<p>Because gradients structure flooded forests in the Amazon, I computed familial floristic and physical structure rank correlations across flooding gradients both within and between várzea (white-water) and igapó (black-water) by sampling trees in 10 plots located at increasing equal distances from Amazonian rivers. I found in várzea the families Cecropiaceae, Moraceae and Myrtaceae formed a nexus and the significant ranks among structural parameters (e.g., richness of all taxa and the diversity and distribution indexes) were mainly positive. I found in igapó Humiriaceae, Clusiaceae and Elaeocarpaceae formed a nexus and the significant ranks among structural parameters were all positive. Physical effects of the water were implicated by ranks between families and between structural parameters across both gradients analyzed together and separately. These results as well as chemical effects never being implicated, leads to the conclusion that physical effects of water are more important than chemical effects in creating várzea and igapó. There was also some evidence that physical effects may be stronger in igapó for the structural parameters. Familial distribution patterns suggested that niches may occur at specific microsites across both gradients for tree species within those families which, if true, must be largely due to the post-dispersal responses of their seeds and seedlings which result from the action of mechanisms (predation, pathogens, herbivory, competition, facilitation, resource availability) and tolerances (germination).</p>