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high flows. The importance of differentiating margins is that marginal areas may contain richer communities of organisms than channel areas as a result of the increased complexity of the stream margins. This circumstance is particularly true of large, deep rivers.

The final level of the habitat hierarchy deals with major channel features that are thought to be important in the distribution of benthic invertebrates and that can be sampled discretely for invertebrates. Six categories are recognized: natural bed, ma nufactured bed, slough, macrophyte bed, woody snag, and bar. Natural bed refers to areas where natural bed materials predominate and where macrophytes are not a dominant feature. Manufactured bed refers to substrates that are created by man, such as revetments, levees, junk cars, riprap, dams, fish weirs, and bridge piers. Sloughs are remnants of abandoned river channels that are connected with the main channel even at normal low flows. Sloughs that are isolated at low flows tend to diverge biologically and chemically from conditions in the river and are not considered here as an instream habitat. Macrophyte beds are areas where emergent or submergent aquatic plants dominate and invertebrate communities are expected to contain organisms dependent upon such plants. Woody snags refer to trees, branches, or other woody debris of terrestrial origin that extend into the water column either from the streambank or streambed. Bars are shallow, gently sloping sand or gravel ridges primarily associated with channel edges or major changes in water velocity. Bars, when exposed at low flows and vegetated, can resemble islands. However, islands typically have woody vegetation and are at an elevation equal to or above that of the surrounding flood plain. Collectively, this three-tiered hierarchy describes 54 possible habitat types. However, sloughs are restricted to channel and island margins, so only 51 habitat types are available for qualitative and semi-quantitative sampling (fig. 2).

Figure 2.--Matrix of instream habitat types defined by the tree-tiered hierarchy of major geomorphic channel units, channel boundaries, and channel features.