information from fixed and synoptic sites. The use of techniques or procedures other than those described for the first- level reach characterization limits the ability to conduct such analyses.
Microhabitat consists of the localized set of conditions that describe where aquatic organisms live. Specifically, microhabitat is considered to be relatively homogeneous patches of such stream habitat features as macrophytes or woody debris, or a bed substrate type within a geomorphic channel unit. Studies have revealed that analyzing patterns in relations between aquatic organisms and habitat at the microhabitat scale can provide insight to patterns of relations between biota and habitat at larger scales (Hawkins, 1985; Biggs and others, 1990).
Collection of microhabitat data is essential for an understanding of the relations among benthic invertebrate and algal communities and physical, chemical, and biological factors. Invertebrate and algal samples are collected at specific locations within geomorphic channel units in a reach. Conversely, fish community samples are collected throughout the reach and are not associated with specific microhabitat data. Procedures for collecting microhabitat data are described in documents detailing the colle ction of invertebrate (Cuffney and others, 1993) and algal (Porter and others, 1993) samples, and are not within the scope of this report.
The NAWQA Program is designed to assess the status of and trends in the Nation's water quality and to develop an understanding of the major factors that affect observed water-quality conditions and trends. Stream habitat is characterized as part of an in tegrated physical, chemical, and biological assessment of the Nation's water quality. The goal of stream habitat characterization as part of NAWQA is to relate habitat to other physical, chemical, and biological factors to describe water-quality conditio ns. To accomplish this goal, environmental settings are described at sites selected for water-quality assessment. In addition, spatial and temporal patterns in habitat are examined at local, regional, and national levels. Evaluating stream habitat as part of the NAWQA Program is based on a spatially hierarchical framework for habitat characterization that incorporates habitat data at basin, segment, reach, and microhabitat scales. This framework provides a basis for national consistency in collection techniques while allowing flexibility in habitat assessment within individual study units.
The GIS data bases are a primary component of basin characterization. Two levels of data-base coverage, national and local, are required for basin characterization to ensure the use of the most complete basin-level information at the greatest resolution for individual study units, as well as to allow comparisons of basin characterizations among study units. GIS data bases that are incorporated into basin characterization include data bases on land use, ecoregion, physiographic province, potential natural vegetation, soil type, and wetlands.