World Animal Protection surveyed the living conditions of 3,000 performing elephants in six Asian nations and found most were treated harshly.
The $2.3 billion project will be located about 1,500 kilometers from the Cambodia-Laos border and is expected to be completed in 2024.
Cambodia generates about half of its electricity from hydropower, while it imports significant amounts from Thailand, Vietnam and Laos.
Ven Vorn has denied he had broken the law during the hearing on Wednesday.
Pak Beng hydropower dam will cost an estimated $2.3 billion and is scheduled to be completed in 2024.
A local human rights monitor said this week that Ven Vorn was seeking legal aid ahead of the hearing, scheduled for June 21.
Cambodia, a member of the UN Framework Convention on Climate Change since 1995, endorsed the Paris deal.
The project hopes to create better marine environment and to protect coral reefs.
Environmental groups have urged “a fair balance” on Mekong dams, reiterating concerns over the new proposed dam.
Cambodia has said it expects to be able to move from importing nearly all of its electricity from neighboring countries to a net electricity exporter by 2030.
Researchers say international trade shifts harmful effects of pollution from consuming to producing nations.
The rescuers offered sugar cane, bamboo leaves and bananas to the elephants after they were freed.