Short-term effects of a summer flash flood on the physiochemical water quality in a restored river

In summer 2021, a flash flood impacted a recently restored river valley in Belgium. Given the anticipated increased frequency and severity of summer flash floods due to climate change, it is imperative to understand their repercussions on aquatic ecosystems. Historical river alterations and nutrient pollution have diminished self-purification, posing significant water quality challenges. This research investigates the effects of a summer flash flood on water quality, with a specific focus on dissolved oxygen, dissolved-, and bound nutrients. Additionally, it evaluates the role of two recently restored meander sections, emphasizing the importance of river restoration projects in enhancing the resilience of the river system. Throughout the summer of the flash flood event, regular water samples were collected, and continuous measurements were conducted. The results revealed that the majority of parameters significantly exceeded the values observed during previous, normal, summers. The considerable influx of organic matter induced anoxic conditions that persisted for approximately a week. For most water quality parameters, more than nine weeks were required for complete recovery post-flash flood. The meanders exhibited a slight improvement of several water quality parameters, induced mostly by the deposition of particles. Near-natural restoration of meanders appeared important. It is probable that the meanders necessitate additional time since reconnection to reach their full potential regarding the enhancement of water quality.