Generic autogenic behaviour in fluvial systems: lessons from experimental studies - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 15

CONCLUSIONS Significant advancement in recognition and quan-

tification of generic autogenic behaviour stems
from experimental research on a landscape scale,
where drivers for stratigraphic architecture can be
verified along known input and boundary condi-
tions. Scaling is by similarity of process, which is
justified by scale invariant architecture and
morphology as channels, sheet flow deposits, bars
and lobes. Such landscape experiments are, as
yet, non-existent for low gradient river systems,
although stretches of low gradient river systems
are presently studied by groups experimenting in
more detail. In addition, field studies with high
resolution age control are required to learn about
the forcing mechanisms of avulsion in fluvial
systems.

Autogenic behaviour is discussed for steep, mod-
erate and low gradient fluvial systems and is
found to be different for each fluvial type. In the
first two types sediment is spread evenly over
wide areas, in contrast with the low-gradient
rivers. Other differences are that:

1 Steep-gradient systems show alternations of
sheet flow and relief steepening, incision and
progradation followed by backfilling. Cycle
duration depends on the total volume of the
fan incision and the backfilling rate, ranging
from years to decades, depending on fan
activity;
2 Moderate-gradient systems show bifurcations
of short duration during high aggradation rates
and bifurcations with slowly lateral migrating
channels during low aggradation rates. Cycle
duration is relative short ranging from days to
years;
3 Low-gradient rivers build their channel belts
up to grade while their floodplains lag signifi-
cantly behind. It is inferred on the basis of
numerical modelling that the origin of avulsion
lies in gradient advantage, channel plugging
and local hydrology in river bends. Details of
the avulsion process are least understood in
this type because experiments that allow study
of their autogenic behaviour are absent. Cycle
durations are relatively long and of the order of
decades to centuries and strongly dependant on
the backwater adaptation length.
4 Aggradation rate in the channel belts is the
most important driver for the frequency of
autogenic behaviour. Aggradation rate decreases
non-linearly when the fluvial system builds up
to grade, so that a full spectrum from high to
low frequency autogenic behaviour is to be
expected in natural systems that build up to
grade.
5 Fast and slow changes in allogenic forcing rela-
tive to the equilibrium time (here related to the
backwater length) of delta plain river systems is
suggested here as the dominant driver of
changes in the rate of aggradation and herewith
the frequency of autogenic behaviour (Fig. 6).
Fast change will increase the frequency of auto-
genic processes and will force rapid vertical
changes in fluvial architecture, whilst a slow
change will result in little or very gradual
change of architecture.

ACKNOWLEDGEMENTS

The ideas presented in this paper evolved during
the PhD study of Maurits van Dijk and through
discussions with Kim Cohen, Esther Stouthamer
and Maarten Kleinhans. In January, 2011 first
draft of the MS was read critically by Kick
Kleverlaan, Maarten Kleinhans and Allard
Martinius, who provided useful comments for
improvement. I would also like to thank journal
reviewers Andrea Moscariello and an anonymous
reviewer for their critical reading, which helped
to clarify the text in places.

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