River Geomorphology Affects Biogeochemical Responses to Hydrologic Events in a Large River Ecosystem

Shifts in the frequency and intensity of high discharge events due to climate change may have important consequences for the hydrology and biogeochemistry of rivers. However, our understanding of event‐scale biogeochemical dynamics in large rivers lags that of small streams. To fill this gap, we used high‐frequency sensor data collected during four consecutive summers from a main channel and backwater site of the Upper Mississippi River. We identified high discharge events and calculated event concentration‐discharge responses for both physical‐chemical (nitrate, turbidity, and fluorescent dissolved organic matter) and biological (chlorophyll‐a and cyanobacteria) constituents using metrics of hysteresis and slope. We found a range of responses across events, particularly for nitrate. Although fluorescent dissolved organic matter (FDOM) and turbidity exhibited more consistent responses across events, contrasting hysteresis metrics indicated that FDOM was flushed to the river from more distant sources than turbidity. Biological responses (chlorophyll a and cyanobacteria) differed more between sites than physical and chemical constituents. Lastly, we found that the event characteristics best explaining concentration responses differed between sites, with event magnitude more frequently related to responses in the main channel, and antecedent wetness conditions associated with response variation in the backwater. Our results indicate that event responses in large rivers are distinct across the diverse habitats and biogeochemical components of a large floodplain river, which has implications for local and downstream ecosystems as the climate shifts. Plain Language Summary How sediment, nutrients, and organic matter enters and moves throughout large rivers has important implications for water quality both locally and globally. One way to study these dynamics is to observe how these materials react to periods of high streamflow. We used this approach to analyze sources and movement of nitrate, organic matter, sediment, and biological indicators in the Upper Mississippi River (UMR). The data indicated that sediment and organic matter likely came from sources that were closer and farther from the river, respectively, and that these patterns were more evident during more extreme high streamflow periods. We also found that materials, particularly biological indicators (chlorophyll a and cyanobacteria), behaved differently in the main channel of the river compared to