Generic autogenic behaviour in fluvial systems: lessons from experimental studies - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 1

Generic autogenic behaviour in fluvial systems:
lessons from experimental studies

GEORGE POSTMA
Utrecht University, Faculty of Geosciences, Department of Geosciences, PO box 80.021, 3508 TA Utrecht,
The Netherlands (E-mail: [e-mail])

ABSTRACT

Substantial advancement in the recognition of generic autogenic behaviour in fluvial
systems stems from recent landscape-scale experimental research, where features of
stratigraphic architecture can be verified along known input and boundary conditions.
Combining experimental work, numerical modelling and field data indicate different
autogenic behaviour for 1) steep-gradient, stream-flow dominated alluvial fan-type
systems, 2) moderate-gradient braided-river type systems and 3) low-gradient single-
river meandering and anastomosing systems. Autogenic behaviour is by cyclic alternating
sheet and channelised flow on alluvial fan surfaces, by avulsions of small bifurcat-
ing streams around migrating mid channel bars in the braided channel belt and by
splitting of the flow from one into two channel belts in case of single thread rivers.
Experimental studies indicate that aggradation rate is directly related to the frequency of
autogenic behaviour and that absolute frequency values depend on the backfilling rate
in the channel and the available accumulation space. Since aggradation rate is the direct
result of the interplay of all allogenic controls, it is more logical to predict autogenic
behaviour directly from aggradation rate than indirectly from sea-level, climate or
tectonic forcing. It is shown that if the change in allogenic forcing is fast relative to the
time that a river needs to come to grade (equilibrium time), then the change in aggradation
rate and the frequency of autogenic behaviour is highest. If the change in allogenic
forcing is slow relative to the river's equilibrium time, then there is also little change in
the frequency of autogenic behaviour.

Keywords: Autogenic, avulsion, aggradation, alluvial fan, braided river, single thread
river, backfilling.

INTRODUCTION discharges (alluvial fans, outwash plains, braided

river systems), than for low-gradient rivers that
carry abundant suspension load. In the first case,
bank stability is low and in the latter it is high,
which keeps the river better in position. If all
alluvial accumulation space is filled and the river
has reached its grade (base level), no net erosion
or deposition along the channel belt occurs and
the spread of sediment and thus avulsion of
streams is halted, while all supplied sediment
bypasses the region.
In nature, boundary conditions that define
base level fluctuate continuously by changes in
discharge, sediment load, grain size etc., affect-
ing the river's slope and its buttress (the point

Generic autogenic behaviour of alluvial systems
refers to the common intrinsic property of alluvial
systems to shift their streams and rivers laterally
by avulsion and bifurcation (Beerbower, 1964;
Allen, 1965; Slingerland & Smith, 2004). In this
way the river spreads its sediment load evenly
over the available surface area to reach the lowest
potential energy level and highest stability (Paola
et al ., 2009). This intrinsic property can be obser-
ved everywhere in nature. The sediment spread
is commonly more even for high-gradient and
moderate-gradient river systems that carry little
fine-grained suspension load and experience high