IN‐STREAM HABITAT UNIT CLASSIFICATION: INADEQUACIES FOR MONITORING AND SOME CONSEQUENCES FOR MANAGEMENT¹

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.

Sampling surface and subsurface particle-size distributions in wadable gravel-and cobble-bed streams for analyses in sediment transport, hydraulics, and streambed monitoring

“…This is performed visually based on recognition of streambed topography, flow patterns, and patterns of spatial bed-material variability. The delineation of geomorphological units may be difficult, particularly for inexperienced operators (Poole et al 1997). Delineation requires training and the results are affected by the stage of flow.…”

Standards for ecologically successful river restoration

“…Classification (the ordering of objects into labelled groups based on common characteristics) has been broadly applied to river channels (Rosgen 1994;Poole, Frissell & Ralph 1997), with more than 40 geomorphically based classification schemes employed or proposed in various parts of the world, based on factors such as channel pattern, gradient, bed material size and sediment load (Kondolf et al . 2003).…”

An Analysis of Potential Stream Fish and Fish Habitat Monitoring Procedures for the Inland Northwest: Annual Report 1999.

“…Previous studies of channel unit-based systems suggested that they were inadequate for monitoring because channel unit identification was subjective and imprecise and hence, the data were biased and highly variable (Roper and Scarnecchia 1996;Poole et al 1997). …”