Chalillo hydroelectric project was first proposed in the early 1990s and later integrated into the Central American Electric Interconnection System (Sistema de Integración Eléctrica para América Central - SIEPAC); it is operating since end 2005, built by the Canadian company Fortis with Chinese participation. The project was the subject of a huge campaign in the late 1990s and early 2000s by environmentalist groups in Belize and elsewhere who were concerned about the project's impacts on the Macal River. Since the first feasibility study there have been warnings of the heavy environmental damage, also due to the peculiarity of the Maya Mountains. Only the head pond behind this dam will inundate 1,000 hectares of riverine habitat, which is, according to some scientists, the only habitat of its type in all of Central America. The dam would further threaten the endangered Scarlet Macaw and other rare species. The dam would fracture the Mesoamerican Wildlife Corridor, a rain forest tract stretching from Mexico to Panama, established to protect migration routes and breeding grounds for wild cats, migratory birds, and other animals. National Geographic News quote Sharon Matola, founder of the Belize Zoo and a principal of a lawsuit against the dam, the first environmental lawsuit of Belize, who said: "This is the cradle for biodiversity in Central America, and arguably the wildest place left in the region. Trading off millions of years of biological evolution for a hydro scheme which, at best, would last 50 years, is an environmental crime of the highest degree." [1] There are also many unexcavated Mesoamerican ruins in the surrounding jungle.

The project is proposed by Belize Electric Company Limited, BECOL, which is 100% owned by Canadian company Fortis, who claimed the project would have minimal impacts on the environment and that it is needed to feed an existing dam during the dry season and to ease dependence on Mexico, which supplies one third of the nation's electricity. Belizean government privatized the energy sector, which allowed Fortis to own both the national distribution company (Belize Electricity Limited, BEL) and the generation company BECOL. Fortis controls this way up to 48% of the electricity sold in Belize, while the rest comes from interconnections with Mexico.  Probe International has criticized the company denouncing that it "continues to be one of the country's highest-earning companies, bringing in more than $186-million revenue last year.! [3], as Belizeans pay US$0.19 per kilowatt-hour, which is more than twice the average price in other Central American countries. "Meanwhile, the Public Utilities Commission (Belize’s regulate public utilities regulator) has reminded BEL that it still owes the country’s power consumers over $40 million in excess earnings realized from unadjusted fuel costs." [3]. Fortis already operates another dam in Belize, the Mollejon. According to an article on The Independent, "When it opened 10 years ago the company claimed it would supply more than enough electricity to meet the growing demands of the 250,000-strong Belizean population without the need for any further construction. While most people agree that the country's electricity needs must be met, those opposed to the new dam say it will operate for just 50 years. They want the government to support the use of alternative, sustainable energy, such as the use of bagasse, a byproduct of the sugar manufacturing process which was once a major industry in Belize, or to buy in power from neighbouring countries, which could cost less over the long term.”

Environmentalists say that the cost of the Chalillo project is too high—environmentally and financially. Fortis's contract with the Belizean government guarantees a 15 to 20 percent profit per year and doesn't even require the company to produce energy. According to Ari Hershowitz, director of Natural Resources Defence Council's Biogems Program, Fortis's geological studies state that the site is granite, when it's really sandstone and shale. "The worst case scenario: The dam breaks, floods communities downstream and kills people." Fortis's contract guarantees that they can sell the dam to the government for $1 without liability, he adds.  Activists took the Belizean government to court numerous times, accusing it of failing to ensure Fortis followed an environmental compliance plan for the Chalillo dam, as required by law. They noted that an earlier court case brought by Belize-based conservation groups prior to the dam’s construction brought to light a secret pact between the government and Fortis that granted the company the rights to build the dam tax-free and without having to obey environmental rules. Fortis was also guaranteed annual rate increases of at least 1.5 percent until 2036. An alliance of environmental justice groups in Belize, the Alliance of Cnservation NGOs (BACONGO) filed the first environmental lawsuit to reach the highest court of appeal in Belize, the Privy Council in London, as BECOL did not have due permission to continue with Chalillo project and lacked  authorization also for the Mollejón dam. Again according to The Independent, "Fortis commissioned an environmental impact study from Amec, the British construction group, which used scientists from the Natural History Museum in London to carry out part of the assessment. But when the scientists concluded that much more work was needed in the region before the dam could proceed, their recommendations were buried in an annexe of the final 1,500-page report.”

In 2009, local inhabitants and environmentalists published alarming pictures of dirty waters coming out of the dam; the water turbidity in the photos was the result of both organic and inorganic dissolved and suspended material in the reservoir [4]. International Rivers quoted the geologist Jean Cormec, who observed that "the problem is really the geological make-up of the Maya Mountains, which are made of materials that during the normal tropical weathering process break down into very large amounts of clays, silt, sand and pebbles." [5] BECOL tried to blame Guatemalan settlements and Xateros for the deforestation in the watershed for making the waters so turbid and threatening therefore drinking water supply for the country.    Candy Gonzalez, president of the Belize Institute of Environmental Law and Policy (BELPO), said that many years after the dam was built, “we still can’t get information about water quality from the company or information on levels of mercury in the fish,” she said, noting that the river has been polluted with sediment. Cracks in the dam have been reported, putting over 16,000 people who live downstream at risk, but there is no emergency plan in case the dam breaks.

The dam itself is 30 meters high (about 150 feet at it's highest point) and will produce a maximum of 5 Megawatts fixed energy, both from Chalillo and by adding storage to the Mollejon Hydroelectric Plant where the water will be stored and converted to energy. It is considered a run-of-river scheme.

In the beginning construction of the project was estimated to cost US$30 million. Though Fortis/BECOL have never released the actual figures, the numbers recently presented at the November commissioning ceremony were in the neighborhood of US$100 million, over three times the initial estimates.