1. IV – Large grains and large floods
Influence of boulders on sediment export during floods
 “Bedrock aggradation”
Only extreme events can mobilize a significant fraction of the boulders
Marsyandi River, Himalayas
Feather River, California
1 m

2. IV – Large grains and large floods
Influence of boulders on sediment export during floods
 “Bedrock aggradation”
Only extreme events can mobilize a significant fraction of the boulders
 long term channel adjustment
Elevation (m)
Distance (km)
French Alps (Brocard and van der Beek, 2006)
Utah (Johnson et al., 2009)
Makes river bedrock more resistant to erosion  Steepening to provide extra power needed

3. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
Exceptional floods (recurrence interval > 100 yrs)
Produce major landscape change (sediment transport, deposition, bedrock erosion)
Caused by powerful storms / typhoons, landslide dam break, glacial lake outburst, subglacial volcanic eruptions
Humla, Nepal, 2011 (
Uttarakhand, 2013 ( 2013/06/24/uttarakhand-flood-2013/)

4. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
Exceptional floods (recurrence interval > 100 yrs)
Produce major landscape change (sediment transport, deposition, bedrock erosion)
Caused by powerful storms / typhoons, landslide dam break, glacial lake outburst, subglacial volcanic eruptions
Full mobility
Net sediment export from mountain to “basin”
Can have a flood-cleaning or flood-depositing effect
Paradise Creek, East Australia (Baggs Sargood et al., 2014)

5. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
Jökulsá a Fjöllum, NE Iceland
(Baynes et al., 2015a, b)
500 m

6. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
Replenishment time is a general term and depends on the context. May be a few years in tectonically active areas with coupled hillslopes, may be on the scale of glacial-interglacial periods in Iceland
Limited by transport capacity
Limited by bedrock weathering
Limited by tools availability

7. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
- Formation of stable boulder bars
Paradise Creek, East Australia
(Baggs Sargood et al., 2014)

8. IV – Large grains and large floods
Influence of extreme flood events on sediment export during floods
 Extreme flood inhibits subsequent sediment export
- Cleaning effect
- Formation of stable boulder bars
 Extreme flood enhances subsequent sediment export
- Hillslope undercutting
- Destruction of resistant landforms
Partnach River, Bavarian Alps, 2005
(Morche and Schmidt, 2012)

9. Conclusions
Boulders are common in mountain rivers; they can inhibit sediment transport and bedrock erosion when abundant
Extreme floods are crucial in mobilising the whole grain population. They can be flood-cleaning or flood-depositing; they can inhibit or enhance sediment export by subsequent floods
Taking episodic supply and transport into account is essential for predicting sediment dynamics and bedrock erosion in mountain rivers, as well as the development of stratigraphy
Amount and grain size of sediment exported during a flood = f (flow competence, transport capacity AND characteristics of sediment available for transport)
Amount and grain size of sediment exported during a flood = f (flow competence, transport capacity AND characteristics of sediment available for transport)
Mountain Rivers
Worth highlighting that sources vary spatially but also in time (climate, tectonics).
Boulders stable yr timescales, can be mobilised by extreme floods.
Depending on sediment availability and sediment supply at the time of the flood  flood cleaning or depositing, leading to very different type of sediment exported
Vary in space and time due to episodic nature of sediment supply + differences in sources