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.

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.

3.2 Incorporating environmental flows

Environmental flows overview

In order to counteract dam's alteration of the natural flow regime, environmental flows have been incorporated into many dams' operations plans, particularly in some western countries (e.g. US, Australia, Canada). Environmental flows manage the quantity, timing and quality of water flows below a dam, with the objective of sustaining ecosystem functions and riverine species. The objective of environmental flows is not reproduce a natural flow regime in whole, but rather to achieve a flow regime that maintains the essential processes required to support healthy river ecosystems. For example, it not usually desirable to reproduce large flood events that can harm people and damage property, but it may still be possible to introduce smaller flood events that achieve the objective of transporting sediment and wood to create essential habitat features. While some dam operations plans included environmental flows from the outset of their operations, including Folsom Dam, in most cases, environmental flows have been retroactively introduced into operations plans.Watch the video from the Natural Heritage Institute on the Global Dam Reoptimization Initiative for more information on the impacts of dams and how redesigning operations can help restore ecosystems and provide climate change mitigation opportunities.

Environmental flows at Folsom (historical)

In addition to the primary operating objectives of flood control, water supply, and hydropower production, other purposes in the initial plans at Folsom dam included fish and wildlife preservation and recreation. Historically, environmental flow regulations focused on minimum flow requirements. For Folsom these included a minimum flow at the mouth of the American River of 250 cfs from January through mid-September and a minimum of 500 cfs required between September 15 and December 31.

Updating Folsom's environmental requirements (present)

Biological, socioeconomic, legal, and institutional conditions have changed substantially since the initial minimum flows at Folsom Dam were adopted in 1958. Many diverse stakeholders involved in various American River actions have agreed that the current regulations are not sufficient to protect the fishery resources within the lower American River. The bullet points below reflect a partial list of some more recent recommendations for an updated flow management standard on the Lower American River. 

Recommendations for lower American River flow management standard (SWRI 2004)

  • Required Flows equal to 2,250 cfs
    • 250 cfs step increases from 1,500 cfs on October 1 to 2,250 cfs on November 9
    • Oct 1 to Oct 24 1,500 cfs
    • Oct 25 to Oct 31 1,750 cfs
    • Nov 1 to Nov 8 2,000 cfs
    • Nov 9 to Dec 31 2,250 cfs
  • Required Flows between 2,250 cfs and 1,500 cfs
    • Incremental step increases from 1,500 cfs on October 1 to Required Flows on November 9
    • Oct 1 to Oct 15 Required Flows = 1,500 cfs
    • Oct 16 to Oct 31 Required Flows -500 cfs, or 1,500 cfs, whichever is greater
    • Nov 1 to Nov 8 Required Flows -250 cfs, or 1,500 cfs, whichever is greater
    • Nov 9 to Dec 31 Required Flows
  • Required Flows less than or equal to 1,500 cfs
    • Implemented on October 1
    • Continue at same level through December 31



During what time of year do the above recommendations apply?
from ____ to ____ (please type full month name)

Display "Correct, the recommendations above only apply during the fall months from Oct-Dec. As such, the recommendations do not address the dam's impact on the natural flow regime for the other 3/4 of the year. Importantly, none of the recommendations attempt to reintroduce controlled high flows or floods into the system, a significant component of the natural flow regime and one with important implications for native fish species."

What dam objectives would be impacted by implementing the recommendations above?

Flood control ____ Hydropower ____ Both ____