Using Return Intervals and Nutrient Spiraling to Examine the 2019 Nebraska Flood

In March of 2019, flooding levels in many parts of Nebraska was the worst that streams and rivers had in decades, and in some locations, on record. Theses historic floods present an optimal case study to examine how current technological resources can be used to enhance our understanding of floods and how these floods impact in situ stream ecosystem processes like nitrogen and phosphorus cycling. Currently, there are only a few resources available to quantify the extent of floods; for my thesis, I will focus on satellite imagery and in situ water level gages. Unfortunately, due to technical issues with climatic phenomena, satellite image analysis did not yield comprehensive information of flooding extent. However, by analyzing the discharge records of 94 stream gages across the state, I was able to evaluate flood stage records and calculated flood return intervals. While the flood stage was reached primarily along streams in the Niobrara, Platte, and Elkhorn River watersheds, 17 streams across the state had a flood return interval greater than 100 years. The average return interval was 48 years ± 73. The nutrient spiraling metrics in ten streams across the state were compared in the summer before and after the flood to evaluate the flood impacts on stream ecosystem processes. In 2018, the mean uptake length (Sw) of NH4 and PO4 was 563m ± 348 and 600 m ± 260. The vertical velocity (Vf) in 2018 for NH4 and PO4 was 0mm/min ± 0.42 and 0.02 mm/min ± 0.42, respectively, while the aerial uptake rate (U) mean was 0.03 ug mm/min ± 0.30 and 0 ug mm/min ± 2.61 respectively. In 2019, the mean Sw of NH4 and PO4 was 248 m ± 349 and 256 m ± 352. The Vf in 2019 for NH4 and PO4 was 0.04 mm/min ± 0.42 and 0.15 mm/min ± 0.90 while the U mean was 0.01 ± 17.16 ug mm/min and 0.57 ug mm/min ± 58.61 respectively. There were no differences (p\u3e 0.05) of NH4 or PO4 in Sw, Vf or U between years. These data suggest that Nebraska stream ecosystems were resilient to the flood. Yet, given the high uptake lengths and low uptake velocity and areal uptake values, these data also suggest that small streams are enriched with N and P well beyond their biological demand for these nutrients. Furthering our knowledge of the quantification of floods and their impacts on stream ecosystem processes can help mitigate the deleterious consequences of floods. Advisor: Jessica Corma