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Large floods on rivers and streams are beneficial for specific life stages of various aquatic species. Floodplain inundation is also necessary for the transport and deposition of sediment and nutrients onto floodplains. However, large floods can be damaging to bridges and other floodplain infrastructure, and they can threaten lives. Communities rely on historical streamflow records to help understand past floods and plan for future ones. Floods are typically studied by analyzing records of the largest flood that occurred each year on a given stream, but these annual events include many smaller flood events that don’t even overtop the riverbanks. Because the conditions that produce large floods may differ from those of smaller floods, it’s important to specifically study the occurrence of large floods. Understanding the seasonality of large floods helps reveal the conditions that generate them. By analyzing historic, climate-driven changes in large floods, potential future changes in large floods can be better anticipated. Climatic changes in floods can result from cyclical climate patterns or longer-term, directional changes like those associated with anthropogenic warming.

A recently published study by NOAA (U.S. National Oceanic and Atmospheric Administration) and USGS researchers examined the seasonality of large floods for regions across the United States for the first time. Changes in the frequency of large floods each year from 1966 to 2015 were also studied for these regions. Because of concerns that flooding could become more frequent with climate change, a focus on watersheds with minimal human alterations was used to isolate climatic signals. The wealth of high quality, systematic streamflow data collected over long time periods at USGS streamgages provided study data for almost 500 rivers and streams.

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At each streamgage, the researchers determined the largest floods by ranking the 50 years of annual peak flows and then selecting the top-2 and top-10 ranking events for the study. They used a novel method to then evaluate if there were significant changes over time in the numbers of these large events at individual gages. The gages were also grouped into regions. For each year from 1966 to 2015, the number of top-ranking floods that occurred at each gage in that year were summed for each region. These regional time series were then used to study changes over time using a traditional method and also to evaluate seasonality.

The study team found that the seasonality of large floods varies considerably across the country. Some regions have most large floods concentrated in a few months of the year while others have more complex or weak seasonality. Large floods in the regions with complex seasonality are often caused by multiple drivers such as snowmelt, tropical cyclones, and (or) convective storms.

There was little evidence that large floods became more or less frequent from 1966 to 2015. Only two U.S. regions showed statistically significant trends over time in the frequency of the 10 largest floods, a decrease in the Gulf Coast and an increase in the Ohio River Basin. The Gulf Coast region was also the only region with a significant change for the two largest floods. It showed a significant decrease in the frequency of the 2 largest floods, meaning more of these large floods in the region occurred in the early years of the study period. The frequency of floods across the country in the later years of the study period at individual rivers and streams generally matched the amount expected by chance, suggesting there is no evidence for increasing or decreasing trends.

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There were some statistically significant regional relationships between large-flood occurrence and known climate patterns such as the Pacific North American (PNA) pattern. During one phase of the PNA, atmospheric moisture transport to the Upper Midwest and other regions of the north-central U.S. is greater, leading to an increased number of heavy rainfall events and floods in this area. There were also significant relationships between large-flood occurrence and the El Niño-Southern Oscillation (ENSO) pattern for three regions of the U.S. from California to Texas. One phase of ENSO is associated with widespread increased precipitation in these areas.

Ultimately, the results of this research represent a novel way to study the seasonality and frequency of large flood events and help bring more context and understanding to the events. This work adds to the knowledge of large floods across the U.S. and is helpful for communities and managers who want to be prepared for future flood events.

The paper, “The occurrence of large floods in the United States in the modern hydroclimate regime: seasonality, trends, and large-scale climate associations” was recently published in Water Resources Research.

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