Hydropower Flow Fluctuations and Salmonids: A Review of the Biological Effects, Mechanical Causes, and Options for Mitigation

This technical report reviews the available research and evaluations on the effects of flow fluctuations on salmonids. It also summarizes how hydropower facilities create flow fluctuations, suggests criteria for mitigation, recommends field procedures, and identifies needs for further research. This technical report is limited to the review of flow fluctuations and does not address flow alterations.

Flow alterations are changes from the natural or unregulated flow that persist for weeks, months, or seasons, either as a result of water storage or as a result of bypassing a section of the river with a penstock. Flow alterations change the amount of habitat available to fish and, thus, change the capacity of the river to produce fish.

Flow fluctuations are unnatural changes in flow over periods of minutes, hours, or days. The biological impacts include immediate mortality, delayed mortality, temporary loss of habitat, reduced reproductive success, loss of food resources, and behavioral responses that could reduce survival or growth. The effects of flow fluctuations are not well-understood by many biologists outside the Pacific Northwest involved in hydropower mitigation, and many site-specific investigations completely ignore the impact of flow fluctuations.

The physical hydraulics of unregulated (i.e., natural) and regulated (i.e., hydropower controlled) rivers are compared to emphasize that unregulated rivers rarely experience drops in stage (i.e., water surface elevation) in excess of two inches per hour, except during floods, whereas regulated rivers may experience a much higher frequency at low and medium flows. Thus, aquatic life forms are not necessarily adapted to stage drops in excess of one or two inches per hour.

The most widely studied biological impact is stranding. Stranding has killed hundreds of thousands of juvenile salmon in single events. The incidence of stranding is affected by the life history stage of the fish, substrate type, river channel contour, range of flow change, rate of flow change, species, and time of day.

Other biological impacts have not been as thoroughly evaluated. These include redd dewatering, invertebrate productivity, fish emigration, and spawning interference. These impacts can be quite significant under some circumstances.

Hydropower facilities cause flow fluctuations in a variety of ways. Successful mitigation requires a thorough understanding of the operation practices and malfunctions that cause flow fluctuations. It is not sufficient to list criteria specifying allowable hydraulic changes.  Developers often fail to recognize or acknowledge all sources of flow fluctuations, and when facilities are built that fail to address all potential sources of flow fluctuations, they will resist unanticipated and often costly alterations of their facilities or changes  to their operation procedures. An overview of mechanical causes and suggested mechanical and hydraulic criteria are provided.

This report ends with a discussion on the significance of biological impacts relative to other types of hydropower impacts. The impact of flow fluctuations has been ignored in many site-specific evaluations and in most comprehensive reviews. Informational deficiencies and additional research needs are also discussed.