4 Fish and Other Aquatic Species

Hydroelectric projects potentially affect many aspects of aquatic communities, as outlined in the Project Effects Matrix (Part I). In addition to affecting aquatic species by disconnecting habitat, hydroelectric projects also potentially impact habitat quality and quantity by altering fluvial processes with disruption of the flow of water, sediment, and woody debris. A variety of approaches are discussed below to evaluate the effects of hydroelectric projects on:

  • Instream flows and aquatic habitat
  • Fish and macroinvertebrate stranding
  • Downstream migration/entrainment
  • Upstream migration
  • Habitat connectivity
  • Fish population dynamics
  • Alteration of macroinvertebrate communities


These are not all of the potential effects of hydroelectric projects on aquatic habitats, and the methods to evaluate them by no means cover all the potential approaches that could be used to study the effects. Rather, this discussion focuses on those effects for which a nexus to project effects is most distinct and where approaches to evaluate effects are not necessarily standardized. For example, basic methods to conduct surveys to identify, characterize, and map the distribution of aquatic resources in the project area are not included. Under the relicensing process and associated National Environmental Protection Act (NEPA) and/or Endangered Species Act (ESA) process, information from these types of surveys is used in a risk assessment to determine if the sensitive resources are, or could be, disturbed by the project. Although these surveys are crucial, the approaches they employ are not unique to hydroelectric projects. This report does not discuss methods for basic resource inventories. Instead, it focuses on approaches that are specifically designed to evaluate effects of hydroelectric projects, while still recognizing the importance of general survey data. The following text provides a description of many of the approaches used to describe fish species composition and distribution:

Nielsen, L. A., and D. L. Johnson. 1983. Fisheries techniques. American Fisheries Society, Bethesda, Maryland.

A more recent approach to determining the historic, or pre-project, distribution of aquatic species uses stable isotope tracing (N15). In this approach, similar methods to those described in Section 2.4.2 are used to determine if marine-derived nutrients were ever deposited upstream of current barriers, thus providing evidence of the historical distribution of anadromous species. Although this method has yet to be widely used in hydroelectric relicensing, it has been widely used in other applications, and is currently being applied at the Lake Chelan Hydroelectric Project (FERC Project No. 637) on the Chelan River, Washington.