Geoscience Reference
In-Depth Information
(Van  Heijst & Postma, 2001). Muto & Swenson
(2005) quantified the maintenance of the fluvial
grade by the specific square-root-of-time depend-
ent rate of relative sea-level fall. The specific coef-
ficient depends on sediment-water supply and
system geometry. Hence, upslope of the knick-
point the alluvial river system can remain aggra-
dational for a wide range of relative sea-level fall
rates and channels can still back fill and avulse.
Downslope of the knickpoint the river profile
steepens, which hinders the back-fill process.
During transgression the shoreline steps back
(PA, see Fig.  4). During the retrogradation brief
stages of progradation occur (delta lobe building),
during which the fluvial system can aggrade (see
the experimental results of Muto & Steel (2001)
and Hoyal & Sheets (2009). The development of a
coastal barrier system forces the shoreline sea-
ward bringing out the base-level point towards
which the fluvial system is going to adjust itself.
In this period of time the system is brought back
close to the start-up stage leading to maximal
aggradation in the alluvial realm and hardly any
sediment bypass. During these periods, backfill-
ing in channels is maximal and avulsion rates
must be at their highest. The regular avulsions
cause regular delta lobe progradation and shifting
that is recognised as parasequences: shallowing
upward sequences developed on top of flooding
surfaces. The precise development of these coastal
sequences can depend strongly on rate of sea-level
rise (e.g. Cattaneo & Steel, 2003).
'buttress' (e.g. sea-level) below which streams
cannot incise and above which streams cannot
aggrade substantially. Upper and lower buffers are
both anchored to this buttress and may diverge for
some distance up-dip as profile variability is
introduced by increasing influence of upstream
base level controls. Upstream controls like cli-
mate and tectonics primarily determine spacing
trends between these upper and lower buffers.
The change in river profile as a consequence of
climate change is relatively fast in the case of a
change in average discharge and much slower in
the case of averaged change in sediment yield, as
shown by experimental studies by Van den Berg
van Saparoea & Postma (2008). These experiments
demonstrate a fundamental difference between
the response of the sediment flux at the river
mouth due to changes in discharge and due to
changes in sediment flux and differences between
the total mass accumulation history in response to
changes in discharge and sediment flux. The first
fundamental difference between a response to
either discharge or sediment input change is the
total sediment budget at the valley outlet, which
is much larger in case of a discharge change. The
second fundamental difference is that the gradient
of the valley floor is correlated positively with
sediment influx and negatively with discharge (cf.
also Mackin, 1948). The third difference is that
the response to changes of discharge is very rapid,
whilst the response to sediment flux changes is
much slower (Van den Berg van Saparoea &
Postma, 2008).
Hence, aggradation rates of the channel belt
(and thus avulsion frequencies) would decrease at
high discharges, as a consequence of reduction of
accumulation space by lowering of the river pro-
file; yet, backwater effects and channel blocking
may temporarily increase the avulsion rate, silting
up adjacent floodplains. If the river system would
be near its grade, deviations in accumulation
space forced by climate change are not likely to be
very large, so the system will remain in the fill-up
stage.
Climate
Holbrook et al . (2006) described the river profile
as being highly variable due to changes in dis-
charge and supply, i.e. by climate change. All
potential river profiles are bounded above by a
profile of highest possible aggradation and below
by the profile of maximum possible incision.
These upper and lower profiles are called 'buffers'
and they envelop the available fluvial preserva-
tion space (Fig. 5). Thickness of the buffer zone is
determined by variability in upstream controls
and should increase up dip to the limit of down-
stream profile dominance.
The buffer model considers fluvial preservation
to be limited to some space between upper and
lower maximum possible profiles: 'buffers' that
move and/or alter shape with downstream base-
level shifts. Downstream, base level is considered
to be controlled by movement of some physical
Tectonics
As also hypothesised by Holbrook et al . (2006),
regional tectonics result in tilting of the river pro-
file, while more local tectonics cause sagging, all
with direct consequences for aggradation rate
(Fig.  5). Channel belts appear not to be attracted
to  the subsidence maximum, unless subsidence
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