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3.1 Impact of Folsom Operations on Natural Flow Regime

Flood control impacts on natural flow

Flood control operations often result in the most substantial alterations to the natural flow regime. By definition, flood control projects eliminate or reduce the magnitude of floods and high flows. In addition, flood control dams often release large volumes of water to empty the reservoir in anticipation of the flood season, a process called reservoir drawdown. The empting of Folsom reservoir begins in October, resulting in downstream flows that are on average higher than those which would naturally occur (Figure 3.1a). In addition, since the released water has been held in the reservoir all summer, it tends to be warmer than the natural flows. As can be seen in Figure 3.1a, Folsom operations completely eliminate high flows and significantly dampen large floods.

Ecologically, floods create important disturbances that rejuvenate aquatic ecosystems. Moderately high flows transport fine sediment and organic resources through the channel, while larger floods transport large wood, gravels and boulders, creating high-quality habitat. In addition, floods that overtop riverbanks connect the channel to nutrient rich and ecologically-productive floodplains. Floodplains provide important nursery grounds for young fish, which use these areas to feed, grow larger and stronger, and hide from predators. Further, the commencement of flood season provides a physiological cue for some species. For example, fall-run salmon depend on early season high flows, which flood control operations often eliminate, to initiate their upstream migration (Figure 3.1a).

Figure 3.1a. Inflows vs. Outflows at Folsom Dam

Folsom natural flow alteration

Hydropower impacts on natural flow

The impact of hydropower releases on the natural flow depends on whether the powerplant is operated to meet base load or peak demands. Base load power is the day-to-day steadfast power we need 24 hrs a day, 7 days a week. While base load demands remain fairly constant throughout the year, many regions experience higher demands in summer months when people use AC to cool their homes. As a result, Folsom releases are higher in the summer than natural flows as water is released through hydropower turbines to meet power demands (Figure 3.1a).

Just as high flows and floods serve important ecological purposes, so do low flow events. Periods of low flow allow riparian plant species to recruit and establish themselves on river banks and floodplains. Also, the tolerance of many native species to prolonged low flows provides them with a competitive advantage over less tolerant species. The elimination of low flows can thus allow less tolerant species, and often exotic, species to displace native aquatic invertebrates and fishes.

Peaking powerplants meet the increase in demand above the baseload that occurs during daytime hours when more people are awake and using power to light, heat, and air condition homes and businesses. The capability to easily switch hydropower plants on and off makes them well suited to meet peak demands. Peak hydropower plants releases fluctuate significantly during the course of a day (Figure 2.0b).

Extreme daily fluctuations in streamflow have important implications for the riverine ecosystem. These fluctuations can result in large areas of river bottom that experience drying and wetting on a daily basis, changing the character of the physical habitat. Fish and invertebrates displaced by the unnatural surges in flows can become stranded when flows decline. In addition, the peaking releases may result in rapid changes in downstream water temperature with producing detrimental effects on fishes and other species.

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