The water level in the Dong Nai River, which supplies HCM City and 11 provinces, is dwindling, allowing seawater to encroach and causing a shortage of potable water for pumping plants situated along its basin.
Truong Khac Hoanh, deputy general director of the Thu Duc Water Plant, said the river was more than 20cm lower than at the same time last year.
As a result, seawater has begun to enter two months earlier than normal.
His plant only drew water from the river during low tide when fresh water flows towards the sea, he said.
The Binh An Water Plant too has stopped drawing water from the river for four to six hours a day during high tides.
The pumping stations warned that if the water level in the river continued to fall they would find it hard to supply water, to households since their technologies cannot treat saltwater.
Nguyen Van Nga, head of HCM City's Department of Natural Resources and Environment's Water and Mineral Resources Division, said competent agencies had repeatedly warned about the decline in water quality in the Dong Nai.
They have also been trying to check pollution along the river basin.
"But the decline in water level has not received proper attention from [the] agencies," he admitted.
The construction of several hydropower plants and the loss of forests in the river's upstream area have mean rain-water keeps running off, causing a severe decline in its water level during the dry season.
Prof Doan Canh of the Viet Nam Academy of Science and Technology's Tropical Technical Institute said forests in the upstream, area needed to be preserved, especially the Nam Cat Tien forest which stores and supplies most of the river's water.
Soil erosion, silting, and release of wastewater into the river should be prevented, he said, adding awareness of protecting the river should be raised among people living along it.
The Dong Nai River supplies water to around 17 million people and irrigates 1.85 million ha of farm land.