The Contemporary Methodology for Quantifying Reserved Instream Flow Water Rights to Support Aquatic Habitat

30 Pages Posted: 19 Jul 2019 Last revised: 27 Mar 2020

Date Written: 2020


This article is the first in a series to provide information related to commonly used technical methodologies in furtherance of legal rights held by American Indian Tribes. Specifically, this article discusses the method for estimating water rights necessary to protect instream habitat to support fish. Unlike reserved water rights for irrigation — which has the well settled practicably irrigable acreage (“PIA”) standard — the United States Supreme Court has not identified a single quantification methodology appropriate for traditional uses of water. Instead, lower courts have fashioned a more ad hoc approach to quantifying traditional reserved water rights. This seems preferable given that unlike irrigation water rights, which are fairly uniform in their nature, each tribe’s traditional uses of water are unique and driven by the physical, historical, and cultural factors present for each tribe. Although the method for quantifying these rights have evolved over time, courts have slowly converged on the Instream Flow Incremental Methodology ("IFIM") and its component part, the Physical Habitat Simulation Model (“PHABSIM”). Although the IFIM/PHABSIM method does not enjoy the de jure recognition of its sister-methodology PIA, the consistent use and acceptance of this method have caused it to become the de facto method for quantifying reserved instream water rights to protect fish habitat. The purpose of this article is to provide an explanation of the IFIM/PHABSIM methodology in an effort to put practitioners in the position to apply and, where necessary, critique the method. It proceeds in a chronological fashion; describing from beginning to end the steps necessary to develop a reserved instream water right claim using the IFIM/PHABSIM method.

Keywords: indigenous rights, reserved water rights, fishing rights, law and science, instream flows, Physical Habitat Simulation modelling, Instream Flow Incremental Methodology, environmental justice, environmental risk

Suggested Citation

Hedden-Nicely, Dylan, The Contemporary Methodology for Quantifying Reserved Instream Flow Water Rights to Support Aquatic Habitat (2020). 50 ENVTL. L. 257, Available at SSRN: or

Dylan Hedden-Nicely (Contact Author)

University of Idaho, College of Law ( email )

P.O. Box 442321
Moscow, ID 83844-2321
United States

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