Welcome to Conservation Gateway

The Gateway is for the conservation practitioner, scientist and decision-maker. Here we share the best and most up-to-date information we use to inform our work at The Nature Conservancy.

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.