Changes to the Ecosystem

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 the section “How The Northwest Hydroelectric System Works,” 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. In addition, more energy can be produced from water falling 100 feet above a turbine than from 10 feet. This height is called “head.” Thus, it is not surprising that the hydroelectric projects producing the most electricity also have the tallest dams and the largest reservoirs.

Run-of-the-river projects allow water to pass at about the same rate that the river is flowing. Generally, the river level upstream of the project is fairly constant, with daily fluctuations limited to only three to five feet at the largest projects.

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.

Reservoirs and 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.

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 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.


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.


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.