Suspended Sed Patterns
Seasonal variability
Winter
Low discharge & inefficient drainage
∴ Little sediment can be transported
Summer
Flow in subglacial channels (faster)
∴ High sed transport capacity
Highest SSC - Spring
Large melt volumes
Access new areas of bed (∴ sed eroded during winter)
Increasing summer sed availability possible as:
Channels migrate & extend
Channel bank collapse ?
Seasonal sed exhaustion (Collins)
= f (mobility of rivers beneath glacier )
If they can move, will keep transporting sediment (move to where sediment is)
Early in season
Pronounced sediment load peaks
Sediment flushing with initial discharge peaks
Later in season
Lower SSCs even when high discharge
Evince sediment exhaustion
--> no more sediment available where meltwater flows
Spring Sed Flush
(Cowton 2012)
If peak SSC doesn't occur with peak discharge
Lake drainage event causes sudden sediment pulse
Large portion of sediment may be flushed out
by individual events
as melt has access to lots more sediment
SSC Residuals (Willis, 1996)
Basal disturbance by ice explains SSC residuals
Whilst sed supply largely controlled by glacier flow/erosion
Residuals (positive outliers) correlate with sudden ice motion events
i.e. when glacier speeds up
Faster flow --> more sed available --> more sed flushed out
Long term sed availability
= Controlled by glacier erosion
whereas short-term basal sed evacuation
--> dictates how much sediment is produced
= f ( subglacial drainage efficiency )
∴ helps sustain erosion
High melt summers
greater flow ∴ more sed produced
more efficient sed evacuation
To sustain erosion...
Need efficient subglacial drainage
to evacuate sediment products
Bedload Transport
e.g. large cobbles etc
-->deposited as outwash
Very difficult to measure
Bedload component of erosion = poorly constrained
∴ Large error in estimates subglacial erosion rate
when based on sediment evacuation