Eroding landscapes:
fluvial processes
Quantifying erosion
in mountainous
landscapes
National Student
Satisfaction survey
(4th year)
Mikaël ATTAL
Acknowledgements: Jérôme Lavé, Peter van
der Beek and other scientists from LGCA
(Grenoble) and CRPG (Nancy)
Marsyandi valley, Himalayas, Nepal
Lecture overview
I. Bedrock erosion processes
II. Quantifying fluvial (and landscape) erosion on the long-term
III. Quantifying fluvial erosion on the short-term
I. Bedrock erosion processes
Plucking (Ukak River, Alaska, Whipple et al., 2000)
Abrasion (bedload impact)
Abrasion (suspended load)
Cavitation (www.irrigationcraft.com)
I. Bedrock erosion processes
Abrasion (bedload impact)
Amount of abrasion is a function of:
kinetic energy = 0.5mv2; angle of impact;
difference in rock resistance between
projectile and target
I. Bedrock erosion processes
Plucking
Amount of erosion is a function of: joint
density; stream power; kinetic energy of
impacts = 0.5mv2; angle of impact.
Whipple et al., 2000
(Ukak River, Alaska)
BEDLOAD
EXERTS A
KEY ROLE
I. Bedrock erosion processes
Abrasion by suspended load
Requires turbulence (eddies)  affects
mostly obstacles protruding in the channel
(e.g. boulders)
Whipple et al., 2000
I. Bedrock erosion processes
V
D
Most of the time, sediment is
resting on the bed and protects it
from erosion  bedrock erosion
(abrasion by bedload impacts +
plucking) happens during floods
I. Bedrock erosion processes
V
D
Consider 1 point in the channel, at a given time, during 1 flow event
Stream power per unit length: Ω = ρ g Q S
Fluvial shear stress: τ0 = ρ g R S
Transport capacity: Qc = k(τ – τc)3/2
where k and τc are constants [Meyer-Peter-Mueller, 1948]
Because sediments in river include a
wide range of grain sizes, some
particles will move while some others
(larger) will rest on the river bed
 TOOLS & COVER
I. Bedrock erosion processes
Whipple et al., 2000: process-based theoretical analysis within the frame of the SPL
Remark:
e = KAmSn = kτa where n = 2a/3
(and m is adjusted to obtain m/n = 0.5)
 if n = 1, m = 0.5 and a = 3/2  Incision  Specific Stream power (law 2).
 if n = 2/3, m = 1/3 and a = 1  Incision  basal shear stress (law 3).
Abrasion (bedload)
Not analyzed
Not analyzed
n = 5/3
a = 5/2
Plucking
n = 2/3 1
a = 1  3/2
Cavitation
n up to 7/3
a up to 7/2
Abrasion (suspension)
II. Quantifying fluvial (and landscape) erosion on the
long-term (103-106 years)
1) Fluvial erosion rates using terrace dating
2) Catchment-wide erosion rates using the fluvial network as an
“age homogenizer”
“Long-term” fluvial erosion rates (103-106 years): fluvial terraces
STRATH
TERRACES
Note: rivers can erode
and form terraces even
without uplift
Courtesy J. Lavé
Strath terraces: thin (or no) alluvium cover, contact alluviumbedrock relatively flat
Central Range, Taiwan
Siwaliks hills, Himalayas
(J. Lavé)
Strath terraces: thin (or no) alluvium cover, contact alluviumbedrock relatively flat
Siwaliks hills, Himalayas
(J. Lavé)
Fluvial incision rates using strath terrace dating
Age = n yr
h
Incision rate = h/n
Age = 0 yr
Dating methods:
- 14C,
- Optically stimulated luminescence (OSL),
- Cosmogenic nuclides.
Fluvial incision rates using strath terrace dating
Bagmati River, Himalayas
(Lavé & Avouac, 2000, 2001)
Fluvial incision rates using strath terrace dating
Bagmati River, Himalayas
(Lavé & Avouac, 2000, 2001)
Terraces are correlated in the field
+ using remote sensing
Fluvial incision rates using strath terrace dating
Reminder: the Quaternary Period
includes the following epochs:
Pleistocene (1.8 Ma  ~12 ka) and
Holocene (~12ka  present)
 Relatively constant incision rates
since the end of the Pleistocene
(PL3 is ~ 22ky old)
Bagmati River, Himalayas
(Lavé & Avouac, 2000, 2001)
FILL TERRACES: usually the result of landslides damming
! the valley (or large alluviation events filling narrow valleys)
Upstream of the dam
Tal, Marsyandi valley, Himalayas
FILL TERRACES: usually the result of landslides damming
! the valley (or large alluviation events filling narrow valleys)
image28.webshots.com
Chame, Marsyandi valley, Himalayas
Thick alluvium, up to hundreds of meters,
contact alluvium- bedrock highly irregular.
Local effect  must not be used to
determine long-term erosion rates.
FILL TERRACES: usually the result of landslides damming
! the valley (or large alluviation events filling narrow valleys)
The events that lead to the formation of fill terraces are relatively frequent in
actively eroding landscapes
Amount of erosion at a
given point along the river
Models, long-term
measurements
Reality
Time (x 105 years)
Terrace dating methods
a) 14C on organic debris in alluvium (up to ~40 ka).
 3 carbon isotopes: 12C (natural abundance 98.89 %), 13C (n.a. 1.11 %) and 14C (n.a.
1 part / trillion).
 When organism dies  no
more exchange with atmosphere
 the number of 14C atoms
decreases due to radioactive
decay.
atm
14C
organism /
 [14C] in the atmosphere is ~
constant (equilibrium between
rate of production and decay) and
is ~ to [14C] in living organisms.
 Age of terraces can be
estimated by counting the
number of 14C atoms in
organic fragments (assuming
that the time between the
organism’s death and its
incorporation into the
alluvium is negligible).
14C
 14C (or radiocarbon) is a
radioactive isotope which decays
with a half-period of 5730 years.
(%)
 14C formed in the atmosphere (interaction between cosmic rays and N molecules):
14N + n  14C + p
http://www.irb.hr/en/str/zef/z3labs/lna/C14/
Terrace dating methods
b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals,
ages from 100 yrs to 350 000 yrs.
 Radioactive isotopes + cosmic rays
 charge carriers (e.g., electrons e-,
electron holes h+) travelling in crystals
 Charge carriers can become trapped
in lattice defects. They progressively
accumulate in these “traps” over
geological timescales.
Charge carriers
http://www.ndt-ed.org
http://www.enigmatic-consulting.com
Terrace dating methods
b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals,
ages from 100 yrs to 350 000 yrs.
 Radioactive isotopes + cosmic rays
 charge carriers (e.g., electrons e-,
electron holes h+) travelling in crystals
 Charge carriers can become trapped
in lattice defects. They progressively
accumulate in these “traps” over
geological timescales.
 Exposure to light, heat, or high
pressures can release charge carriers
from trapping sites  reset the system
 The release process is associated
with a photon release. Number of
photons released = f (number of
trapped charge carriers released).
Charge carriers
http://www.ndt-ed.org
http://www.enigmatic-consulting.com
Terrace dating methods
b) Optically stimulated luminescence: burial ages of quartz or feldspar crystals,
ages from 100 yrs to 350 000 yrs.
 Sunlight releases trapped charge carriers.
 If a crystal gets buried, charge carriers are
going to accumulate in trapping sites.
 The longer the burial, the larger the number
of trapped charge carriers.
Optical stimulation (light)
 release of
charge carriers 
release of photons
 light emission
The older the terrace, the longer the
burial, the higher the number of
trapped charge carriers  the larger
the number of photons released with
the charge carriers  the higher the
http://suppelab.gl.ntu.edu.tw
intensity
of the light emitted!
Terrace dating methods
c) Cosmogenic Nuclides: exposure ages.
 Cosmic rays interact with atoms in the atmosphere and in the rocks exposed at the
surface of the Earth  nuclear reactions  cosmogenic nuclides.
3He,
Examples:
Stable
10Be,
14C,
21Ne,
T1/2 = 5730 a
T1/2 = 1.5 Ma
26Al,
T1/2 = 0.73 Ma
Stable
Cosmogenic nuclides accumulate in minerals
in the 1-2 m thick layer at the top of the Earth.
T1/2 = 0.3 Ma
Cosmogenic nuclide production rate
The longer the rock exposure, the higher
the amount of cosmogenic nuclides
Concentration in cosmogenic nuclides in
minerals = f (EXPOSURE TIME, latitude,
altitude, topography, type of mineral, type
of cosmogenic nuclide).
36Cl.
1-2 m
Depth
Terrace dating methods
c) Cosmogenic Nuclides: exposure ages.
Beryllium: 9Be = stable isotope; 10Be = cosmogenic isotope formed by interactions
between cosmic rays and O, N, Si, Mg, Fe. Beryllium in Quartz frequently used in
geomorphology to date objects up to millions of years old.
Chlorine: 35Cl and 37Cl = stable isotopes; 36Cl = cosmogenic isotope formed by
interactions between cosmic rays and Ar, Fe, K, Ca, Cl. Chlorine in calcite is a
method which begins to be reliable to date objects up to millions of years old.
Boulders on terraces
Bedrock strath terrace
© Scott T. Smith/CORBIS
http://web.ges.gla.ac.uk/~jjansen
II. Quantifying fluvial (and landscape) erosion on the
long-term (103-106 years)
1) Fluvial erosion rates using terrace dating
2) Catchment-wide erosion rates using the fluvial network as an
“age homogenizer”
Photo Eric Gayer
“Detrital methods”
http://www.futura-sciences.com/
Assumption: time spent in the
fluvial network is negligible
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Courtesy Eric Gayer
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
Uplift
1-2 m
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
Uplift
1-2 m
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
Uplift
1-2 m
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
Uplift
1-2 m
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
Uplift
1-2 m
Catchment-wide erosion rates
a) Cosmogenic ages on fluvial sands (Q + Grt).
Main limitation: assumption that landscape is eroding at a constant rate through time
Erosion
1-2 m
1-2 m
Landslide
Uplift
 gives the impression that
the catchment includes zones
with low, moderate and
extremely high erosion rates!
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
pangea.stanford.edu
Fission tracks in zircon or apatite
Bernet & Garver, 2005
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Deposition age (age of sediment td) (Ma)
td = 30 Ma
0
10
20
tc = 50 Ma
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Deposition age (age of sediment td) (Ma)
td = 20 Ma
0
10
20
tc = 40 Ma
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Deposition age (age of sediment td) (Ma)
td = 10 Ma
0
10
20
tc = 30 Ma
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Deposition age (age of sediment td) (Ma)
td = 0 Ma
0
20
tc = 20 Ma
Long
lag-time
Slope 1:1
10
Short
lag-time
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Let’s imagine that erosion
rate increases at 30 Ma
 Lag-time = 15 Ma
Deposition age (age of sediment td) (Ma)
td = 15 Ma
0
10
20
tc = 30 Ma
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: fission tracks
If erosion rate is constant, lag
time is constant.
Example: lag-time = 20 Ma
Let’s imagine that erosion
rate increases at 30 Ma
 Lag-time = 15 Ma
Deposition age (age of sediment td) (Ma)
td = 5 Ma
0
10
20
tc = 20 Ma
30
40
Bernet & Garver, 2005
FT age
(tc)
50
0
10
20
30
40
50 Ma
Catchment-wide erosion rates
b) Detrital termochronology: Ar/Ar or K/Ar methods (very simplified here)
39K
is stable.
40K
decays into 40Ar (gas) with a half-life of 1.25 billion years.
Degassed at high temperature, accumulates in
minerals at temperatures < closure temperature
40Ar
accumulates in mineral. Amount of
40Ar = f (time since crossing the isotherm)
T = closure temperature
 clock starts
T > closure temperature:
40Ar degassed
Isotherm corresponding to
closure temperature
Biotite: 300 ºC
Muscovite: 400 ºC
Hornblende: 550 ºC
Catchment-wide erosion rates
b) Detrital termochronology: Ar/Ar or K/Ar methods (very simplified here)
Central Himalayas, Nepal (Wobus et al., 2005)
Ar/Ar ages
on detrital
muscovite
Isotherm 400 ºC
 Migration
of the MCT?