ABSTRACT
This chapter argues that the physical and biological characteristics of large rivers are unique to the extent that agencies must evaluate the underlying assumptions of these systems before defining narrative or numeric biological expectations. It focuses on midwestern large rivers, operationally defined as river systems having a drainage area of greater than 20,000 sq. km. Increasingly, water resource agencies are expanding biological monitoring programs to include large rivers. Water resource agencies should understand the potential magnitude of variability and habitat effects. The Ohio River Ecological Research Program is a long-term study of aquatic life near once-through cooled power plants on the Ohio River. Fish are sampled using several gear types and, because sampling is conducted three times from June to October, seasonal variability of population and community parameters can be documented. The increased physical dimensions of large rivers reduce fish sampling effectiveness; the combined effects of habitat zonation and small proportion of surface area sampled account for this problem.
