Step Changes in Flood Magnitudes Across the Pacific Northwest, USA

Recognizing climatically driven variability in hydrologic systems is an important element in both understanding the past and projecting into the future. To test for climatic trends in flood events across the Pacific Northwest region of the US, annual peaks from 134 streamflow gages with at least 50 years of record and largely unaltered watersheds were analyzed. Mann-Kendall tests indicated that 25% of gages had statistically significant linear trends, with 14% increasing and 11% decreasing. Western Washington showed increasing flood magnitudes, while Oregon, Idaho and Western Montana had mixed but generally decreasing magnitudes. A 25-year moving window analysis was used to examine non-linear trends in flood magnitudes. The results reveal two step-changes, stepping up in the late 40's, and then stepping down in the late 70's. These dates have been noted in other studies regarding climate and hydrology of the US, and seem to track phases of the Pacific Decadal Oscillation. Western Washington has responded in anti-concert to these changes, with relatively low flood magnitudes from the 1940's to 1970's, and a dramatic increase post-1970's. Analysis to be completed will investigate whether the linear trends observed are true trends or a result of step changes and periods of record that only occasionally extend back further than 60-80 years.