Hydroelectric

Hydroelectric Dam. Photo credit: Hydroelectric energy, 2011

CONSTRUCTION

Hydroelectric power is generally produced through the construction and operation of dams. These large-scale dams use large tracts of land, particularly if they’re situated in flat areas rather than naturally hilly or canyon-like landscapes. Unlike many other types of renewable energy, dams’ most detrimental effects often aren’t directly associated with construction, but are products of merely the unnatural presence of the dams. However, dam construction still has extremely harmful effects on the surrounding areas.

Flooding

First, the creation of a dam causes backup of water behind the dam, resulting in massive flooding. This destroys forests, wildlife habitat, agricultural land, and even scenic lands (Environmental Impacts of Hydroelectric Power, 2013). As the area behind the dam floods, portions of the river below the dam experience decreased water levels, and sometimes even complete drying out of the river system (further effects of this problem will be discussed in more detail below). 

Reservoirs

Shasta Dam under construction
Photo credit: Lee, 1942

Additionally, flooding behind the dam results in a river environment turning into a lake-like reservoir, whose slow-moving waters and higher surface temperatures can be extremely detrimental to many different species. One of the most prominent effects is that warm, slow-moving reservoirs favor natural predators of native river species. This causes higher population losses of certain species, specifically fish, which is particularly harmful to already endangered or threatened populations (Electricity from Hydro, 2000). Higher temperatures can also affect survivability and/or reproductive success of many river species, as even slight changes in temperature can dramatically alter physiological processes. Further, because reservoirs are more stagnant than naturally flowing rivers, evaporation off the reservoir occurs at a much faster rate, worsening the effects of dried out segments below the dam. Slow-moving water can also affect certain species that require a naturally flowing river for migration, because they rely on the river flow to carry them downstream. These fish types can then become disoriented, thereby lengthening the duration of their migration (Electricity from Hydro, 2000). Additionally, certain species need quick-moving water purely for survival purposes, like the net-spinning caddisfly, which requires a specific water velocity in order to capture food (Cushman, 1985). Some insects also need water currents in order to replenish their oxygen supply, and slow-moving reservoirs do not provide those currents, nor do they have as much dissolved oxygen than a naturally-flowing river (Cushman, 1985).

Carbon Emissions

In tropical environments, the flooded area behind dams can also experience vegetation and soil decomposition, which results in the release of carbon dioxide and methane (Environmental Impacts of Hydroelectric Power, 2013). Construction of dams often necessitates carbon emission from construction equipment, though carbon release rates are not comparable to traditional fossil-fuel power plants.

OPERATION & MAINTENANCE

The effects of dams on wildlife are widely researched, and are particularly well known in the Pacific Northwest, as we not only get most of our power from hydroelectric dams, but we have strong cultural and economic connections to salmon runs. Unfortunately for us, those salmon runs are being degraded due to the presence and operation of dams.

Variable Water Flow

Sockeye salmon. Photo credit: Hines, 2013

As discussed above, dam construction causes flooding behind the dam and decreased river runs below the dam. As water levels downstream drop, plants and animals can dry out and die, or can become stranded as the water recedes, particularly if the river slope is shallow (Cushman, 1985). In response to this problem, dam operators are required to release certain amounts of water at certain times of the year. However, variable water levels can have just as detrimental effects as consistently low water levels, as operators are then unnaturally changing water velocity, river depth, river width, and the wetted perimeter (distance along stream bottom from one shoreline to the other) (Cushman, 1985). When operators only release water occasionally, the river habitat experiences times when there is little to no water, then sudden, powerful surges that erode soil and vegetation and flood shoreline plants and wildlife. This also causes disruption of natural seasonal variations of rivers that would have originally triggered growth and reproduction cycles of river species (Electricity from Hydro, 2000), and can decrease the survivability of fish eggs (Cushman, 1985). Studies have also shown that dramatic variations in water flows can have major effects on benthic organisms, including reduced species diversity, biomass, density, and average weight. They also found that the benthic organisms present tended to be more “tolerant” species (can endure wider ranges of conditions related to temperature, water velocity, dissolved oxygen, etc.) and were therefore of lesser quality as food for native fish species (Cushman, 1985).

Photo credit: Oil Peak, 2011

Further, the resulting changes in reservoir levels can degrade shorelines and disturb waterfowl and bottom-dwelling organisms around the reservoir, rather than just along the downstream portions of the river (Electricity from Hydro, 2000). Additionally, dam operators can either release water from the top of the reservoir or the bottom, depending on the setup of the dam. Water released from the top of the reservoir is warmer than naturally flowing rivers, and water released from the bottom is much cooler. Either temperature difference can disrupt downstream habitat and can negatively affect the growth rate and survival of species (Electricity from Hydro, 2000).

Sediment

Sediment cloud released after removal of Glines Canyon Dam. Photo credit:
Fish hide from heavy sediment flow in newly freed Elwha River, 2012

Flowing rivers naturally transport sediment along the river, often resulting in beach habitats at the mouth of the river. Because dams cause water stagnation, the sediment that ordinarily would have been caught up in the downstream current is able to settle to the bottom of the reservoir. Not only does the downstream loss of sediment result in a loss of beach at the mouth of the river, but settled sediment also disrupts natural habitat for fish spawning. Further, when sediment and nutrient levels are higher than normal, algae and other aquatic weeds are able to flourish and crowd out other river animals and plant life (Environmental Impacts of Hydroelectric Power, 2013). Further, if toxic elements are contained within the sediment, otherwise harmless levels of sediment that would ordinarily have been swept up in the river and deposited in very small amounts can begin to accumulate on the floor of the reservoir and reach extremely toxic levels. This poses a problem especially when dealing with dam removal, as management of the large sediment buildups poses questions of how to dispose of the toxic elements safely. If removal of sediment is not a viable option, the problem then becomes managing large sediment releases once the dam is removed, which can bury species at the mouth of the river. (In the Pacific Northwest, the Elwha dam removal project is getting a lot of attention for this and many other reasons. Check out the Dam Removal Blog for more information on the removal process.)

Direct Effects

Finally, many species are affected through pure contact with the dam and its moving parts. Turbine blades can disorient, bruise, stress, injure, or even kill migrating fish and other organisms; small and juvenile fish are particularly vulnerable (Electricity from Hydro, 2000). If a river has multiple dams – even if they have implemented fish passage devices like fish ladders – the success rate of those devices is greatly reduced, as it is a great effort for fish to use those devices (Electricity from Hydro, 2000). Some dam operators have taken to manually transporting fish through the dam via barges, but this can increase stress and disease transmission, and can decrease homing instincts of fish species (Electricity from Hydro, 2000).

Original Figure, stats from Hydropower Statistics, 2014