catchments and channels
- less seasonal flow regime
- not capable of eroding from top of vegetated slope
- accumulation of fine grained sediments
the end of the last glacial
- Periglacial conditions
- lower sea level = rivers with higher energy regimes
- sea level ca. 100m below present
- less vegetation on the landscape
- energy provided = erosion of Solent river
- as sea level rose = raised base level = drowned river valleys
sediment assemblages
- coarse grained (gravel, pebble transport and deposition)
- erosional features
- high energy
- periglacial processes (ice wedge cast)
Thames terrace
- the present day course of the Thames was only arrived at after the Anglian ice overrode its palaeochannels
- excavation of terraces
- interglacial deposits contain; pollen, insect fauna, mammalian fauna
- provide valuable information about the environments of past interglacial periods
- archaeological material is often found alongside interglacial deposits
