Floodplain sediment disconnectivity at a tributary junction and valley constriction site in the Fitzroy River basin, Queensland, Australia

Many catchment-scale sediment models assume connectivity from the top to the bottom of the catchment. The purpose of this study is to provide evidence that particular topographic features can affect catchment connectivity and the transfer of sediment through the catchment. This study investigates floodplain sediment deposition in a valley bottom constriction and tributary junction site within the Fitzroy River basin, a large semiarid–subtropical catchment in NE Australia. These sites can cause backwater effects during flooding that enhance sediment deposition. Isotope analysis of floodplain sediment cores for cesium-137 and plutonium-239 + 240 is used to determine deposition rates since the 1960s. Results show overall higher deposition rates across the lower tributary floodplain proximal to the confluence and within the valley constriction reach. Deposition rates in the upper constriction reach were less than in the constriction reach but similar to rates reported by other studies in the Fitzroy River basin. However, evidence suggests that the region of influence of the valley constriction on sediment deposition is dependent on flood size with large floods depositing sediment further upstream than the sampling sites of this study. Nevertheless, the study shows evidence that backwater-inducing features (such as confluences of disparate channels) can promote sediment deposition and cause a disconnection in the transfer of sediment from tributary to main channel. The effect of these processes on storing sediment needs to be considered in catchment-scale sediment budget models.