Fluvial Geomorphology

1. Fluvial Landscapes Fluvial Landscapes

2. Overview Overview
„„
Fluvial Processes Fluvial Processes
„„
Basel Level Basel Level
„„
Down Down--cutting and meandering cutting and meandering
„„
Drainage Drainage
„„
Watersheds Watersheds
„„
Drainage Patterns Drainage Patterns
„„
Stream flow and fluvial processes Stream flow and fluvial processes
„„
Meandering Streams Meandering Streams
„„
Floodplains Floodplains
„„
Flooding Flooding

3. Base Level and Fluvial Landscapes Base Level and Fluvial Landscapes „„
A Profound Concept: Water flows downhill A Profound Concept: Water flows downhill
„„
Abrades and erodes the landscape as it happens Abrades and erodes the landscape as it happens
„„
Abrasion = erosion process Abrasion = erosion process
„„
Creates Creates erosional erosionallandforms landforms ––drainage patterns, cut banks drainage patterns, cut banks
„„
Carries eroded material downstream as either dissolved, Carries eroded material downstream as either dissolved,
suspended or bed load suspended or bed load
„„
Load = Transport Load = Transport
„„
Load becomes deposited, creating depositional landforms Load becomes deposited, creating depositional landforms
„„
Depositional landforms: Deltas, point bars Depositional landforms: Deltas, point bars
„„
Base Level: The lowest level that a river can erode Base Level: The lowest level that a river can erode
down to. down to.
„„
Gravity sets the water in motion Gravity sets the water in motion
„„
Ultimately, the water is trying to reach sea level Ultimately, the water is trying to reach sea level
„„
Local impediments to flow create Local impediments to flow create
local base levels local base levels

4. „„
Higher above base level, water has more potential Higher above base level, water has more potential
energy, which gets converted to kinetic energy as energy, which gets converted to kinetic energy as
it flows it flows downslope downslope
„„
Above base level, more energy directed downwards, Above base level, more energy directed downwards,
more able to erode bedrock more able to erode bedrock --
downcutting downcutting
„„
Closer to base level, energy directed outward with Closer to base level, energy directed outward with
motion, erosion occurs more laterally motion, erosion occurs more laterally --
meandering meandering

6. „„
Landforms associated with rivers vary by how Landforms associated with rivers vary by how
high above base level the river is high above base level the river is
„„
Above base level Above base level
„„
Steep V Steep V--shaped valley walls shaped valley walls
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Narrow stream channels Narrow stream channels
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Distinct drainage patterns based on underlying Distinct drainage patterns based on underlying
geography geography
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Downcutting Downcutting(erosion) and transportation are the main (erosion) and transportation are the main
geomorphological geomorphologicalforces acting forces acting
„„
Closer to base level Closer to base level
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Wide stream channels (floodplains) Wide stream channels (floodplains)
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Valley walls characterized by terracing Valley walls characterized by terracing
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Meandering river patterns, river deltas Meandering river patterns, river deltas
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lateral abrasion, transport and deposition lateral abrasion, transport and deposition
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Tectonic forces can cause areas close to base level Tectonic forces can cause areas close to base level
to become uplifted to become uplifted
„„
Meandering gives way to Meandering gives way to downcutting downcutting
„„
Entrenched meanders, canyons Entrenched meanders, canyons

11. Drainage Drainage
„„
When rain falls on a landscape, it drains When rain falls on a landscape, it drains
from higher to lower elevations from higher to lower elevations
„„
erodes surfaces as it happens erodes surfaces as it happens
„„
largely an largely an erosional erosionalprocess process
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Occurs above base level Occurs above base level --downcutting downcutting
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over time, forms regions of consistent over time, forms regions of consistent
drainage patterns that concentrate runoff drainage patterns that concentrate runoff
into specific streams and rivers into specific streams and rivers
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watersheds or drainage basins watersheds or drainage basins
„„
drainage pattern drainage pattern

12. „„
Watersheds Watersheds
„„
topography directs flow of surface water topography directs flow of surface water
into various into various downslope downslopeareas areas
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at high points in the topography, surface at high points in the topography, surface
flow separates down different slopes, and flow separates down different slopes, and
channels water into separate drainage channels water into separate drainage
systems systems
„„
these high points are called drainage divides or these high points are called drainage divides or
interfluves interfluves
„„
drainage divides form the boundaries between drainage divides form the boundaries between
watersheds watersheds

20. Drainage Patterns Drainage Patterns „„
Pattern relates to Pattern relates to
underlying bedrock underlying bedrock
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Type of rock Type of rock
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Soft sedimentary layers tend to Soft sedimentary layers tend to
produce produce dendritic dendriticpatterns patterns
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Igneous or hard sedimentary Igneous or hard sedimentary
bedrock tends to produce bedrock tends to produce
rectangular patterns rectangular patterns
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streamflow streamflowdirected through directed through
orthogonal rock fractures orthogonal rock fractures
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Folding Folding
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produces trellis patterns produces trellis patterns

21. „„
Dendritic Dendritic
„„
soft underlying soft underlying
sedimentary rock sedimentary rock
exerts no strong exerts no strong
directional influence directional influence
on the flow of water on the flow of water
„„
most common most common
stream pattern stream pattern
„„
a radial pattern a radial pattern
occurs when soft occurs when soft
layers are uplifted layers are uplifted
by an igneous by an igneous
intrusion (dome) or intrusion (dome) or
on mountain or on mountain or
volcanic peaks volcanic peaks

24. „„
Trellis Trellis
„„
Occurs where there is intense folding Occurs where there is intense folding
„„
antiforms antiformsand and synforms synforms
„„
alternating layers of soft and resistant rock alternating layers of soft and resistant rock
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Common in Appalachia Common in Appalachia

30. „„
Rectangular Rectangular
„„
Occurs on areas with Occurs on areas with
resistant bedrock resistant bedrock
„„
Surface flow follows Surface flow follows
rectilinear fracture rectilinear fracture
pattern of the rock pattern of the rock
„„
produces an annular produces an annular
pattern when this pattern when this
occurs on a dome occurs on a dome

31. „„
Parallel Parallel
„„
Occur on steep Occur on steep
slopes slopes
„„
Deranged Deranged
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Occur where Occur where
something has something has
disrupted normal disrupted normal
surface patterns and surface patterns and
processes, such as processes, such as
glaciation glaciation

32. Stream Flow and Fluvial Processes Stream Flow and Fluvial Processes
„„
Streams run down Streams run down
slopes slopes
„„
Potential energy is Potential energy is
changed to kinetic changed to kinetic
energy energy
„„
Discharge Discharge
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Speed Speed
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Channel depth Channel depth
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Channel width Channel width
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Fluvial Processes Fluvial Processes
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Erosion: abrasion Erosion: abrasion
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Transport: load Transport: load
„„
Deposition Deposition

34. Load Load
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Dissolved Load Dissolved Load
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Suspended Load Suspended Load
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Greatest part Greatest part
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Bed Load Bed Load
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Traction Traction
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Saltation Saltation
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General Principle: The greater the speed, the General Principle: The greater the speed, the
more load a stream can carry more load a stream can carry
„„
Slowing causes deposition Slowing causes deposition
„„
Largest and heaviest materials deposited first Largest and heaviest materials deposited first

35. Meandering Streams Meandering Streams
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Erosion and Erosion and
Deposition Deposition
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Undercut bank and Undercut bank and
point bar point bar

37. Floodplain Floodplain
„„
Formation Formation
„„
Widening of valley through meandering Widening of valley through meandering
process process
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Features Features
„„
Floodplain Floodplain
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Natural levees Natural levees
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Oxbow lakes and swamps Oxbow lakes and swamps
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Meander scars Meander scars
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Terraces Terraces

48. Entrenched Meanders Entrenched Meanders
Landscape was once near base Landscape was once near base
level, and meanders formed. level, and meanders formed.
Later, tectonic uplift caused the Later, tectonic uplift caused the
river to be raised above base river to be raised above base
level, recommencing down level, recommencing down--
cutting, and carving these cutting, and carving these
canyons canyons

53. Deltas Deltas
„„
Deposition Deposition
„„
River reaches sea level (base level) River reaches sea level (base level)
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Velocity decreases dramatically Velocity decreases dramatically
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Load is deposited Load is deposited
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Distributaries Distributaries

58. Flooding Flooding
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Rating (10 Rating (10--year flood, 100 year flood, 100--year flood, etc) year flood, etc)
„„
an expression of probability (10%, 1%, resp.) an expression of probability (10%, 1%, resp.)
„„
Contributing factors Contributing factors
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Rainfall, of course Rainfall, of course
„„
Oscillations, hurricanes Oscillations, hurricanes
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Human activities that increase runoff and reduce Human activities that increase runoff and reduce
soil moisture storage capacity soil moisture storage capacity
„„
urbanization (increasing paved surfaces) urbanization (increasing paved surfaces)
„„
agricultural practices agricultural practices
„„
Midwest Flood of 1993, North Carolina flood Midwest Flood of 1993, North Carolina flood
of 1999 of 1999
„„
1000 year flood events (0.1% probability of 1000 year flood events (0.1% probability of
occurring) occurring)
„„
Contributed to formations of Contributed to formations of ““Dead Zones Dead Zones””
„„
Dead Zone Dead Zone
(The Zone of Hypoxia in the Gulf of Mexico) (The Zone of Hypoxia in the Gulf of Mexico)
„„
Pamlico Sound, NC Pamlico Sound, NC