Nutrient and sediment dynamics change following a major flood event on a large, grassland river

River ecosystems can impact the quantity of materials and nutrients flowing into receiving waters. However, these processes may change with climate change, particularly with respect to the impacts of extreme flooding events. In this study, we examined the long-term impacts of an extreme flooding event, the 2019 U.S. Midwest floods, on nutrient loads and concentrations in an unconfined, fourth-order grassland stream, the Niobrara River, Nebraska. To compare historical trends to modern conditions, archived data and contemporarily sampled data were used in conjunction. Prior to the flood (1990–2018), NO₃-N concentrations showed a strong seasonal pattern with no relationship with discharge, indicating a groundwater NO₃-N source. Total phosphorus (TP), dissolved inorganic phosphorus (DIP), and total suspended solids (TSS) showed slight and individual seasonal patterns relating to discharge, following expected patterns of sediment sourced P. Post-flood, NO₃-N concentrations declined by 91.5% (F_{[2, 269]} = 34.96, p <.0001). NO₃-N depression in the system remained even once discharge returned to pre-flood levels. TP, DIP, and TSS (F _{(2, 236)} = 38.14, ANOVA p <.0001; F_{(2, 176)} = 44.06, ANOVA p <.0001; F_{(2, 205)} = 25.98, ANOVA p <.0001) each saw elevation of 100%, 117%, and 66% respectively in 2019 with an expected drop in 2020 when discharge returned to pre-flood. It is unclear why NO₃-N concentrations, unlike phosphorus, did not return to pre-flood levels in the 2 years of this study. The observation suggests extreme flooding events may cause complex, unexpected shifts in biogeochemical cycling in river ecosystems.