Tributary mitigates river discontinuity by dam depending on the distance from dam to the tributary confluence

Abstract

AbstractRiver ecosystems host a continuum of various discontinuities, such as dams and tributary inflows, which shape the habitats of aquatic organisms; this makes it important to evaluate the interactions among discontinuities and effects of a single discontinuity point. Herein, we focused on tributaries in dammed rivers to elucidate a manner to mediate discontinuities and hypothesized that the effects of the tributary inflow on the downstream river were mediated by the (1) distance between the dam and tributary inflow, (2) size ratio between the dam and tributary, and (3) ability of the tributary to transport fine sediments, such as sand and gravel. We conducted field investigations to measure 24 environmental factors, including the water column, riverbed, and river topography, along eight dammed rivers to test the three hypotheses. Only five factors differed significantly between the upstream and downstream sites of the tributary confluence, whereas 12 factors differed significantly between the tributary and mainstem. We also classified the variation pattern using principal component analysis (PCA). The correlation between each PCA axis and environmental factors suggests that the modifications of the water quality factors by tributary inflow were significantly correlated with the distance between the dam and the tributary inflow, and that the flux of various riverbed factors, that is, was significantly correlated with the ability to resupply of fine riverbed sediments by the tributary. Overall, tributary‐inflow downstream of a dam has the potential to be applicable in environmental restoration, especially in terms of resupplying fine sediment.