Predicting Salmonid Habitat–Flow Relationships for Streams from Western North America

Abstract

One of the most widely applied methodologies for developing instream flow recommendations is the instream flow incremental methodology (IFIM) and its component microhabitat model, physical habitat simulation (PHABSIM). In this paper we reviewed over 1,500 habitat–flow curves obtained from 127 PHABSIM studies from western North America to develop predictions for flow needs for salmonids in this region and to test whether habitat–flow relationships for salmonids were related to watershed characteristics and geographic location. We present regressions that predict PHABSIM optima for four life history stages of four salmonid species and for all salmonid species in the database as a group, and we quantify the uncertainty in these estimates. Mean annual discharge (MAD) was the best predictor of optimum flow. The general form of the regressions was log _e (optimum flow) = A × log _e (MAD), where A < 1. Minor improvement in predictive power was sometimes possible with addition of latitude and longitude coordinates to the regression. This relationship is asymptotic and differs considerably from the fixed flow percentages recommended by Tennant. Our results are presented as a planning tool to (1) allow managers and project proponents to conduct a preliminary assessment of proposed water-use development projects, (2) optimize research efforts for instream flow studies and experiments, and (3) set experimental boundaries for adaptive management of stream flow.