The spatial extent and timescales of bank infiltration and return flows in an upland river system: Implications for water quality and volumes.

The temporary storage of river water in riverbanks and its subsequent return to the river is an important part of the hydrological cycle. Detailed multi-year head and electrical conductivity (EC) variations document the spatial and temporal scales of bank infiltration and return flows in the Ovens catchment, southeast Australia. The study represents one of only a few that has integrated geochemical data and hydraulic heads over several hydrological cycles in a catchment unimpacted by groundwater extraction. The river water infiltrates only a few metres to tens-of-metres into the riverbanks and dilution of regional groundwater in the banks is commonly observed for several weeks to months following high river stages. Two-dimensional numerical modelling shows that the extent and timescales of bank infiltration and return flows are governed mainly by the regional head gradients, hydraulic conductivities, and the height of the stream pulse. Variable topography in river catchments will result in significant variations in the volumes of bank storage and the timescales that these bank waters are retained. The combination of field observations and numerical modelling illustrates that parts of the riverbanks may store water for several months to years and reconnection of rivers to the regional groundwater will occur in different parts of the catchment at different times. In much of the catchment the bank storage zones act as a buffer against contaminated regional groundwater and bank return flows will sustain streamflow and riverine ecosystems during dry periods.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.