Fluvial geomorphology
ESS 400a
Summer 2014
The fluvial system
Headwaters
Straight channel
Tributary
Meandering channel
Delta
The fluvial system
Gravity! The engine
for geomorphic
work
Headwaters
Straight channel
Tributary
Meandering channel
Delta
The fluvial system
Gravity drives changes….but the amount of water and
sediment is controlled by basin size.
Drainage basin (watershed):
the area contributing water and
sediment to the channel
The fluvial system
Quiz time!
1
Where is the river flowing
fastest – point 1 or 2?
2
The fluvial system
Quiz time!
1
Where is the river flowing
fastest – point 1 or 2?
Point 2 – why
is this?
2
The fluvial system
The volume of water increases downstream as more
tributaries and run-off join
1
This volume of water is termed discharge, and is the
volume of water flowing through a channel per time
unit.
Therefore, Q = v * A
So as Q increases, velocity and area
increase!
2
The fluvial system
The volume of water increases downstream as more
tributaries and run-off join
1
This volume of water is termed discharge, and is the
volume of water flowing through a channel per time
unit.
Therefore, Q = v * A
So as Q increases, velocity and area
increase!
2
Note: At the reach scale, such as BSC, Q is constant
The fluvial system
How do we talk about it’s components?
Stream order: A numerical classification of stream
segment size within a larger network
Headwaters
Straight channel
1
1
2
Tributary
Meandering channel
3
Delta
The fluvial system
What do these streams look like?
Straight channels:
The fluvial system
What do these streams look like?
Meandering channels:
The fluvial system
What do these streams look like?
Braided channels:
The fluvial system
What do these streams look like?
The fluvial system
Pool-riffle features in meandering or sinuous rivers
Pool: over-deepened channel from
scour or high discharge events
Riffle: shallow channel with rough lowflow water surface
Bar: above flow, loci of sediment
deposition during high flows
Thalweg: German for valley, deepest,
and often fastest, part of the channel
The fluvial system
Pool-riffle features in meandering or sinuous rivers
Hydrology
Q = area * velocity = width * depth * velocity
Water velocity
0.6 depth = depth average
velocity
Laminar sub-layer (very
very thin)
With the exception of the laminar sublayer, flow is turbulent!
Water velocity
0.6 depth = depth average
velocity
Laminar sub-layer (very
very thin)
With the exception of the laminar sublayer, flow is turbulent!
Velocity in a meander
Factors:
- Drag from the bed
- Drag from banks produces a parabolic velocity shape
across the width
- Super-elevation and pressure gradients?!
Velocity in a meander
Higher elevations = higher pressure = pressure gradient
inwards = helical flow
Lower velocity on
inside = deposition
of gravel bar
Higher velocity on
outside = erosion of
bank
Meander migration
1. Cut banks erode
2. New material on bar
3. Meander migrates
4. Planes off a valley
Incision
Planation forms the valley but how does a river erode down?
Rivers are at a “graded equilibrium”, where their profiles
approximate an inverse log curve
A change to the shape of this
curve – i.e. fault uplift – can
provoke a reaction – such as
incision and knickpoint formation
Incision
Planation forms the valley but how does a river erode down?
Incision
Planation forms the valley but how does a river erode down?
Incision
Planation forms the valley but how does a river erode down?
The river upstream of the fault is now going to
erode down to return to its previous
equilibrium, and there is now a knickpoint
(zone of steepened slope)
Knickpoint
Incision
Planation forms the valley but how does a river erode down?
Knickpoint will move
upstream like a wave
Migration style of
knickpoint will depend on
bedrock and substrate
Pool generate at base of
knickpoint
Knickpoints
There are multiple ways to generate
a knickpoint, including:
1. Differential bedrock strength creating
zones of different erosion
2. Base level drop from sea level changes
or rock uplift
3. Base level drop from faulting across the
river
4. Changes in discharge from stream
capture, piracy, or beheading
5. Anthropogenic influences – redirecting
streams, changing surrounding
surfaces
6. Natural or artificial damming
Landforms
Floodplain: the lowest surface adjacent to the river that is composed of
overbank flood deposits overlying laterally accreted alluvium
Levee: overbank deposits immediate to the river formed during rapid
settling during floods – builds up bank height
Terrace: abandoned floodplain, product
of incision and lateral planation
Oxbow: abandoned meander bend in
the floodplain
Alluvial fan (not shown):
conical in shape, loose,
coarse sediments from
small channels on steep
hillslopes
Floodplain
Terraces
Fill
terraces
Alluvium only, bedrock
buried
Strath
terraces
Erosion & planation into
bedrock with thin gravel
cap
Paired
terraces
Un-paired
terraces
Incision and planation
occur at different times
Incision and planation occur
together (more common)
Terraces
A record of the incision and planation history
What happened in this valley?
Terraces
A record of the incision and planation history
What happened in this valley? – 6 or more incision, 9+
planation, and 3 aggradation periods!
You will be asked to do this, or something similar, for Assignment 4!
Response timescales
• Terraces form due to incision, which we learned has
multiple triggers
• Thus, a younger terrace might record recent 103 year
fault movement, while an older terrace might be due to
climate variations on a 104 to 105 timescale
Response timescales
Terraces aren’t the only features left from the adjustment of rivers to
environmental changes
Your site map:
• Read HARRELSON ET AL. 1994
• Recording all the features we just talked
about will help you constrain the geomorphic
history: oxbows, slumps, modified areas,
terraces, alluvial fans…
• Channel features such as pools, riffles,
knickpoints, strath exposure, will also help
your history – note these on a site map or on
the long profile notes!
Additional readings
ESS326 – Geomorphology (WTR)
ESS426 – Fluvial geomorphology (SPR)
ESS541 – Applied fluvial geomorphology (AUT)