Conventional hydropower is one of the oldest and the nation's leading renewable energy resource. Power is produced by the fall of water which spins a turbine. The amount of power that can be generated depends on both the head (height of the drop) and the volume of water. At most conventional hydropower facilities, power production varies with seasonal water flow increases and decreases. Most projects are constructed on dams but some rely on water diversions at natural falls and through conduits.
The two main types of hydropower facilities are peaking/pulsing and run-of-river. Peaking facilities store water to be released during peak energy use times when demand and prices are highest. Pulsing facilities store less water and generate power until a lower limit is reached and then shut down to refill, but the pulse is not necessarily coordinated with peak energy use. Run-of-river facilities pass flows on a real-time basis, with outflow from the impoundment equaling inflow to the impoundment at all times.
The Federal Energy Regulatory Commission (FERC) regulates the development of non-federal hydroelectric projects. These projects comprise about half of the nation's developed hydroelectric capacity. The remainder are federally-owned projects that do not require FERC licensing. FERC has the exclusive jurisdiction to authorize non-federal hydroelectric projects that are:
- on navigable waters in the United States;
- on non-navigable waters over which Congress has jurisdiction under the Commerce Clause, were constructed after 1935, and affect interstate or foreign commerce;
- on U.S. public lands or reservations; or
- use surplus water or water power from any federal dam.
Hydropower represents about 2.5 percent of the nation's energy use. The largest concentrations of hydropower in the United States are on the West Coast and in the Northeast, and the best sites in the continental United States have already been exploited.
Fish and Wildlife Considerations
Water impounded by dams can create ponded lake habitat when and free-flowing-river habitats are inundated. This may result in changing the composition of the fish community and productivity of the ecosystem.
Dams block both upstream and downstream fish access to habitat, interfering with fish movement (e.g., migratory movement to spawning habitat, seasonal movement to overwintering habitat or daily feeding or territorial movements). Habitat fragmentation degrades the ecosystem and disrupts life cycles of fish and other aquatic species. A variety of fishway designs can safely pass fish upstream or downstream of dams, but many are species-specific and not as effective as movement in a free-flowing river. Fish may be killed when entrained (sucked into the turbines) or impinged (pinned against structures). Entrainment mortality varies by fish size and species, turbine type, head (heights of the drop) and other factors. Mortality ranges between five percent and nearly 100 percent. Various screens and barriers may help to minimize fish entrainment.
Hydropower operations may dewater a section of river (bypassed reach) when water is diverted through a pipeline or power canal. Minimum conservation flow into the bypassed reach may be required to protect fish and wildlife habitat. These flows may vary by season or try to mimic the natural hydrograph. Water levels in the project impoundment can vary considerably on a daily or seasonal basis, resulting in a relatively barren zone along the shoreline where few organisms can survive. Downstream fluctuations may also occur when flow through the turbines is varied. Water level concerns and downstream fluctuations may be minimized with run-of-river operations. Some projects are required to maintain protective impoundment levels during fish spawning or nursery seasons, or waterfowl or shorebird nesting seasons, and conservation flows below the project may be required.
Water quality may be impacted by increased water temperatures in the impoundment and downstream, sediment releases (turbidity), gas supersaturation, resuspension of toxic substances and reduced dissolved oxygen levels. These are all factors that impact the viability of fish and other aquatic life. These issues may often be addressed, at least to some extent, by modifications to project operations.
