How a Hydroelectric Project Can Affect a River – FWEE – Foundation for Water & Energy Education

Hydropower has traditionally been considered environmentally friendly because it represents a clean and renewable energy source. The term renewable refers to the hydrologic cycle that circulates water back to our rivers, streams, and lakes each year. At hydroelectric projects, the force of falling water is used as “fuel” to generate electricity. In contrast, fossil fuels like coal, natural gas, or oil must be extracted from the earth and burned to produce electricity.

The term clean is also used because production of electricity with hydropower does not pollute the air, contribute to acid rain or ozone depletion because of carbon dioxide emissions, or (like nuclear power) leave highly toxic waste that is difficult to dispose of. Visit Northwest Sources of Electricity to learn more.

As Hydropower in the Northwest graphically illustrates, over 80% of the Northwest’s renewable energy to generate electricity comes from hydropower. And while there are many benefits to using hydropower as a renewable source of electricity, like any energy resource there are also environmental impacts. These impacts generally relate to how a hydroelectric project affects a river’s ecosystem and habitats.

Because there are over 370 hydroelectric projects in the Northwest, understanding such ecosystem and habitat issues is vitally important. Examining these issues, however, needs to be done in a broad context for three reasons. First, no two hydroelectric projects are exactly alike, and many are very different. Thus, while issues can be examined in general terms, one should not draw conclusions that all or even most projects have similar environmental impacts.

Second, while this discussion focuses on hydroelectric projects, one should not conclude that all dams are used to produce electricity. Nationally, for instance, only three percent of the nation’s 80,000 dams are used to produce electricity. Most dams are used for purposes such as irrigation, flood control, and water treatment. Further, many dams support a combination of activities. For example, dams on the mainstem of the Columbia River are used for irrigation, flood control, transportation, recreation, and the production of electricity.

Third, this section does not provide detailed information about a host of other activities that can significantly impact a river’s ecosystem and the species that rely on it for survival. Examples of other non-hydropower related impacts include grazing, logging, agricultural activities, mining, land development, and the harvesting of fish. Determining the relative impact of these activities versus hydroelectric projects is very complex and the subject of ongoing debate.

Protection, Mitigation, and Enhancement strategies are how hydropower generators meet their environmental impact responsibilities.

Specific ecosystem impacts caused by a single hydroelectric project largely depend on the following variables: 1) the size and flow rate of the river or tributary where the project is located, 2) the climatic and habitat conditions that exist, 3) the type, size, design, and operation of the project, and 4) whether cumulative impacts occur because the project is located upstream or downstream of other projects.

The first two variables depend on a complex set of geologic, geographic, and weather conditions. For the Northwest, the bounty and beauty of these phenomena are described in the sections What Makes The Columbia River Basin Unique, and The Columbia River Basin and Its Ecosystems.

Engineers typically determine the type, size, design, and operation of a project based on these natural dynamics. As described in Hydropower in the Northwest, the two most common hydroelectric facilities are storage projects and run-of-the-river projects.

Storage projects hold water in a reservoir or lake to adjust a river’s natural flow pattern to release water when the demand for electricity is highest. Run-of-the-river projects allow water to pass at about the same rate that the river is flowing. At some projects, “pondage” behind a dam may modestly raise the water level and effect the rate of water flow.

Although no two storage or run-of-the-river projects are the same, let’s take a look at some of the ecosystem changes that may occur because of their presence.

Stratification

Reservoirs, also called lakes, are created when storage projects are built. Reservoirs can significantly slow the rate at which the water is moving downstream. Surface temperatures tend to become warmer as the slower moving or “slack” water absorbs heat from the sun. This same effect may also occur to some degree at run-of-river projects with pondage behind a dam.

In addition to surface water warming, the colder water sinks toward the bottom because of its higher density. This causes a layering effect called stratification. The bottom layer is the coldest and the top layer the warmest.

When stratification occurs, there is also another ecosystem effect. Specifically, the colder water that sinks toward the bottom contains reduced oxygen levels. Further, at some sites when water is released from the colder, oxygen-depleted depths, downstream habitat conditions change because of the reduced oxygen level in the water.

Supersaturation

Supersaturation occurs when air becomes trapped in water spilled over a dam as it hits the pool below, creating turbulence. Because air is comprised of 78% nitrogen, the level of nitrogen dissolved in the water can increase dramatically. The affected water does not lose the excess nitrogen quickly. For fish and other species, supersaturated water can enter tissues. If fish swim from an area supersaturated with nitrogen to a lower pressure area, a condition similar to “the bends” in scuba diving can occur. This effect causes injury and can even cause death to fish.

Changing Water Levels

Building a storage project can raise the water level behind a dam from a few feet to several hundred feet. When stream banks and riparian areas become covered by the reservoir’s higher water level, the result is called inundation. Habitat conditions change and a new equilibrium emerges. As this occurs, a different set of dynamics begin impacting species that traditionally grow, nest, feed, or spawn in these areas.

Once built, storage projects can also raise and lower the level of water in a reservoir on a daily, weekly or seasonal basis to produce electricity. One term used to describe this process is “power peaking.” This occurs when, for instance, more water is released in the morning because electricity demands increase as people wake up and begin taking hot showers, using kitchen appliances, etc. In a riparian zone, (the area where moist soils and plants exist next to a body of water) this may result in shoreline vegetation not being effectively reestablished.

Sedimentation

Sediments, which are fine organic and inorganic materials that are typically suspended in the water, can collect behind a dam because the dam itself is a physical barrier. From the time a project is built, man-made and natural erosion of lands adjacent to a reservoir can lead to sediment build-up behind a dam. This build-up can vary based on the ability of a river to “flush”the sediments past the dam. It can also vary based on the natural conditions specific to the river and its upstream tributaries.

When sediments collect, the ecosystem can be affected in two ways. First, downstream habitat conditions can decline because these sediments no longer provide important organic and inorganic nutrients.

Second, where sediment builds up behind a dam, an effect called “nutrient loading”can cause the supply of oxygen to be depleted. This happens because more nutrients are now available, thus more organisms populate the area to consume the nutrients. As these organisms consume the nutrients, more oxygen is used, depleting the supply of oxygen in the reservoir.

Similarly, gravel can be trapped behind a dam in the same way as sediment. In cases where the movement of gravel downstream is part of establishing spawning areas for fish, important habitat conditions can be affected.

Erosion

Changing water levels and a lack of streamside vegetation can also lead to increased erosion. For example, the lack of vegetation along the shoreline means that a river or reservoir can start cutting deeply into its banks. This can result in further changes to a riparian zone and the species which it can support. Increases in erosion can also increase the amount of sedimentation behind a dam.

Just as the changes that occur to ecosystems vary greatly from project to project, so do changes in habitat. Indeed, painting a picture of all or even most hydroelectric projects as having more or less the same impacts is a serious mistake. For a given project, learning what habitat conditions exist and the extent of ongoing impacts requires a good deal of investigation. Likely sources of information include the project owner, information provided to the Federal Energy Regulatory Commission, state and federal fish and wildlife agencies, and local environmental groups.

When ecosystem changes occur at a project, a new pattern of biological activity and equilibrium is likely to emerge. As this happens, a new and dynamic equilibrium takes hold. With this new equilibrium comes changes to the plants, fish, and wildlife that populate these areas.

To examine these possible habitat changes, let’s begin with fish.

Fish

Salmon are the most well-known species of fish that migrate up and down streams. As a species, salmon are called anadromous because they migrate from fresh water to the oceans and then back again. Resident fish, on the other hand, spend their entire lives in fresh water streams, tributaries, and rivers. Some migrate from streams to lakes (adfluvial), others migrate from streams to rivers (fluvial), and some remain in the same reach of water. Brook trout and bull trout, for instance, are well-known resident fish that migrate up and downstream.

Depending on the species, these migration patterns can vary dramatically. In the case of salmon, for instance, coho tend to spawn in small streams and prefer shaded pools with overhanging trees and shrubs; sockeye salmon can migrate hundreds of miles to spawn in large lakes where, as fry, they live for two years before migrating to the sea; and chinook can spawn in large rivers like the mainstem of the Columbia River.

Salmon are called “fry”when they emerge from their spawning area and begin swimming freely and feeding in the stream. As they increase in size and maturity, they become “smolts”or “juveniles.”During their time in the ocean, they become “adults.”As a result, adults only exist in the river when they are migrating upstream to their spawning grounds.

Based on their life cycle and migration and spawning patterns, fish can face a number of different and changing ecosystems. Listed below are common fishery impacts that relate to hydroelectric projects.

Slower moving waters in a reservoir can strongly affect salmon for two reasons. First, they can become disoriented in slower moving waters; and second, the length of time it takes smolts to reach the ocean may increase. With disorientation and lengthened travel time comes an increased exposure to predators.
Fish passing through or around a dam can become stressed, injured, disoriented, or die because of contact with turbines, the walls of the dam, deflection screens, or other bypass facilities. They then exit into a relatively small area where their exposure to predators is increased. While fish passage rates are often better than 95%, fish that must pass through multiple dams face proportionally greater risk.
Supersaturation is a danger for fish going over a dam or through its spillway. If too much nitrogen is absorbed in the bloodstream, air bubbles form and create the equivalent of what divers call “the bends.” At high nitrogen levels, fish and some other aquatic species die. Also, if supersaturation conditions exist, fish passing through or around a dam will absorb greater nitrogen levels and suffer the effects as they continue downstream.
When adult salmon and other fish migrate upstream, the dam can again present itself as a physical barrier. If a “fishway “does not exist, then passage to spawning grounds is lost. Areas upstream of these dams are often referred to as “blocked.” While fish ladders are the most common fishways, other examples include fish locks, fish elevators and, trap and haul (transportation of fish upstream via truck). Where ladders are used as fishways, fish can find it difficult to find them if sufficient attraction flows are not provided at their base. Once up the ladder, they can again become disoriented and be sucked back over a dam or through its spillway. Salmon do not feed during their migratory journey back to their spawning grounds, so loss of energy and time become critical survival issues.
Reservoirs can create changes in downstream habitat conditions. For instance, macroscopic plants in the Columbia’s estuary are an important food source for salmon migrating to the sea. These plants, however, are no longer as prevalent in the estuary as they once were. Part of this decline has been traced back to conditions caused by reservoirs in the mainstem of the Columbia and on the Snake River.

There can also be effects to fish from loss of riparian vegetation, sedimentation, erosion, and temperature changes. Unlike the impacts listed above, however, these effects are also caused by non-dam activities such as farming, logging, and land development. As a result, when studying the health of habitat along a particular reach of river or tributary, all sources of environmental impacts must be reviewed.

Further, while fish migrating down and upstream may encounter altered ecosystems and barriers that impact their ability to survive, predation from other species also has an impact. Squawfish, for instance, live below a dam where (as predators) they can easily feed on smolts as they come through the tailrace of the dam’s powerhouse.

Likewise, slower moving waters and temperature changes caused by reservoirs can provide improved environments for warm water fish such as smallmouth bass and walleye. These resident fish also prey on salmon smolts moving downstream. And as smolts enter the ocean, the increased presence of sea lions makes survival more difficult.

The introduction of non-native fish to Northwest rivers further complicates the situation. Warm water fish such as smallmouth bass and walleye are examples of non-native species introduced to Northwest rivers by humans. While many anglers enjoy catching these fish, it is important to note that their improved conditions are at the cost of poorer conditions for native stocks of salmon and trout.

Wildlife

Riparian vegetation and its bordering waters provide critical habitat for birds, waterfowl, and small and large mammals. When a hydroelectric project results in inundation of a free-flowing river, the nesting, forage, and cover provided by these areas is temporarily or permanently lost.

When habitat is lost, animals are forced to move to higher ground or other areas where habitat conditions may be less suitable, predators are more abundant, or the territory is already occupied. As an example, ground birds like pheasant and grouse require cover and cannot successfully move to higher, more open, ground.

In cases where water levels stabilize at a new height, vegetation in riparian zones can re-emerge and species can re-populate an area. With storage projects, the riparian zone that re-emerges has conditions that now reflect that of a reservoir or lake rather than a free-flowing river. When such conditions occur, certain species will begin to decline, others will become more abundant, and some will populate these areas for the first time.

Ducks and geese are examples of waterfowl that are strongly attracted to the habitat conditions found in reservoirs. For some of these species, reservoirs are providing an important alternative to the wetland areas that they formerly occupied. Canada geese are one example of birds that now frequent reservoirs as part of their migration pattern.

Hydroelectric projects do affect the ecosystems of rivers and their surrounding areas. The degree, however, to which any one project affects a river varies widely. As discussed, one of the most important variables is whether a dam is part of a storage or run-of-the-river hydroelectric project. Other variables include the size and flow rate of the river or tributary where the project is located; the existing habitat and climatic conditions; the type, size, and design of a project; and whether a project is located upstream or downstream of other projects.

As changes in habitat occur, observation and time make it increasingly clear which plants, fish, and wildlife are affected. Some species end up doing quite well, others sharply or completely decline, and some are minimally affected.

Protection, Mitigation and Enhancement strategies are measures being taken to address environmental impacts. It is important to remember these measures are part of a much larger and complex whole. As such, their perceived success or failure is often dependent on a number of non-hydroelectric project activities.

For instance, extreme weather like droughts and climate change can dramatically affect the health of a river’s ecosystem and habitat. Indeed, in drought years rising water temperatures and slower stream flows have reduced salmon runs. And climate change has dramatically affected salmon survival and abundance in the ocean.

Beyond these natural conditions are a host of man-made conditions. Such conditions include but are not limited to:

Reduced streamflows from irrigation withdrawals and altered vegetation from land development, e.g., parking lots, roads, etc.
Loss of riparian zone, changes in water temperature and quality, loss of large woody debris, erosion, and sedimentation can come from various types of logging practices, agricultural activities and other land uses, e.g., grazing of cattle, mining and the building of roads.
Changes in estuary conditions from wetland drainage, diking, and navigational improvements.
The harvesting (catching) of fish by both commercial and sport fishermen.

While this section focused on impacts from hydroelectric projects, understanding how to maintain the health of rivers and tributaries throughout the basin requires investigating these much broader impacts as well. Additional sources of information on these impacts and what is being done to mitigate them include federal, state, local, and tribal agencies; public and private organizations that contribute to these impacts; and non-profit groups interested in these environmental issues.

Updated February 2022