Generic autogenic behaviour in fluvial systems: lessons from experimental studies

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 bifurcating 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.