As opposed to meandering or channelized fluvial beds, braided rivers are characterized by a morphological activity starting at very low flows, since fluxes are concentrated in a limited number of small channels. With increasing discharge, more channels are involved, up to the situation in which the complete alluvial plain is flooded. As a consequence, there is an intermediate range of flows for which pattern complexity is maximum and braided indices are highest, representing essential conditions for the coexistence of a large variety of habitats and for ecosystems prosperity. In this paper, a new methodology for a quantitative assessment of the complexity of braided rivers at a reach scale is introduced. It is based on the application of the box-counting algorithm to flooded areas identified through a two-dimensional (shallow water) hydrodynamic simulation model, in order to derive an estimate of the fractal dimension with varying flow rate. The identification of the range of discharges for which the fractal dimension is highest is of particular importance in river restoration projects. An application to the River Tagliamento (North-East Italy) is illustrated.
