Hydroelectricity: Environmental Costs, Benefits, and Outlook

Hydroelectricity is a significant source of power in many regions of the world, producing roughly 24% of the world's electricity. Brazil and Norway rely almost exclusively on hydropower. In Canada, 60% of electricity generation comes from hydropower. In the United States, 2,603 dams produce 7.3% of electricity, almost half of which is produced in Washington, California, and Oregon.

The use of hydropower to generate electricity pits two environmental concerns against each other: while hydroelectricity is renewable and lower in greenhouse gas emissions than fossil fuels, its impact on the environment is destructive of native lands and wildlife habitats. Finding the right balance between these concerns is necessary to confront the twin crises of climate change and biodiversity loss.

How Hydroelectricity Works

Hydropower involves using water to activate moving parts, which in turn may operate a mill, an irrigation system, or a turbine to produce electricity. Most commonly, hydroelectricity is produced when water is held back by a dam, then channeled through a turbine that is coupled to an electricity-producing generator. The water is then released into a river below the dam. Rarer run-of-the-river hydroelectric plants also have dams, but no reservoir behind them. Instead, turbines are moved by river water flowing past them at a natural flow rate.

Ultimately, the generation of hydroelectricity relies on the natural water cycle to refill reservoirs or replenish rivers, making hydropower a renewable process with little input of fossil fuels. Fossil fuel consumption is associated with a multitude of environmental problems: for example, the extraction of oil from tar sands produces air pollution; fracking for natural gas is associated with water pollution; the burning of fossil fuels produces climate change-inducing greenhouse gas emissions.

Costs

However, like all sources of energy, renewable or not, there are environmental costs associated with hydroelectricity. As the need to combat climate change makes hydroelectricity increasingly attractive, weighing the environmental costs and benefits is essential to determining hydro's future role in the electricity mix.

Destruction of Indigenous Homelands

Nothing could be more environmentally devastating than the loss of one's ancestral homeland. Looking at the issue from an environmental justice perspective, hydroelectric dams have long been seen among many indigenous people around the world as “a colonization of their land and their cultures,” since hydropower projects have often involved the involuntary displacement of indigenous people from their homelands. Protecting indigenous lands is not only a human rights concern, it's an environmental one, as indigenous people are caretakers of 80% of the world's biodiversity. As representatives to the COP26 summit in Glasgow, Scotland, testified, respecting the land rights of indigenous people is essential to preserving indigenous knowledge and indigenous practices of environmental management. Defending indigenous rights is central to, not separate from, environmental protection.

Barriers to Fish

Many migratory fish species swim up and down rivers to complete their life cycle. Anadromous fish, like salmon, shad, or Atlantic sturgeon, go upriver to spawn, and young fish swim downriver to reach the sea. Catadromous fish, like the American eel, live in the rivers until they swim out to the ocean to breed, and the young eels (elvers) come back to freshwater after they hatch. Dams obviously block the passage of these fish. Some dams are equipped with fish ladders or other devices to let them pass unharmed. The effectiveness of these structures is quite variable.

Changes in Flood Regime

Dams can buffer large, sudden volumes of water following spring melt of heavy rains. That can be a good thing for downstream communities (see Benefits below), but it also starves the river of a periodic influx of sediment and natural high flows that renew habitats for aquatic life. To recreate these ecological processes, authorities periodically release large volumes of water down the Colorado River, with positive effects on the native vegetation alongside the river.

Downstream Impacts

Depending on the design of the dam, water released downstream often comes from the deeper parts of the reservoir. That water is therefore much the same cold temperature throughout the year. This has negative impacts on aquatic life adapted to wide seasonal variations in water temperature. Similarly, dams trap nutrients coming from decomposing vegetation or nearby agricultural fields, reducing nutrient loads downstream and affecting both river and riparian ecosystems. Low oxygen levels in the released water can kill aquatic life downstream, but the problem can be mitigated by mixing air into the water at the outlet.

Mercury Pollution

Mercury is deposited on vegetation downwind from coal-burning power plants. When new reservoirs are created, the mercury found in the now submerged vegetation is released and converted by bacteria into methyl-mercury. This methyl-mercury becomes increasingly concentrated as it moves up the food chain (a process called biomagnification). Consumers of predatory fish, including humans, are then exposed to dangerous concentrations of the toxic compound. Downstream from massive Muskrat Falls dam in Labrador, for example, mercury levels are forcing indigenous Inuit communities to abandon traditional practices.

Evaporation

Reservoirs increase a river’s surface area, thus increasing the amount of water lost to evaporation. In hot, sunny regions, the losses are staggering: more water is lost from reservoir evaporation than is used for domestic consumption. When water evaporates, dissolved salts are left behind, increasing salinity levels downstream and harming aquatic life.

Threats From Climate Change

Increased evaporation also leaves reservoirs subject to dramatic climate change losses. Drought is a major factor in Earth's rising temperatures, as areas once blessed with rainfall adequate for hydroelectric power are increasingly faced with low dam levels and loss of electricity generation. In 2021, historic droughts across the Western United States dramatically lowered reservoir levels behind hydroelectric dams. In California, the Oroville Dam fell to just 24% of its normal capacity. Declining hydroelectricity has forced California utilities to increase natural gas generation, further exacerbating global warming.

Methane Emissions

The nutrients trapped behind hydroelectric dams are consumed by algae and microorganisms, which in turn release large amounts of methane, a powerful greenhouse gas. This is especially the case in newly built hydroelectric projects, as methane emissions decrease over the lifespan of a dam.

Benefits

The main benefit of the massive amounts of relatively reliable electricity that hydroelectric dams provide is that the electricity is both renewable and low in carbon emissions.

Clean(er) Renewable Electricity

Hydroelectricity is renewable, supplying 37% of all renewable electricity generation in the United States. Examining the entire life-cycle of hydroelectricity from dam construction to electricity consumption, hydropower produces roughly one-fifth the greenhouse gas emissions of fossil fuels. Hydropower may be seasonably variable, but it is far less intermittent than solar and wind power, and it is projected to play a significant role as a reliable source of clean, renewable energy into the foreseeable future.

Energy Independence

As part of a portfolio of energy sources, using hydroelectricity means a greater reliance on domestic energy, as opposed to fossil fuels mined overseas, in locations with less stringent environmental regulations.

Flood control

Reservoir levels can be lowered in anticipation of heavy rain or snow melt, buffering the communities downstream from dangerous river levels.

Recreation and Tourism

Large reservoirs are often used for recreational activities like fishing and boating. The largest dams also generate income for local communities through tourism.

The Future of Hydroelectricity

While the heyday of building large-scale hydroelectric dams dates to the 1930s and 1940s, hydropower is expanding in the developing world. The future of hydroelectricity will involve new construction, dam removals, upgrades, and the declining costs of even cleaner alternatives.

Dam Removal

More than half of the dams built before the 1970s in the United States are reaching or beyond the end of their 50-year expected lifespan, part of the country's decaying infrastructure. Dam decommissioning and removals have increased as the economic benefits of older dams wane while their environmental costs mount. Dam removals, though infrequent, have been habitat success stories, with rapid renewals of migratory fish stocks.

Re-purposing and Upgrading Existing Dams

Increasing the efficiency of existing hydroelectric dams and re-purposing existing non-hydro dams are two ways to expand hydroelectricity generation without increasing its environmental impact (though not decreasing it either). In a pilot program, the U.S. Department of Energy's Water Power Program increased the efficiency of three hydroelectric plants, adding more than 3,000 megawatt-hours per year to local electricity grids. Of the dams in the world today, no more than 10% are used for electricity generation. Re-purposing them to produce electricity could provide an additional estimated 9% of current global hydroelectric power.

Cleaner Alternatives

Evaluating the environmental impact of hydroelectricity involves not just comparing it to fossil fuels, but also to less impactful clean-energy alternatives to fossil fuels. No form of electricity production is without negative impacts, yet the greenhouse gas emissions from hydroelectricity are roughly ten times that of nuclear, solar, and wind power.

One recent study estimated that solar photovoltaic (PV) panels could produce the same amount of electricity as all 2,603 hydroelectric dams in the United States using roughly one-eighth of the existing reservoir area. Replace those dams with solar PV and 87% of the land would return to wildlife, while the remaining 13% could support solar electricity.