Alto Maipo is a large hydroelectric project owned by the American company AES Gener S.A., and it’s located in the southeast of the city of Santiago, in San José de Maipo commune, in the upper basin of the Maipo river.

The purpose of the project is to produce electricity through the construction and operation of two run-of-river hydropower plants, the Alfalfal II and Las Lajas power plants, which together will produce a maximum power of 531 MW and deliver the energy to the Sistema Interconectado Central (SIC) [Interconnected Central System]. This would be done by using the water from the upper area of the Volcán river, the Yeso river, the intermediate basin of the Colorado river, and the turbinated water of the Alfalfal power plant.

The foreign megaproject has been trying to establish in Chile since 2007 with fierce opposition of the inhabitants of the Metropolitana region and the entire country, as it’s a business initiative that hasn’t been exempt from a series of events that have damaged and marked a before and an after in the history of the Maipo river.

Part of the construction of the Austrian construction company Strabag, near the Yeso reservoir / © Paulina Jimenez.


This report will let the readers know about the extensive history of the Alto Maipo hydroelectric project of more than 13 years to the present; taking into consideration the most relevant milestones shown in chronological order for a better understanding.

  • 2007: 
  • June: Mr. Derek Edward Martin, representing AES S.A., submits the first Estudio de Impacto Ambiental (EIA) [Environmental Impact Study] of the Proyecto Hidroeléctrico Alto Maipo (PHAM) [Alto Maipo hydroelectric project] to the Sistema de Evaluación de Impacto Ambiental (SEIA) [Environmental Impact Assessment System], which is accepted for processing.
  • October: AES Gener sends a letter to the SEIA to cease the project’s evaluation, which is approved by the SEIA the same day.


  • 2008: 
  • May: AES Gener requests to remove the PHAM from SEIA, which is approved the same day. The Sociedad Alto Maipo SpA introduces a new Estudio de Impacto Ambiental for the PHAM Exp. N° 105.
  • June: SEIA sends the EIA of the PHAM to the San José de Maipo city hall to allow the citizens to make relevant questions and comments.

  • 2009:
  • March 30: The Comisión de Medioambiente [Environmental Commission] of the Metropolitana region approves the Estudio de Impacto Ambiental from the PHAM (EX. RES. N°256-2009).
  • December: The Comisión de Recursos Natualares [Natural Resources Commission] of the Deputies Chamber concludes that the PHAM shouldn’t have been approved by the Comisión Regional del Medio Ambiente (COREMA) [Regional Metropolitan Environmental Commission] of the Metropolitana region under the conditions it was presented because of the several irregularities in the project itself. They point out that the PHAM could impact the safety and quality of the drinking water supply of Santiago, the sedimentology of the natural resources of the area, the irreversibly deterioration of Maipo river basin, the destruction of the Volcán, Yeso, and Colorado sub-basins and affect the El Morado National Monument.


  • 2011: 
  • June 6: A secret agreement is signed between AES Gener and Aguas Andinas (AA) [water supply and sanitation Chilean company] in which the use of water for human consumption is promised to generate electricity for 40 years, thus disposing of 70 % of the Metropolitana region’s drinking water. In compensation for the AA’s duties, a monthly payment is established by the hydroelectric company to the sanitation company to use water rights from the basin of Negra lake, Lo Encañado lake, and El Yeso reservoir, which supplies the Maipo river basin. In the agreement, AES Gener is obliged to conduct the collected water through the tunnel system considered in the PHAM and return it through the same flow to the Maipo river in front of the Las Lajas substation, upstream of the Aguas Andinas independent water inlet or, eventually, in the Colorado river in El Alfalfal.

It should be noted that the water that AA is pledged to deliver to AES Gener (a flow of 2.5 m3/sec of water) that comes from Negra and Lo Encañado lake is exceptionally pure, clean, and crystalline, which represent the most valuable water resource of the AA concession, and even more, of the entire basin of the Maipo river.


  • 2012:
  • February: AES Gener begins the construction of the deforestation works of the PHAM.
  • August: AES Gener closes contracts of building works for the PHAM.

The tenders favored Voith Hydro, which provides the supply, assambly, and generation equipment of the project; Strabag, which executes all the civil and underground works that are developed in the Colorado river valley; and Hochtief/CMC, which does the same but in the Yeso and Volcán river valleys.

 The Coordinadora No Alto Maipo calls for a multicultural march against the PHAM.

  • November: The Austrian construction group Strabag, based in Vienna, was awarded a first partial contract for the construction of the hydroelectric power plant.


  • 2013:
  • January 28: A unanimous decision of the fifth chamber of the Court of Appeals of Santiago allows the community to partially access the contract signed in 2011 between AES Gener and AA, as its content is public interest. Initially, access to the information regarding the contract was denied by the Superintendencia de Servicios Sanitarios [Sanitary Services Superintendency]. This case was received by the Consejo para la Transparencia[Council for Transparency], which also denied access, and then the case went to the Court of Appeals.
  • March: A series of civil organizations call for a national march to protect the water due to the water shutoffs that occurred in January and February of that year.
  • June: The Coordinadora Ciudadana No Alto Maipo, along with senators and city councilors, requested the Superintendencia del Medioambiente (SMA) [Environmental Superintendency] to suspend the Resolución de Calificación Ambiental [Environmental Assessment Resolution] N°256/2009 of the project (RCA N°256/2009).
  • July: 
  1. Antofagasta Minerals (Luksic) incorporates into the partnership that develops PHAM. Two contracts regarding electricity were signed in which 40 % of the project goes to the Luksic family, and 60 % goes to AES Gener. Under the condition that for 20 years all the energy generated goes to the Los Pelambres mine based in the Choapa valley, fourth region.

  2. SEIA fines AES approximately CLP 40,000,000 for using other people’s water rights, cutting down endangered species by law, and performing unapproved works by the RCA N°256/2009.

  • August: Around 5,000 individuals come together at La Obra park, located at the entrance of the Cajón del Maipo commune, in protest of the construction of the PHAM.
  • November: The video “Voces por los ríos libres – No Alto Maipo” [Voices for free rivers – No Alto Maipo] is spread, with the participation of actors, social leaders, and activists, who call on citizens to oppose the PHAM.
  • December: The construction of the PHAM begins.


Alto Maipo tunnels, Valle Engorda, Maipo river basin / © Hans Fernández.


  • 2014:
  • February: The Coordinadora Ciudadana Ríos del Maipo files a petition for annulment against The Dirección General de Aguas (DGA) [General Water Authority] to the Court of Appeals for the permissions given by the institution to AES Gener related to Alto Maipo.
  • June: Remains of an indigenous cemetery are found in the Los Maitenes sector in the excavation works of a tunnel for the PHAM. Such remains belong to the prehispanic culture of the Chiquillanes.


  • 2015:
  • September:
  1. The maximum levels allowed of the parameters of aluminum, manganese, and suspended solids are exceeded by the industrial liquid waste treatment plant, VL7-VL8, of the PHAM.

  2. The Coordinadora No Alto Maipo starts an international campaign to stop the construction of PHAM, represented by Juan Pablo Orrego, Director of the NGO Ecosistemas, with a delegation to appear before the Inter-American Development Bank and others who financed the project, in Washington D.C.

  • December 5: The large family march No alto Maipo is done peacefully in Santiago, attended by over 30,000 individuals.


  • 2016:
  • January:
  • The wastewater discharges out of authorized season triggered the SMA to decree temporary measures. Charge N° 13 of the penalizing procedure.

  1. The Red Metropolitana No Alto Maipo presents two new complaints before the SMA to stop or revoke the construction permits for the project.

  2. The toxicological team of the Colegio Médico [Medical Association of Chile] and Dr. Andrei Tchernitchinda made public the study “Informe aguas del Cajón del Maipo y Alto Maipo – muestras CMA” [Report on waters of Cajón del Maipo and Alto Maipo – MCA samples] which reported high concentrations of toxic elements in the water near the tunnel excavation works that belong to PHAM.

  • February: The Comptroller General Office ordered to check the legality of the agreement between Aguas Andinasand AES Gener.


  • May 12: The Court of Appeals of San Miguel accepts protection measures against Alto Maipo SpA and the Comisión de Evaluación Ambiental [Environmental Assessment Commission] of the Metropolitana.
  • June 28: Alto Maipo evacuated the workers’ camp in Aucayes due to the risk of landslide and mountain collapse.


  • July: A complaint is filed by CONAF [National Forest Corporation] against an illegal quarry named “Predio Piedras Coloradas” before the Local Police Court. From this quarry, closed for operating without a municipal license, Alto Maipo SpA obtained a big part of the necessary rock fill for their construction sites. The extraction of such material caused the loss of a large part of a small population of cypresses of the mountain range and other Andean trees.


  • 2017:
  • January 19: Having a 40 % progress of the project, the Los Pelambres mine owned by the Luksic group was no longer a minority partner. The reasons were cost overrun, energy rate changes, and problems with the owner AES Gener.
  • January 20: SMA opens a punitive administrative proceeding against Alto Maipo Spa, filing 14 charges for breaching obligations stated in the RCA N°256/2009. Nine charges were classified as serious, according to Article 36 N°2 letter e) and N°3 of the LOSMA [Organic Law of the Environmental Superintendency] (EX. RES. N°1/ROL D-001-2017).
  • February 16: The company presented its compliance program before the SMA, which was subject to many comments by the supervisory body.
  • July 6: Alto Maipo SpA presented a consolidated, coordinated, and systematized compliance program, which also was subject to many comments by the SMA.
  • August 21: There was an emergency because of water infiltrations in the section of the tunnel L1 of Las Lajas, located in the hydrogeological sector of El Manzano. The infiltration flow in this tunnel is considerably higher than its natural recharge capacity, which exceeds 26 l/s, with peaks of 70 l/s, being almost 3 times its natural recharge process, producing a meaningful environmental impact. Information contained in EX. RES. N°528 of the DGA of the Metropolitana region from April 10, 2018.
  • November 23: The city councilor Maite Birke filed a complaint against Alto Maipo due to wastewater discharges out of authorized season onto Maipo river (RCA N°256/2009) that took place on August 21.
  • December 13: The complaint is filed again before the DGA.


Alto Maipo tunnel route, in the San José de Maipo basin.

  • 2018: 
  • January 6: Alto Maipo SpA presents a third version of its consolidated, coordinated, and systematized compliance program.
  • January 8: SMA’s inspector officers visit Las Lajas 1 and confirm that the emerging water volume was higher than the aquifer natural recharge, which was stated in the Exempt Resolution of the DGA N°528.
  • March 26: Alto Maipo SpA gives SMA a fourth version of the consolidated, coordinated, and systematized compliance program and its annexes.
  • April 6: SMA approves the Compliance Program of Alto Maipo, which is an option given by the supervisor to the owners to amend their irregularities, but it implies the suspension of the sanctioning process (EX. RES. N°29-2018/ ROL D-001-2017).
  • 3
  • May 3: The city councilor Maite Birke filed before the Segundo Tribunal Ambiental [Second Environmental Court] of Santiago a claim action against the SMA, looking for the specialized judiciary to declare the resolution that has approved the compliance program of Alto Maipo as illegal and to reject that plan. The case was admitted with the role R-183-2018, and then two cases were added, R-184-2018 (filed by the Coordinador Ciudadano No Alto Maipo) and R-185-2018 (filed by Red Metropolitana No Alto Maipo).
  • August: Alto Maipo receives the Environmental Prize “Energy Globe” in Austria for contributing to the construction design and the executed employability policy.


  • 2019:
  • January: The Segundo Tribunal Ambiental orders Alto Maipo to ask the Servicio de Evaluación Ambiental (SEA) [Environmental Assessment Service] for the revision of the RCA N°256/2009, which is an innovative interim equitable relief that seeks to clarify whether there is any meaningful change in the variables evaluated in the environmental impact study of the project. This was decreed after analyzing and rejecting the request of the city councilor of San José de Maipo, Maite Birke, to stop the construction’s works.
  • November: Alto Maipo achieves 82 % of the construction progress with 59 kilometers of excavated tunnels and only 6.5 kilometers left to excavate.
  • December: The Coordinadora Ciudadana No Alto Maipo and Chile Sustentable file a civil nullity complaint for the agreement entered into with an illegal purpose by the company AES Gener and AA in 2011.


  • 2020:
  • June 25: The Segundo Tribunal Ambiental conducts the hearing, with lawyers Álvaro Toro on behalf of María Martínez and others, Alejandra Donoso on behalf of Maite Birke, Macarena Soler on behalf of Marcela Mella (Coordinadora Ciudadana No Alto Maipo), Benjamin Muhr on behalf of SMA, and Mario Galindo on behalf of Alto Maipo SpA.
  • August: Alto Maipo achieves 90 % of the construction progress and only 3.4 kilometers of Las Lajas discharge tunnel excavation are left.
  • October 7: More than 70 civil society organizations, both national and international, send a letter to AA to finish the agreement entered with AES Gener for endangering the water of Santiago.
  • October 16: The Segundo Tribunal Ambiental conducted a personal inspection at the area where the PHAM takes place under the judicial claim R-183-2018. The procedure is carried out by the Ministers Cristian Delpiano Lira (President of the Court), Alejandro Ruiz Fabres, and the relator Ricardo Pérez Guzmán, who served as certifying officer. The evidentiary procedure is performed with the entrance of the Ministers to the tunnel and the other facilities and premises of the Las Lajas L1 sector.
  • October 23: The Court resumed the personal inspection with the presence of the President of the Court, Cristian Delpiano Lira, who visited the El Yeso water inlet, the discharge of the emerged waters of the denominated area V5, Las Vegas EY1, and the tunnel located on the V1 sector, finishing the evidentiary procedure.


“The Alto Maipo hydroelectric project will produce desertification of the aforementioned area, speeding up the melting of glaciers and snow and impacting natural areas and national monuments.” Nicolás Leblanc, Lawyer of the Fundación Glaciares Chilenos.


The Maipo river is the primary drinking water supplier of the Metropolitana region, providing water resources to the entire Maipo valley and contributing exponentially to the local economy.

Nowadays, the central area of Chile is facing a shortage of water because of climate variability, the decrease in rainfall, the increase in temperatures, and the decrease of the Maipo’s flow.

The above has not been responsibly considered by the PHAM, which has presented irregularities in its EIA from the beginning and has caused the creation of an Investigation Commission in the Chamber of Deputies that notifies the anomalies of the project.

The PHAM directly infringes on the safety of the drinking water reserves and threatens the continuity of water flow for the irrigation systems of more than 120,000 hectares of the Maipo valley.

From a touristic point of view, the PHAM also creates uncertainty in that area since it degrades an important recreation area for the Metropolitana region that receives over 2,000,000 tourists annually, which critically threatens the source of work in this field.

The 70 km private tunnel, owned by AES Gener, intercepts underground aquifers, and it also drains and alters the course of the flowing water, which causes the drying of slopes and estuaries.

The PHAM will produce the desertification of the area mentioned before, speeding up the melting of glaciers and snow and impacting natural areas and national monuments.

This is a poorly designed project with unforeseen or evaluated impacts by the environmental institution, which is an accomplice of the conflict. They have created an atmosphere of great opposition, including water channel owners associations, federations, citizen assemblies, and other social organizations.

Seventy civil society organizations encouraged Aguas Andinas to conduct the termination clause of the agreement with AES Gener since the sanitary company is entitled to do so, as Alto Maipo once again presented a delay in its entry of operation until the end of 2021. The sanitary company finished the agreement in 2019 but signed an extension with AES Gener for USD 10 million, which maintains the agreement valid.

In addition, dozens of complaints have been piling up in the Subsecretaría del Medioambiente [Undersecretary of Environment] that have not been successfully resolved, which causes a long list of unfulfillments and inconsistencies.

The battle on the part of the citizens has extended for over a decade now. The resistance to the project has been notorious. Undoubtedly, 2021 will be decisive for the future of the project in our country. With an environmental institution focused on the lobby and the foreign economic progress that has been complicit in violating the human rights to the water and health of its population, this is a common course of action for our country when we review the Chilean history of non-compliance with international obligations.



  • Cristian Donoso. (2018). [Article] Link
  • Patricia San Juan. (2020). [A piece of news] Link
  • Coordinadora por la Defensa del Agua y la Vida. (2016). [A piece of news] Link
  • Coordinadora por la Defensa del Agua y la Vida. (2018). [A piece of news] Link
  • Coordinadora por la Defensa del Agua y la Vida. (2018). [A piece of news] Link
  • Coordinadora por la Defensa del Agua y la Vida. (2019). [A piece of news] Link
  • Jessica Esturillo. (2019). [Article] Link
  • Felipe Delgado. (2020). [A piece of news] Link
  • Carolina Mardones Labraña. (2017). [A piece of news] Link
  • Rodrigo Alarcón L. (2013). [A piece of news] Link
  • Paula Correa. (2013). [A piece of news] Link
  • Paula Correa. (2013). [A piece of news] Link
  • Diario Uchile. (2013). [A piece of news] Link
  • Paula Correa. (2013). [A piece of news] Link
  • Héctor Areyuna. (2014). [A piece of news] Link
  • Raúl Martínez. (2016). [A piece of news] Link
  • Natalia Figueroa. (2016). [A piece of news] Link
  • Segundo Tribunal Ambiental. (2019). [A piece of news] Link
  • Segundo Tribunal Ambiental. (2020). [A piece of news] Link
  • Segundo Tribunal Ambiental. Role R-183-2018. Link
  • Segundo Tribunal Ambiental. Sentence R-183-2018. Link
  • Court of Appeals of San Miguel, First Courtroom. Rol N° 512-2016 PROT. 12-05-2016. [A piece of news] Link
  • Comisión Regional del Medioambiente of the Metropolitana region. Exempt Resolution N° 256/2009. Link
  • Felipe Larrain Aspillaga and Vicente Javier Giorgio. (2011). Agreement between AES Gener and Aguas Andinas. Link


Featured image:
  • Mesón Alto Glacier, Cajón del Maipo, Metropolitana region, Chile / © Marco Aguilar. Location link