The Basic Fixed-Site sampling is designed to provide an integrated assessment of spatial and temporal distribution of general water-quality conditions and the transport of major inorganic constituents of streamwater in relation to hydrologic conditions and major sources. Data from Basic Fixed-Site sampling are the primary source of information for meeting water-column assessment objectives for temperature, salinity, suspended sediment, major ions and metals, nutrients, and organic carbon (Gilliom and others, 1995, p. 14). Site selection and sampling strategy for Basic Fixed Sites are based on balancing needs and priorities for assessing water-column conditions, constituents in bed sediment and fish tissue (see Bed-Sediment and Tissue Studies), and aquatic ecological communities.
The ALMN Study Unit has five Integrator and five Indicator Basic Fixed Sites (two of which are classified as Intensive Fixed Sites). The Integrator Basic Fixed Sites are chosen to represent the range of large streams in each study unit and include major nodes in the drainage system for large-scale transport analysis. Collectively, the basins of these Integrator Sites usually include all or most of the study unit. The Indicator Basic Fixed-Sites primarily are chosen to represent each of the environmental settings in the study unit that are most relevant to streamwater quality and, thus, also are most likely to represent environmental settings that most influence the water quality of the downstream Integrator Site. Commonly, the collective basin areas of Indicator Sites may represent only a small percentage of the total study-unit area.
The water-column sampling strategy at each Basic Fixed Site consists of three types of sampling activities designed to yield a core of consistent data on hydrologic and general water-quality conditions. The three types of sampling activities -- continuous monitoring, fixed-interval sampling, and extreme-flow sampling -- are conducted for at least 2 years (Gilliom and others, 1995, p. 15). Basic (and Intensive) Fixed Sites are the only sites in the NAWQA study design where streamflow and selected additional characteristics are continuously measured.
There are four major components of the Fixed-Site Reach Assessment conducted by NAWQA at all of the Basic (and Intensive) Fixed Sites. These components include; algal, benthic invertebrates, fish-community assessments and aquatic-habitat evaluations. These data are used to improve understanding of relations among aquatic biological communities and the physical, chemical and hydrologic conditions associated with selected environmental settings (Gilliom and others, 1995, p. 24). In addition, each aquatic community provides an indication of what has been occurring in the stream over a different time scale. Algal communities tend to respond to changes in water quality relatively quickly (hours to days), aquatic insects somewhat slower (days to weeks), and fish communities, which are responding to changes in algal and invertebrate communities as well as directly to water-quality changes, over a longer period (weeks to months). An evaluation of aquatic habitat is necessary to determine if community responses are due to chemical changes in the water or physical changes in flow, channel morphology, or other habitat changes.
Algae and aquatic insects are sampled concurrently in fast flowing water in a known sample area. Additionally, a qualitative sample of both communities is collected throughout the stream reach to include a broader range of habitats and taxa. Fish were sampled with electrofishing gear appropriate for the stream reach being sampled. The entire stream reach was fished except in the largest rivers where near shore habitats were sampled.
Habitat measures were collected along transects. Both instream and riparian measures were made and include instantaneous velocity and depth, substrate analysis, canopy cover, erosion patterns, flood plain and bank composition and structure, and instream fish-habitat components. Microhabitat measures were also taken at each algae and aquatic macroinvertebrate sampling point and included instantaneous velocity, embeddedness, and substrate particle sizes at each algae and aquatic macroinvertebrate sampling point.
Additional information regarding sampling methodology is available in NAWQA Method and Guideline Protocol publications. A map of the drainage basins and site locations of the Basic Fixed Sites is available for viewing: