Swiftness of biomorphodynamics in Lilliput- to Giant-sized rivers and deltas

Physical experiments of self-formed river channels and floodplainswith live vegetation are pathways for understanding
that complement numerical modelling. Recent experiments succeeded in creating braided rivers and
dynamicmeandering systemswith clastic and vegetated floodplains. However, application of the insights gained
from such experiments to natural systems depends on understanding potential scale effects, temporal, and spatial.
Here we combine review, analysis, and experiments to identify fundamental problems of biomorphological
river pattern formation that are open for further research in experiments. We first show by review and analysis
that physics-based, linear bar theory predicts negligible spatial scale effects in bar and bend wavelength relative
to channel width. Time scaling, on the other hand, remains problematic because it integratesmultiple processes
of sediment transport, floodplain formation, and bank failure affected by bank stratigraphy and riparian vegetation.
As a tentative solution, we secondly present experimentalmethods to assess bank strength effects that can
be used in the design of river pattern experiments. The third issue is that riparian vegetation has often been
represented in experiments by uniformly seeded sprouts of a single plant species, whilst spectacularly different
patterns are obtained with contrasting seeding protocols, showing the need for other experimental procedures,
and alternative riparian species. The main challenge for future experiments is better understanding of temporal
scaling of biomorphodynamics.