Structural Features

When a hydroelectric project is built, its physical structure can address specific needs that arise. For instance, fish passage facilities may require improvements or water temperature further regulated. Sometimes, modifications to a project’s physical structure can effectively address such issues.

Strategy: Surface Collectors, Fish Screens, Turbine Modifications and Guidance Devices
Issues: Fish Migration, Fish Populations and Maximizing Generation

A fish passage is a waterway which allows fish to pass a dam or other obstruction. At hydroelectric projects, examples include lifts or elevators, barging, fish pumps, locks, screens and other bypass devices.

For juvenile fish migrating downstream, there are essentially three “in-river” ways for fish to pass from one side of a hydroelectric project to the other. These are for fish to follow the path of water:

  1. through the turbines and exit at the tailrace of the project,
  2. to fish screens or surface collectors, which then route fish to a bypass system that channels them around a dam, or
  3. over a project’s spillway.

These are called “in-river” paths because they do not rely on barging or trucks to “taxi” fish downriver before being released. For the Columbia and its tributaries, these paths primarily affect juvenile salmon (also called smolts) migrating downstream. These paths may also affect fluvial fish (those who migrate up and downstream but not to and from the ocean). Examples of fluvial fish receiving close attention because of declining populations include bull trout and cutthroat trout.

The success of traveling each path also varies from project to project. The reason is that the design of each project, the flow rate of the river, and the particular geologic and topographic circumstances of projects widely differ. Some variables that affect successful migration via each path include:

  1. When fish flow through turbines, they can die as a result of being bruised, stressed, and/or disoriented. The U.S. Department of Energy and a consortium of hydropower industry participants are addressing this issue by designing “fish friendly turbines” for both large and small projects. Currently, the large turbine design has been lab tested and is now ready to be field tested. The small turbine design is expected to be ready for lab testing in 1998. Such turbine modifications maximize both fish passage and the amount of electricity a project can generate.
  2. Bypass systems use surface collectors, fish screens, and other devices to guide fish through or around a hydroelectric project. Limitations to these systems sometimes occur because the natural instincts of fish draw them toward areas where the water flow is strongest. Often, this is the path that will lead fish into a project’s penstock and turbine area.
    • At some projects, “behavioral” guidance devices are used to guide fish to a bypass system. Examples include lights, sound repulsion, and air bubble curtains. By avoiding lights, sound, and other unattractive disturbances, the behavior of fish is affected to encourage them to swim away from hazards and/or toward a bypass system.
    • As with any path, issues of stress and disorientation are also present with bypass systems. Finally, such systems tend to release fish into a relatively small area. Here, they can become easy prey for predators such as squawfish.
  3. When fish pass over a spillway, they fall into the pool of water below. Changes in pressure can cause gas-bubble disease (which is the equivalent of “the bends” for divers). Stress and disorientation represent additional challenges to their mortality. Spill deflectors are sometimes used to help minimize these issues.

Strategy: Fish Ladders and Upstream Fish Passage
Issues: Fish Migration and Fish Populations

Other major structural features to assist fish passage relates to the upstream journey of fish. Along the Columbia and its tributaries, this journey often relates to adult salmon migrating upstream to spawn. Examples of fluvial fish that may also need such assistance include bull trout and cutthroat trout.

Fish ladders are the most common structural modification designed to assist this journey. Different designs are used to maximize a ladder’s effectiveness at each project. For instance, design modifications can help fish find the small attraction flows at the entrance of a ladder, or reduce the possibility of fish falling back over a spillway when they exit a ladder.

Strategy: Spill Deflectors and Selective Water Withdrawl
Issues: Water Temperature and Oxygen Levels

Beyond fish migration, structural features at a project can be used to assist with addressing water quality issues that affect aquatic life and plants. As mentioned above, spill deflectors (also called flip lips) can help reduce nitrogen supersaturation caused by water plunging into a pool several to a few hundred feet below a spillway.

In addition, some projects have gates built at different depths of water. By allowing water to pass through these gates, adjustments (often called selective withdrawl) can be made to regulate changes in water temperature and oxygen levels.

Strategy: Boat Ramps and Other Shoreline Recreational Access
Issue: Reservoir or River Access

The building of boat ramps, camp grounds, picnic areas and similar improvements are done to support the social interests of a community. These modifications do not directly affect the project itself. Rather, they are made up and downstream of a project to help ensure a river reach is as accessible and friendly to people as possible. Such improvements can also be aesthetic in nature and help the local economy by supporting tourism.