Guri Dam, officially the Simón Bolívar Hydroelectric Plant (Central Hidroeléctrica Simón Bolívar), is a concrete gravity and embankment dam on the Caroní River in Bolívar State, eastern Venezuela, about 100 km upstream from where the Caroní joins the Orinoco. It is the plant that powers most of Venezuela. Its wall is 162 m high and 7,426 m long, it holds back the 4,250 km² Guri Reservoir, and its two powerhouses carry an installed capacity of roughly 10,000 MW (10,235 MW in Wikipedia's list of the world's largest hydroelectric stations), making it the largest power station on Earth until Itaipu on the Brazil–Paraguay border overtook it. Built in two stages between 1963 and 1986, it was fully inaugurated on 8 November 1986. Because the national grid leans so heavily on this one plant, its failures have repeatedly plunged the country into darkness.
The Guri Reservoir: 4,250 km² of Caroní Blackwater
The Caroní runs 952 km from the Kukenan tepui on the Gran Sabana to its confluence with the Orinoco, making it the longest river that lies entirely inside Venezuela. Its basin covers about 95,000 km² of the Guiana Highlands, and for a basin that size its flow is exceptionally high: an average of 4,850 cubic metres per second, rising to an average seasonal maximum of 6,260 m³/s and with a recorded extreme of 17,576 m³/s. The upper Caroní and its tributary the Paragua fall in a staircase of rapids and waterfalls, including Angel Falls on a tributary in the Canaima region, so the river arrives at the dam site with both volume and height to spend.
The water itself is dark. Humic acids leached from decomposing forest vegetation stain the Caroní the colour of strong tea, placing it among the world's blackwater rivers alongside Brazil's Rio Negro. When the first-stage dam closed the Necuima Canyon in the late 1960s, the lake behind it held its surface at 215 m above sea level, already the largest body of fresh water in Venezuela and one of the largest artificial blackwater lakes ever created. The second construction stage raised the operating level to 272 m. At that height the reservoir spreads over 4,250 km², an area larger than the US state of Rhode Island, and stores about 138 billion cubic metres, a volume that ranks it among the ten largest reservoirs anywhere.
That storage is the plant's real asset. The Guiana Highlands have a pronounced wet season from May to October, and the reservoir lets operators bank the flood months' water and release it steadily through the dry season. The cost of flooding the valley was paid by the forest and its wildlife: in 1968 the Venezuelan zoologist Pedro Trebbau led Operación Rescate, an expedition that rescued about 10,000 animals of 53 species from land that the rising water was about to drown.
Necuima Canyon Wall: Concrete Gravity Core and Earth-Fill Wings
The site was chosen after feasibility studies begun in 1961 by the US-based Harza Engineering Company. An international consortium of six firms, four of them American companies taking part under the Alliance for Progress, won the construction contract, and excavation in the Necuima Canyon began in 1963. By 1969 the first dam stood 106 m high and 690 m long.
The second stage, launched in 1976 because Venezuela's demand for power was growing so fast, transformed that structure. Engineers built a new concrete gravity dam about 1,300 m long and raised the old gravity section and spillway to the new crest. A gravity dam works by sheer mass: the concrete is heavy enough that the reservoir's push cannot slide or tip it, so the central section had to grow in thickness as well as height, reaching the 162 m that the dam stands today. A tall concrete wall alone could not close the whole valley, however. On both banks the builders added earth and rockfill embankments and a series of closing dikes, so that the full crest, from one end to the other, runs to 7,426 m (some sources give as much as 11,409 m by counting every dike). Only the central concrete block, where the spillway and powerhouses sit, needed the strength of mass concrete; the long flanking wings simply had to be watertight barriers across lower ground.
The Three-Channel Spillway and the May–October Floods
A reservoir that rises every wet season needs a safe way to shed what the turbines cannot use. Guri's answer is a spillway built into the concrete gravity section and divided into three channels, which releases surplus water during the rainy months from May to October. The second construction stage raised the spillway to the new 272 m crest level along with the gravity dam, and also excavated a second discharge channel below the plant.
When the gates open, water falling from a reservoir held behind a wall up to 162 m high turns the release into a white plume of spray that rolls down the gorge below the dam. Taming that energy matters as much as generating it, because every cubic metre that leaves through the spillway still has to pass the smaller plants downstream on its way to the Orinoco.
Powerhouse I and Powerhouse II: Twenty Turbines on the Caroní
The electricity comes from two separate powerhouses. Powerhouse I belongs to the first stage: when it went into service the plant had a combined capacity of 1,750 MW, and by 1978 ten generating units together delivered 2,065 MW. Powerhouse II, built at the foot of the new gravity dam on the right bank during the second stage, added ten far larger units, rated at about 725 to 730 MW each depending on the source. Hitachi was selected to make the turbines, Westinghouse the generators and Alstom the power transformers, according to the Spanish-language Wikipedia account of the project.
Water from the reservoir drops through penstocks to the turbines, and the head, the difference between the lake surface at up to 272 m above sea level and the river below, sets how much energy each cubic metre carries. Raising the lake from 215 m to 272 m in the second stage did more than add storage: it gave every unit in both powerhouses a greater head to work with. Twenty units fed by roughly 4,850 m³/s of average river flow are what put Guri in the 10,000 MW class.
Time has worn that machinery. A refurbishment programme running since 2000 set out to extend the plant's working life by 30 years with new runners and main components for Powerhouse II, and work on four units in Powerhouse I began around the end of 2007. Its progress has been uneven: a March 2020 report cited by Spanish-language Wikipedia said only 13 of the 20 turbines were working, and in August 2021 former deputy energy minister Víctor Poleo said only nine were running.
Macagua, Caruachi and Tocoma: The Lower Caroní Cascade
Water leaving Guri is used again on its way to the Orinoco. Downstream sit the other plants of the lower Caroní scheme, each placed to catch the river's remaining fall:
- Caruachi Dam, about 59 km below Guri, a concrete gravity dam whose twelve Kaplan units, the last commissioned in February 2006, give 2,160 MW and about 12 terawatt-hours a year.
- Macagua Dam (Antonio José de Sucre), 81 km below Guri and 10 km above the Orinoco confluence inside Ciudad Guayana. It is 69 m high and 3,537 m long and feeds three power stations with 3,167.5 MW between them; Macagua I, built from 1956 to 1961, is older than Guri itself.
- Tocoma Dam (Manuel Piar), between Guri and Caruachi, begun in 2006 for 2,320 MW. Its reservoir began filling on 16 November 2015, its budget had tripled to US$9.4 billion by 2017, and as of 2019 it was still unfinished.
Because Guri's reservoir regulates the whole river, it is the cascade's storage tank as well as its biggest generator. The flow it releases decides how much the plants below can produce, which is why the scheme was planned as a single system run by the state company CVG Electrificación del Caroní (EDELCA), now part of Corpoelec.
Ciudad Guayana's Smelters and the Lines to Caracas and Boa Vista
Cheap hydroelectricity reshaped Bolívar State. Ciudad Guayana, founded in 1961 by merging San Félix and Puerto Ordaz where the Caroní meets the Orinoco, grew around power-hungry iron, steel and aluminium industries and reached a metropolitan population of about one million. Long-distance transmission lines carry Guri's output hundreds of kilometres north-west to Caracas and the industrial centre of the country, and until September 2018 part of it was also sold to Brazil over the Guri–Boa Vista line into Roraima state.
The plant's name reflects that political weight. For decades it honoured Raúl Leoni, president of Venezuela from 1964 to 1969 and a native of El Manteco in Bolívar State, before it was renamed for Simón Bolívar. Venezuelan policy from the 1960s onward favoured hydropower so that more oil could be exported, and in 2021 hydroelectricity still supplied 64% of the country's power, with Guri alone accounting for 64% of the hydro capacity.
Drought, Rationing and the 7 March 2019 Blackout
Dependence on one reservoir is a risk whenever the rains fail. In January 2010 a prolonged drought pulled Guri's level so low that the government imposed rolling blackouts. In April 2016 low water forced cuts of four hours a day for 40 days; state employees were told to stay home on Fridays and three days were added to the Easter holiday to shut down public services and businesses for a week.
The worst failure came without a drought. Shortly before 5 pm on 7 March 2019 the plant tripped, leaving most of Venezuela's roughly 32 million people without power. Repeated attempts to restart the San Gerónimo B substation, which distributes about 80% of the country's electricity, failed, and power was not restored to much of the country until 14 March; at least 43 deaths were attributed to the outage. The government called it sabotage, while engineers and former energy officials blamed years of deferred maintenance and the loss of skilled staff. Estimates of Guri's share of national supply range from more than a third in 2006 to 70–80% in accounts of the 2019 crisis, and either way the lesson was the same: when the Necuima Canyon plant stops, Venezuela stops. For the dam that took Guri's world title, see our guide to Itaipu Dam; for the river the Caroní feeds, read about the Orinoco River, and for the highlands that supply its water, follow the Caroní's tributaries up to Angel Falls.