A HIERARCHICAL APPROACH TO CLASSIFYING STREAM HABITAT FEATURES

We propose a hierarchical system of classifying stream habitats based on three increasingly fine descriptions of the morphological and hydra ulic properties of channel geomorphic units. We define channel geomorp hic units as areas of relatively homogeneous depth and flow that are b ounded by sharp gradients in both depth and flow. Differences among th ese units provide a natural basis for habitat classification that is i ndependent of spatial scale. At the most general level of resolution, we divide channel units into fast- and slow-water categories that appr oximately correspond to the commonly used terms ''riffle'' and ''pool. '' Within the fast-water category, we identify two subcategories of ha bitats, those that are highly turbulent (falls, cascades, chutes, rapi ds and riffles) and those with low turbulence (sheets and runs). Slow- water habitats include pools formed by channel scour (eddy pools, tren ch pools, midchannel pools, convergence pools, lateral scour pools and plunge pools) and those formed behind dams. Dammed pools include thos e obstructed by debris dams, beaver dams, landslides and abandoned cha nnels. We consider back-waters as a type of dammed pool. Fishes and ot her stream organisms distinguish among these habitats at one or more l evels of hierarchy. Habitats defined in this way represent an importan t habitat templet on which patterns of biological diversity and produc tion form. We believe that a hierarchical system of classification wil l facilitate understanding of biotic-habitat relationships in streams and lead to more effective methods of evaluating the effects of enviro nmental change on stream ecosystems. Refining the criteria by which ha bitats are distinguished, quantifying how different species use differ ent habitats, and integrating the ways biota respond to habitat variat ion should facilitate the emergence of a theory of stream habitat orga nization.