chemical, and biological characteristics of water quality in the study unit and to be of importance locally, regionally, or nationally. Two distinct types of sampling sites are established as part of NAWQA--fixed sites and synoptic sites.
Fixed sites are typically located at or near USGS gaging stations where continuous discharge measurements are available. At these sites, broad suites of physical, chemical, and biological characteristics are measured, along with characterizations of fish, benthic invertebrate, and algal communities. Synoptic sites are typically nongaged sites where one-time measurements of a limited number of physical, chemical, and biological characteristics are made with the objective of answering questions regarding source, occurrence, or spatial distribution.
The methods used for evaluating stream habitat at the various scales combine existing information from sources such as GIS data bases and maps with field-collected data on instream and riparian features. These methods provide a basis for national consistency yet allow flexibility in habitat assessment at all scales of the hierarchy.
The term "basin" denotes the area drained by all surface waters located upstream of a selected site. The development of stream habitat is influenced to a large degree by basin-scale spatial factors including physiographic province, geology, and climate (Schumm and Lichty, 1965; Frissell and others, 1986; Klingeman and MacArthur, 1990). Thus, the basin is a convenient unit for characterizing the environmental setting of selected sites (Leopold and others, 1964). Evaluation of basin-wide natural and human factors also enhances an understanding of the comparative biogeographic patterns in biological communities (Biggs and others, 1990; Quinn and Hickey, 1990).
The GIS data bases are a primary component of basin characterization. For the aggregation of data among study units, data related to basin-level variables must be gathered from data bases with national coverage. Scales for national coverage maps generally range from 1:24,000 to 1:7,500,000 resolution for many data bases of basin-scale characteristics. For analysis of data at the study-unit level, local coverage maps and aerial photographs generally provide better resolution per data base and are more current than national coverage maps. Also, nondigital coverages may provide valuable information for individual study units. Thus, two levels of data-base coverage 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.
A basin characterization is conducted at fixed and synoptic sites using a form such as the one presented in figure 2. Instructions for completing a basin characterization form are detailed below, with the numbers corresponding to the items as presented in figure 2.