LAUSR

This study presents high‐resolution flow‐through experiments investigating transport processes in a laboratory setup mimicking an aquifer affected by hydropeaking (i.e., abrupt fluctuations in the river stage by the release or storage of water in reservoirs). Highly transient flow conditions were experimentally generated by sudden changes of the water level in two rivers in hydraulic contact with an unconfined aquifer. High‐resolution image analysis and depth‐resolved, high‐frequency sampling at the outlet allowed monitoring of the spatio‐temporal evolution and the breakthrough of a dye tracer plume in the porous medium. The plume spreading and mixing were quantified by moment analysis and entropy‐based metrics of the scalar field. We show that hydropeaking strongly enhances spreading and mixing in the subsurface (up to 249.5% and 41.8% in these experiments) and demonstrate the relevance of considering highly transient flow regimes to properly capture transport and mixing‐controlled biogeochemical reactions at the surface water ‐ groundwater interface.