• Geoffrey C. Poole,

  • Christopher A. Frissell,

  • Stephen C. Ralph

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      Respectively, Staff Scientist, Flathead Lake Biological Station and School of Forestry, Numerical Terradynamic Simulation Group, The University of Montana, 311 Bio Station Lane, Polson, Montana 59860–9659; Research Assistant Professor, Flathead Lake Biological Station, The University of Montana, 311 Bio Station Lane, Polson, Montana 59860–9659; and Regional Salmon Ecologist, Environmental Protection Agency, Region 10, Office of Ecosystems and Communities, 1200 Sixth Avenue (ECO-088), Seattle, Washington 98101–1128.

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    Paper No. 96137 of the Journal of the American Water Resources Association (formerly Water Resources Bulletin). Discussions are open until April 1, 1998.


ABSTRACT: Habitat unit classification can be a useful descriptive tool in hierarchical stream classification. However, a critical evaluation reveals that it is applied inappropriately when used to quantify aquatic habitat or channel morphology in an attempt to monitor the response of individual streams to human activities. First, due to the subjectivity of the measure, observer bias seriously compromises repeatability, precision, and transferability of the method. Second, important geomorphic and ecological changes in stream habitats are not always manifested as changes in habitat-unit frequency or characteristics. Third, classification data are nominal, which can intrinsically limit their amenability to statistical analysis. Finally, using the frequency of specific habitat unit types (e.g., pool/riffle ratio or percent pool) as a response variable for stream monitoring commonly leads to the establishment of management thresholds or targets for habitat-unit types. This, in turn, encourages managers to focus on direct manipulation or replacement of habitat structures while neglecting long-term maintenance or re-establishment of habitat-forming biophysical processes. Stream habitat managers and scientists should only use habitat unit classification to descriptively stratify in-stream conditions. They should not use habitat unit classification as a means of quantifying and monitoring aquatic habitat and channel morphology. Monitoring must instead focus on direct, repeatable, cost-efficient, and quantitative measures of selected physical, chemical, and biological components and processes spanning several scales of resolution.