Evidence of Groundwater Seepage and Mixing at the Vicinity of a Knickpoint in a Mountain Stream.

Streamflow generation and biochemical hotspots are significantly influenced by groundwater contributions distributed along the drainage network. However, identifying the geomorphic landscape features that drive groundwater‐surface water interactions remains challenging. In this study, we investigate the role of knickpoints in controlling these interactions in a mountainous stream in Switzerland. We employ a combination of synoptic sampling of environmental tracers, endmember mixing calculations, and groundwater flow simulations. Our findings reveal substantial groundwater seepage concentrated near the knickpoint of the main river stem. Using parsimonious groundwater flow modeling, we validate the hypothesis that the topographical shape of the knickpoint enhances local groundwater discharge rates. We quantify that approximately 20% of the total catchment streamflow originates from around the knickpoint. These results indicate that knickpoints are significant hotspots for groundwater seepage and physicochemical mixing, providing a clear method for identifying major localized sources of streamflow generation. Plain Language Summary: In ice‐free catchments, groundwater is the main water source maintaining streamflow during extended dry periods. In addition to its importance for water resources, locations where groundwater contributes to surface stream water are important hotspots where interactions between biological and chemical processes occur, that have major implications for aquatic ecosystems. However, those hotspots remain difficult to identify. Specifically, our understanding on the relationships between landscape and groundwater discharge in streams remains limited. In our study of a Swiss Alpine catchment, we sampled a stream for water chemical analysis and used statistical and numerical models to find where groundwater emerges into the stream. We discovered that most groundwater enters the stream around a "knickpoint", a location where the stream profile becomes steeper. According to our calculations, this knickpoint, although of minimal extent, contributed one fifth of the total streamflow in the catchment. Although our results are site‐specific, knickpoints are ubiquitous in mountain areas, thus they might be important when predicting water availability and biogeochemical fluxes at regional scale. Key Points: We highlight the sporadic nature of groundwater seepage hotspots along a river profileKnickpoints influence groundwater flow patterns, creating preferential mixing zones between groundwater and surface watersBiogeochemical hotspots are influenced by knickpoints in mountainous hydrographic network [ABSTRACT FROM AUTHOR]