Controls of groundwater floodwave propagation in a gravelly floodplain

Interactions between surface water and groundwater can occur over a wide range of spatial and temporal scales within a high hydraulic conductivity gravelly floodplain. In this research, dynamics of river–groundwater interactions in the floodplain of the Matane River (eastern Canada) are described on a flood event basis. Eleven piezometers equipped with pressure sensors were installed to monitor river stage and groundwater levels at a 15-min interval during the summer and fall of 2011. Results suggest that the alluvial aquifer of the Matane Valley is hydraulically connected and primarily controlled by river stage fluctuations, flood duration and magnitude. The largest flood event recorded affected local groundwater flow orientation by generating an inversion of the hydraulic gradient for 16 h. Piezometric data show the propagation of a well-defined groundwater floodwave for every flood recorded as well as for discharges below bankfull (<0.5 Qbf). A wave propagated through the entire floodplain (250 m) for each measured flood while its amplitude and velocity were highly dependent on hydroclimatic conditions. The groundwater floodwave, which is interpreted as a dynamic wave, propagated through the floodplain at 2–3 orders of magnitude faster than groundwater flux velocities. It was found that groundwater exfiltration can occur in areas distant from the channel even at stream discharges that are well below bankfull. This study supports the idea that a river flood has a much larger effect in time and space than what is occurring within the channel.