Lessons from and assessment of Boussinesq aquifer modeling of a large fluvial island in a dam-regulated river

The suitability of a numerical Boussinesq aquifer model for representing groundwater dynamics in a fluvial island surrounded by a regulated river is assessed and the model is used to compare exchange fluxes for varying configurations of island hydraulic conductivity (K) and diffusivity. The model results are qualitatively similar to field observations of the water table although there is obvious dissimilarity between modeled and observed heads suggesting that a Boussinesq aquifer model may not be the best option for representing the island. Nonetheless, the simulations show that pronounced ridges and valleys form in the water table and that their spatial configuration may change drastically with small variations in diffusivity. Stage fluctuations significantly increase exchange flux across the island relative to the case where stage is constant. The flux increases non-linearly when island K is low but then becomes quasi-linearly dependent on K with further increases in K. Regulated river stage fluctuations due to dams significantly affect surface water–groundwater interactions between a fluvial island and a river. This deserves further inquiry for both scientific and management reasons.