Assessing the expansion of wadeable benthic macroinvertebrate collection methods in large semiwadeable rivers

Benthic macroinvertebrates are used commonly as water-quality indicators in wadeable streams and rivers. However, increased depth and overall complexity of large rivers may cause inaccessibility or unacceptable index and metric variability when attempting a single wadeable method. Significant increases in sampling effort and time expenditure have been recommended to compensate for the increased variability when assessing large rivers. However, the transition between wadeable streams and nonwadeable rivers is not well defined, especially for free-flowing systems. We evaluated the utility of a modified wadeable-stream collection method for sampling large and complex semiwadeable rivers. Our results suggest that water-quality measurements across the width of a river can be used to identify influences of multiple water sources. This spatial information and a single-habitat (e.g., riffle-run) approach could be used with a modest increase in sampling effort to reduce metric variability and provide important source-tracking information. These supplementary data and a grasp of the complex ecological processes that exist in large rivers are essential for recognizing why biological communities differ among sites and for using macroinvertebrates to assess large river systems.