The destruction of Turkey’s 12,000-year-old Hasankeyf settlement and ancient citadel has moved a step closer as authorities have begun to collapse cliff faces around the ruins of the citadel.

The move, linked to the construction of a highly controversial dam about 50 miles downstream, is also expected to damage the rich ecosystem of the Tigris river basin.

Local authorities have announced that the rocks were broken off “for safety reasons” and that 210 caves – a fraction of thousands of manmade caves in the area – would be filled before the town’s inundation in order to prevent erosion.

The Ilisu dam, part of the Southeast Anatolian project (Gap) and one of Turkey’s largest hydroelectric projects to date, has been mired in controversy ever since it was first drafted in 1954. The dam will raise the level of the Tigris at Hasankeyf by 60m, submerging 80% of the ancient city and numerous surrounding villages, including more than 300 historical sites that have still not been explored.

Environmental engineer Ercan Ayboga of the Initiative to Keep Hasankeyf Alive warns that close to 80,000 people will be displaced. Many of them will lose their land and their livelihoods. Because of additional debts taken up to purchase new homes, thousands face impoverishment.

Germany, Austria and Switzerland withdrew financial support for the Ilisu dam in July 2009, citing concerns about the social, cultural and environmental impact. The Turkish government, arguing that the dam will help produce much needed energy and irrigation, has secured domestic financing of the €1.1bn (£1.02bn) project and is pushing ahead despite a pending court decision at the European court of human rights.

The Ilisu dam has a life expectancy of less than 100 years, but the destruction of the fragile natural environment will be irreversible.

“The Tigris river basin is one of the last areas where a river runs freely in Turkey without having been dammed,” Ayboga says. “The dam will completely destroy the river banks. The microclimate will change due to the dam, a phenomenon we have already seen after the dams on the Euphrates. The biodiversity will suffer; the rich variety of plant and animal life will be severely diminished.”

Numerous vulnerable and endangered species are threatened by the construction of the dam, including the Euphrates softshell turtle, the red‐wattled lapwing, and many other rare birds, bats and mammals. While the environmental impact on Turkey will be severe, the effect on neighbouring Iraq is expected to be catastrophic.

Toon Bijnens, international coordinator for the Save the Tigris and Iraqi Marshes Campaign in Sulaymaniyah, said downstream water levels are expected to decrease by 40%: “This means that the water quality of the Tigris will worsen. There will be increased salt water intrusion, making the water unfit for drinking or irrigation.”

Ilisu, once operational, will also be detrimental to the Mesopotamian marshes, a wetland area in southern Iraq declared a Unesco world heritage site in 2016. “The dam will dry up a considerate part of the marshes,” Bijnens said. The marshes were drained by Saddam Hussein in the 1990s and the community of the Marsh Arabs has only recently returned to their land. Their livelihoods are now again endangered by the Turkish dam.

An official source from the Turkish ministry for Forests and Water Works told the Guardian that “all dammed up water is sent downstream via the turbines”. Because Ilisu was a hydroelectric dam, there would be no decrease of water levels. “The importance of reservoirs as a safe water source in the fight against global warming and drought has increased,” the source said. “For that reason the Ilisu dam has to be seen as an advantage for Iraq, not a threat.”

However, Ankara has not yet ratified the 1997 UN Watercourses Convention, a treaty that seeks to establish a law for governing freshwater resources shared across international borders that entered into force in 2014. Since no formal agreement was signed, the sovereignty over how much water is released downstream rests with Turkey, Bijnens warned.

The tomb of Zenyel Bey at Hasankeyf is carried on a rolling structure as it is moved to stave off the risk of flooding from the dam project. The tomb is a 15th-century memorial to one of the key figures in the Ak Koyunlu tribe who controlled much of eastern Anatolia and the Caucasus and vied for supremacy with the emerging Ottomans. Photograph: Ilyas Akengin/AFP/Getty Images

Ayboga stressed that despite the controversies surrounding the construction of the Ilisu dam, all protests and public meetings were banned under the current state of emergency, declared just over a year ago.

“There has always been a serious lack of transparency and accountability,” he said. “But now the Turkish government uses the conflict in the region and the state of emergency to speed up the project and to silence all opposition. Many locals are scared to protest now.”

Ayboga added that it was very difficult for journalists, both local and international, to access the area. “This makes it easier for the government to push ahead with extremely controversial measures,” he said.