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3.0 Environmental flows

Overview of natural flow regime

In recent decades, the ecological impact of dams has taken a forefront in many discussions regarding the cost and benefits of dam building, decommissioning, and removal. One of the most prominent concerns is the impact dam operations have on the natural flow regime. The natural flow regime refers to the changing quantity and timing of water flows throughout the course of a year, as well as inter-annual variation, which would occur under natural conditions. Figure 3.0a provides an example of the different components of a natural flow regime. Each component is characterized by its magnitude, duration, frequency, timing, and rate of change.

Figure 3.0a. Components of an example natural flow pattern (Poff et al. 1997)

Poff-Natural flow pattern

Streamflow plays a critical role in determining many principal characteristics of rivers, including channel geomorphology, water temperature, water quality, and habitat diversity, leading Poff et al. (1997) to define the natural flow regime as the ‘master variable’ that regulates the ecological integrity of rivers and controls the distribution and abundance of riverine species.

Impact of dam operations on natural flow regime

Most dams alter the natural flow regime. However, the extent and nature of flow alteration depends on the dam size, type, operating objectives, and location. For example, a run-of-river dam without significant storage capabilities will release water at essentially the same rate it enters the reservoir, with only minimal alteration of the natural flow regime downstream. In contrast, the storage and release schedule of a large storage dam can radically alter the natural flow and a diversion dam can remove large volumes of water from the natural flow regime. The following pages describe how flood control and hydropower operations at Folsom Dam impact the natural flow regime, as well as some potential methods to incorporate environmental flows into dam operations.

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