In this study,“naturalized” daily streamflow records, created by the U.
S. Army Corps of Engineers and the Bureau of Reclamation, or were used to compute 1-,3-, 7-, and 10-,15-, 30-, or 60-day annual maximum streamflow durations,which are running averages of daily streamflow for the number of days in each duration. Once the annual maximum durations were computed, the floodduration frequencies could be estimated. The estimated flood-duration frequencies correspond to the 50-, and 20-,10-, 4-, or 2-,1-, 0.5-, and 0.2-percent probabilities of their occurring or being exceeded each year. For this report,the focus was on the Willamette River Basin in Oregon, which is a subbasin of the Columbia River Basin. This study is fragment of a larger one encompassing the entire Columbia Basin.
Source: usgs.gov