Hydraulic complexity at a large river confluence in the Amazon Basin

Confluences are a classic feature in riverine networks with important ecological and morphological functions. A method to characterize the hydraulic complexity of a river based on velocity gradients was applied, for high and low flow conditions, to the Negro and Solimões Rivers confluence in the Amazon Basin. The applied metrics M1 and M2 approximate the drag forces imposed on aquatic organisms moving between two locations, and may identify potential habitat zones and edges. Metric M2 corresponded best with the hydraulic and morphological patterns in the confluence hydrodynamic zone, with the largest M2 values in the entrance of the confluence, centered at the mixing interface, and M2 values generally decaying laterally toward the banks and longitudinally with downstream distance. Seasonal decreases in discharge magnitude in the Amazon, and decreases in discharge between other river basins analyzed in this study, led to increases in hydraulic complexity metric M2. The hydraulic complexity metrics can characterize some aspects of habitat heterogeneity and contribute to an explanation for observations of increased species richness at Amazon basin confluences, and the larger ecological patterns of diversity increasing at nodes in riverine networks.