Upstream migrating fish can be delayed in project tailraces and fish ladders, or injured in draft tubes (exit for turbine discharge) when they attempt to enter the draft tubes because of a false attraction to the discharge, or to use the draft tubes as “cover” (Figure 1 shows typical facilities). Injury and delay are observed typically under the following circumstances:
- turbine discharge has better water quality than mainstem river,
- turbine discharge is a large proportion of total flow,
- turbine discharge is rapidly changing,
- fish ladders are too long, or
- fish are imprinted (chemically and return to their natal stream) on water from turbine discharge.
The initial evaluation that is typically conducted consists of comparing the swimming speeds of fish species of concern with the water velocities in draft tubes. If draft tube velocities are in excess of swimming speeds, injuries are not likely, except during turbine start-up. However, delay may still be an issue. The following reference provides a good overview of potential risks to upstream migration, and methods to evaluate effects.
NMFS (National Marine Fisheries Service). 1993. The use of barriers to prevent adult salmon delay and injury at hydroelectric powerhouses and wasteways. Working Paper. NMFS, Environmental & Technical Services Division, Portland, Oregon.
Various approaches for evaluating and monitoring the risks to upstream migrants are used by researchers, the most common of which are discussed below, including radio tagging and underwater observations.
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