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Preliminary IHA Analysis for the Middle Fork Willamette River at Jasper OR

Opperman, Jeff

For this IHA analysis, unregulated flow data were provided by the Army Corps of Engineers. I acquired regulated flow data from the USGS website for gauge 14152000. Based on information on the Website that the last major dam upstream was completed in 1966, I began the analysis on 10/1/1967 (water year 1968) for both the unregulated and regulated data. The unregulated flow data spanned from 10/01/1967 to 9/30/2004 while the regulated flow data spanned from 10/01/1967 to 8/31/2006.

A primary function of the IHA software is to compare to hydrological data sets and calculate a variety of statistics to assess the degree of hydrological alteration between them. The program is set up to process a single data set and the user is asked to input the year of the ‘impact.’ The simplest case is for a long hydrological record that has a single dam built at some point of time; the IHA then divides the data set into a ‘pre-impact’ period (before the year of dam completion) and ‘post-impact’ period. The Willamette data sets represent a different approach: comparing unregulated and regulated hydrological data from the same period of record. Within the IHA, I defined the unregulated data as ‘pre-impact’ and the regulated data as ‘post-impact.’ However, because the IHA requires a single data set with a user-defined year of impact, I created ‘dummy’ years for the post-impact data (regulated data) with an impact date of 10/1/2004. Within the analysis, you will see that the post-impact flows are represented by the water years 2005 to 2043. Keep in mind that theses post-impact years are the same years as the pre-impact data (with two extra years in post-impact) but for the purposes of the IHA analysis they’ve been labeled with future years.