The Instream Flow Incremental Methodology (IFIM) was originally developed by the Instream Flow Group (IFG) of the U.S. Fish and Wildlife Service (USFWS) in Fort Collins, Colorado, as a decision-making framework (Figure 1) for assessing the impacts of water development projects on aquatic ecosystems Bovee , Bovee et al. . Computer models provided a mechanism for quantifying aquatic habitat per unit length of stream by linking stream channel hydraulics with habitat suitability criteria to create an index to habitat called weighted usable area (WUA). Additional models then could link the habitat index to hydrology to put the index into the context of flow variability and perform a time series analysis of total habitat.
In addition to the hydraulic habitat modeling program PHABSIM, the IFG created TSLIB for time series analysis of baseline and alternative hydrology, SNTEMP for stream network temperature analysis, and LIAM for legal-institutional analysis, but did not write any software to implement the complete IFIM. Even though the IFG constantly reminded U.S. and international users of the PHABSIM approach that it should be applied within the context of IFIM, there was no software written to do so, and PHABSIM the model eventually came to be perceived as synonymous with IFIM the process. There are other reasons besides the lack of software, of course, some psychological, some driven by cost, some due to inexperience, but the end result was the same: the IFIM was criticized as being only a physical fish habitat model instead of being correctly known as a multi-disciplinary, multi-component framework for alternatives analysis and negotiation.
Consequences of the conflation between PHABSIM and IFIM include the multiplication of alternative and often similar methods, fragmentation of analysis among scientific disciplines, loss of study integration, and ultimately weaker protection for natural resources. Correction of the misperception is very difficult in the absence of a fresh approach, especially since the original staff of the IFG is nearly all retired and the group has been given a different direction by the parent agency. In addition, the IFG has no plans to program PHABSIM to function under the newest operating systems, RHABSIM and EVHA require similar upgrading and recompiling, related software packages have not been widely distributed, translated, or adopted, and no national or international organization appears ready to step in and replace the function of the IFG.