Geochemical and sediment dynamics during an experimental high flow pulse event on the Allegheny River: Lessons for river system management

High flow and flood events are essential for sustaining river ecosystems, driving nutrient cycling, habitat diversity, and species dispersal. However, widespread flow regulation via dams and reservoirs has disrupted natural hydrological processes, leading to river fragmentation and homogenization of flow regimes. While previous research has largely focused on the hydraulic and biological impacts of engineered flow events, less attention has been given to their influence on solute mobilization, transport, and biogeochemical transformations. This study addresses this gap by evaluating the geochemical and sediment dynamics of the first experimental spring high flow event (i.e., pulse event) on the Allegheny River (Pennsylvania, USA), conducted by the United States Army Corps of Engineers under the Sustainable Rivers Program. The pulse event, initiated on March 30, 2023, involved a sustained release of 451 cms from Kinzua Dam over 21 h. We hypothesized this experimental spring pulse would mobilize organic-rich sediment and nutrients stored behind Kinzua Dam, while also altering the geochemical signature of downstream waters through interactions with hyporheic zones, sediment scouring, and channel connectivity. To assess these impacts, we collected hourly water grab samples over a 48-h period spanning pre- and post-pulse conditions at multiple downstream locations. Samples were analyzed for dissolved metals, nutrients, total suspended solids, and nitrate isotopes. Results reveal distinct temporal shifts in water chemistry, with observed fluctuations in total suspended solids, dissolved metals, and nutrient concentrations highlighting hydrological connections between the main channel and riparian zones, reinforcing the importance of experimental pulse events in ecosystem restoration. Based on these findings, we propose a conceptual model linking controlled flood pulses to sediment and solute fluxes, which can be tested in other regulated river systems to evaluate the effectiveness of flow restoration strategies. These results provide key insights into the role of controlled high flow pulses in shaping sediment and solute dynamics, filling an important knowledge gap in understanding the biogeochemical implications of large-scale flow experiments.