Introduction
Many criteria have been proposed for classifying standing waters . Some typological systems are widely accepted ; others are of limited usefulness . The classification systems for lotic habitats, however, are more varied, uncertain, and unreliable . Few have had anything more than local acceptance . ILLIES & BOTOSANEANU (1963) have briefly reviewed the widely scattered literature bearing on classification of lotic habitats, and this literature will therefore not be reviewed in the present paper. ILLIES & BOTOSANEANU point out the several basic kinds of classification systems . At one extreme are those proposals which utilize a single chemical factor to classify running waters, e .g . OHLE (1937) who uses the calcium content of the water . Among physical measurements, substrate composition has been most frequently used, especially by fisheries biologists and aquatic insect ecologists .
Many authors, working in restricted areas, subdivide flowages, from headwaters to mouth, on the basis of the longitudinal distribution of various invertebrate indicator species within a single large taxon, such as Trichoptera, Plecoptera, and Ephemeroptera . Such zonation classifications have often been coupled with altitude, latitude, and stream flow. A representative study is that of KNIGHT & GAUFIN (1967), who worked out the species distribution of Plecoptera in the Gunnison flowage in Colorado . Other classifications are based on bottom fauna associations, and may include such combinations as insects, oligochaetes, and mollusks . Some biological classification systems purport to be so ambitious as to characterize *Contribution No .