One-fifth of the world’s power generation is supplied by hydropower. Hydroelectric power is created as water passes through a dam and runs into a river. Large amounts of water are stored behind dams in reservoirs. As more water passes through a dam it creates more and more energy. Energy is then harnessed as electricity with the use of a turbine. This electricity is then used in several different settings, from households to industries. Hydropower is the most reliable source of electricity and offers the lowest cost of electricity across all major options.
SUSTAINABLITY
The power of hydroelectricity is a renewable resource that has been used successfully for collecting, storing, and managing enough water to sustain civilization. Hydropower dams can store water during the rainy season and release it during the dry seasons, helping to control floods and droughts. The loss of floods however results in sediments accumulating in the streambed. Floods naturally move sediments, which restore riparian habitats for fish species (Keskinen, 2012). Hydroelectric power plants have been found to have higher productivity than 90% of thermal plants (Yuksel, 2008). The use of hydroelectricity saves approximately 200 million metric tons of carbon from entering our atmosphere each year (Frey, 2002).
Compared to solar and wind power, hydroelectricity has the lowest cost per kilowatt-hour. It is reliable year round, while solar and wind powers are only so reliable based on their location and time of year. Alternate backup energy sources are often necessary for solar power and wind power. Hydroelectric dams can encounter issues in colder climates, such as freezing pipes (Bakos, 2002). As our energy consumption grows our need for renewable energy sources such as these follows.
POLLUTION
Hydroelectric dams have been known as a clean, pollution free energy source, until recently when it was found that they release CO2 and CH4 (Fearnside, 2004). Hydropower turbines release between 7-12 ton/gwh of Carbon Dioxide, which is a very small amount compared to coal at 830-920 ton/gwh. The effects of pressure and temperature by the turbines release almost all of the methane contained in the water. Downstream from hydroelectric dams, emissions resulting from the rapid depressurization of water from turbines are significant. As water rich in methane passes through the turbines and becomes exposed to the atmosphere, hydrostatic pressure drops and a large amount of gas is released, leaving almost no opportunity for oxidizing bacteria to consume the methane (Kemenes, 2007). Anoxic environments are also being created due to decay of vegetation left in reservoirs. These environments produce methane, provide conditions for methylation of mercury, and even have the possibility to corrode turbines (Fearnside, 2001).
In the Lancang River in China heavy metals were tested upstream and downstream of a hydroelectric dam. The results of the test showed that the concentration of six heavy metals (arsenic, chromium, cadmium, copper, nickel, lead, and zinc) in the soil upstream were significantly lower than the soil downstream from the dam. Tests showed no contamination was found upstream for any of the heavy metals except for Cadmium with low levels. Downstream low contamination levels were found for Arsenic, Chromium, Copper, and Nickel, while moderate contamination was found for Cadmium. This indicates that some contamination is occurring from hydroelectric dams (Bai, 2009).
MIGRATION
Hydropower dams were not built with the intent to help fish pass safely downstream and as a result many species of migratory fish encounter problems directly with their turbines or effects from their presence. A 25-year study at the Columbia-Snake River System found mortality rates to be between 7% and 13% with juvenile salmon passing through turbines (Kareiva, 2000). A dam can also block or delay the upstream migration of fish. During low flows in the Columbia Basin, juvenile Chinook salmon reach the estuary approximately 40 days later than they did before the construction of the dams. Some dams delay and concentrate fish, making this area attractive to predators and the fish more susceptible to them (McClure, 2003).
Hydroelectric dams can alter an environment through discharge modification, water quality changes, and water temperature changes. Reduction or modification in river discharge encourages loss of migration routes, loss of spawning grounds, decreased survival of eggs and juveniles, and reduced food production. Thermal and chemical water changes effect spawning success and can cause fish mortality. Population stability can be affected to the point of extinction if the dam prevents migration between the species feeding and breeding zones (Larinier, 2001).
IMPROVING FISH PASSAGE
The operation and design of turbines has begun modification in order to improve the survival of several fish species attempting to pass through hydroelectric turbines during migration and feeding. Fish that pass through hydroelectric turbines encounter injuries due to several means, such as rapid and extreme pressure change, cavitation, turbulence, and grinding. Low impact turbines that maintain efficient electrical generation while having environmental benefits are more “fish-friendly” and advanced. Development of a new turbine runner minimizes the number of blade leading edges and maximizes the flow passage size. The development of new screening systems guide fish around turbines to increase passage success (Cada, 2001).
The Coumbia-Snake River Basin located in the Northwestern United States has found success with fish ladders. Fish ladders are generally quite long with a gentle uphill sloping, baffled raceway or a natural slope with structures designated for resting. These guide upstream migrants into downstream ends and then coax the fish into continuing upstream for migration. Another method is a fish lift where fish are held in one area and then raised up and over the dam, where the fish are released (Schilt, 2007).
Executive Summary
Although hydroelectric dams produce very little greenhouse gases and are a cost-effective source of energy they do alter the environment around them. Hydroelectric dams block the migration of fish, modify normal river flow, and disrupt natural spawning grounds. The International Rivers Network estimates that more than 60% of the world’s major rivers have been dammed, leading to irreversible loss in species and ecosystems (Namy, 2007). As long as advancements to assist migrating fish in successful dam passage continues, the positives seem to outweigh the negatives for hydropower. The reliable availability of water for hydropower creates an option that is less detrimental than thermal energy. Continuing progress on minimizing environmental impacts is the most important factor when it comes to the future of hydroelectric power.