Substrate characteristics are estimated either visually in shallow, clear waters or from examination of sampler contents. Characterization consists of identifying the dominant and co-dominant substrate size fractions and embeddedness class. The substra te classifications and two-letter substrate codes are listed at the bottom of the "MICROHABITAT CHARACTERIZATION" section. The substrate size and embeddedness classifications are identical to those set forth in the habitat protocol (Meador, Hupp, and ot hers, 1993).
Snag sampling differs from other types of semi-quantitative sampling activities. Consequently, there is a separate data sheet for snag sampling (fig. 16). This data sheet differs from other semi-quantitative data sheets in that the "SAMPLER CODES" secti on contains only samplers appropriate for snags, and the "MICROHABITAT CHARACTERIZATION" section includes space for recording approximate length and width of the branches sampled and the depth to the snag and streambed. More than one snag may be collecte d from a given location, such as several branches collected from a single large tree or debris accumulation. The dimensions of each of these branches are entered on the data sheet and associated with the same location number. At least five snag locations (if available), numbered sequentially from the downstream to the upstream end of the reach, are sampled within the sampling reach.
Benthic invertebrate sample processing, enumeration, and taxonomic identifications are done by outside contract laboratories under the direction of the USGS Quality Management Group's Biological Quality-Assurance Unit (BQAU), located at the National Water Quality Laboratory (NWQL) in Arvada, Colo. The BQAU oversees and coordinates all contracts for the processing and identification of benthic invertebrate samples according to standardized qualification, processing, and QA/QC criteria (Cuffney and others, 1993). It is responsible for overseeing the quality of samples processed by outside contract laboratories, in terms of the accuracy of enumeration and taxonomic identifications, and for resolving taxonomic issues within and among study units. The BQAU also oversees the entry of contractor data into the National Water Information System-II (NWIS-II) data base, maintains reference collections, and deposits voucher specimens in outside museums.
Study-unit personnel send sample components to the contract laboratory (large-rare and main-body sample components) and the BQAU (elutriate and split-sample components) as soon as possible, preferably directly from the field. This procedure helps to mini mize storage of formalin-containing samples and reduces the damage or loss of specimens and samples during shipment and storage. The study-unit biologist contacts the BQAU, prior to collection of the invertebrate samples, to determine which contract labo ratory will receive the samples. The contract laboratory receives only the large-rare and main-body sample components and is not apprised of the existence or identity of quality-assurance samples.