Hydropower produces 16% of the world’s electricity and plays a strong role in satisfying the national energy needs of certain countries. Paraguay produces all of its electricity from hydropower, while exporting most of its excess supply to neighboring Brazil and Argentina. Albania, Bhutan and Lesotho also depend exclusively on hydroelectricity. Norway produces more than 98% of its electricity from hydropower, Brazil draws upon it for 85%, Iceland for 80%, Venezuela for 69%, Colombia for 65% and Canada for 61%. Austria (60%), Switzerland (56%) and New Zealand (53%) are other countries that generate the majority of their electricity from hydro sources. (The U.S. draws only 6% of its electricity from hydropower.)

In overall terms, China is the largest hydroelectricity producer, generating 721 terrawatt-hours (TWh) in 2010, which represented 17% of its electricity consumed. Canada ranks second, at 369 TWh in 2009, and Brazil is third, at 363 TWh, with the U.S. fourth at 250 TWh and Russia fifth at 167 TWh.

But hydropower is a resource governed by geographic restraints. “You need water and a change in elevation. That cuts out a lot of the world right away,” says Chris Bergesen, Editorial Director, UDI Products, Platts, a unit of McGraw Hill Financial, of which ENR is a part.

“Hydropower development is taking place disproportionately in developing countries, because the U.S., Western Europe and Japan are built out, essentially,” says Bergesen. The six nations that are advancing hydropower projects most ambitiously are China (82,492 MW worth of projects currently in the pipeline), Brazil (20,562 MW), India (15,328), Turkey (7,904 MW), Ethiopia (6,874 MW) and Pakistan (6,143 MW), according to Platts UDI World Electric Power Plants Data Base.

The largest-capacity hydropower plant being built in the world is the Xiluodu Dam, on the Jinsha River (the upper Yangtze River) in southwest China’s Yunnan Province. The 285-m-tall, 700-m-long concrete arch dam will contain eighteen 770-MW turbines, for a total capacity of 13,860 MW. The first turbine was commissioned in July 2013, with the remainder expected to go online later in 2014.

Brazil has one of the most robust dam-building programs of any country. The largest dam currently under construction there, Belo Monte, on the Xingu River in northeastern Brazil, is a complex of three dams, numerous dikes and a set of canals that will supply water to two power stations. The planned capacity of 11,233 MW will rank it as the third or fourth largest-capacity hydropower plant in the world when it is completed. It is being built by the Belo Monte Construction Consortium, at an estimated cost of $14.4 billion. CCBM is led by contractor Andrade Gutierrez, along with Odebrecht and Camargo Corea, and seven other smaller contractors. Construction began in 2011, but has been halted several times by courts reacting to environmental lawsuits and legal actions by indigenous peoples. The current workforce is 28,000. Most of its turbines are expected to begin producing power in 2015.

The Jirau Dam is taking shape on the Madeira River, in western Brazil near the border of Bolivia. It is a 1,500-m-long, 63-m-high rock-fill dam with an asphalt-concrete core, which will produce 3,750 MW. It will supply electricity to São Paulo, 1,400 miles away. Camargo Correa is the primary contractor, with 20,000 workers currently engaged. It is expected to be completed in 2015, at a cost of $8 billion.

Ethiopia’s current dam-building effort will lift its generating capacity from approximately 2,000 MW at present to 10,000 MW within the next three to five years.

The Gilgel Gibe III Dam, under construction on the Omo River by Salini Costruttori of Italy, will be the largest hydroelectric plant in Africa when completed, with a capacity of 1,870 MW. It is 75% finished, with no completion date announced.

The Grand Ethiopian Renaissance Dam, currently being built on the Blue Nile near the Sudan border, also by Salini Costruttori, will be the largest hydropower plant in Africa upon its completion in 2017, leapfrogging over Gilgel Gibe III. A roller-compacted concrete dam measuring 170 m tall and 1,800 m long, it will have powerhouses on each side of the spillway and will produce 6,000 MW. Much of the power generated by Ethiopia’s new dams is likely to be sold to its neighbors, including Kenya, Sudan, Djibouti and possibly Egypt. Its construction cost is $4.8 billion.

The Grand Ethiopian Renaissance Dam has engendered political tensions with downstream neighbors Sudan and Egypt. Egypt in particular fears that a temporary reduction of water flow in the Nile during the filling of the dam may hurt its farmers. Water loss via evaporation from the dam’s reservoir is another concern. It may also permanently lower the water level in Lake Nasser, thereby reducing the generating capacity of Egypt’s Aswan Dam. Egypt, Ethiopia and Sudan have established the International Panel of Experts to study the impact of the dam and make recommendations for cooperative approaches.

The benefits of large dams are well known: irrigation, flood control, navigation improvements and power production. But they also engender a wide range of environmental consequences. They degrade water quality and block sediment flowing downstream, harming ecosystems and agriculture. They block fish migration. They flood fertile river-valley farmland and sometimes displace large numbers of residents. In tropical regions shallow reservoirs submerge large quantities of plant life, which generates large volumes of methane, a potent greenhouse gas. “Tucurui Dam [in the Amazon basin], by flooding a rainforest, triggered a new scientific discipline,” says Bergesen.