The Tanzania government on Monday issued a warning to its citizens in the Kirumi, Kwibuse, Marasibora and Ryamisanga villages of Rorya District to stop using water from the Mara River over chemical pollution.
Officials also banned fishing and other uses for the water, a day after a minister of state, Mr Selemani Jafo, formed a team of experts to investigate the pollution.
The Mara River has a foul smell, its water turning dark and killing fish.
The river starts in Narok County in Kenya, passes through the Maasai Mara National Reserve, sustains wildlife and is renowned for the annual wildebeest migration spectacle.
Nation.Africa observed that the river's tributaries Talek and Sekenani are also emitting a foul smell that wafts to the wildlife sanctuary, which attracts several local and international tourists.
Residents told the Nation they suspect the effluent is caused by several factors, including human activities in villages around the Talek and Sekenani streams, improper disposal of animal carcasses and climate change that has resulted in low water levels.
Narok County Natural Resources Network chairman Nicholas Murero blamed human activities such as irrigation and use of chemicals in the surrounding areas for the river's pollution.
"The waters in the river have now turned green and a foul smell is all over the place. During the recent drought, hundreds of livestock and wildlife such as buffaloes got stuck in the river's banks, died, and decomposed, thus polluting it," he said.
He added: "There are irrigation activities in the upper side of the river and some chemicals drain down."
The river's pollution raised concerns on the Tanzania side earlier this week, when residents reported massive fish deaths and a change in the river’s water colour.
Subsequently, Mr Jafo created a team to investigate the phenomenon.
Mr Jafo told the news media that the river’s water was contaminated and was releasing a bad smell.
"This alarming situation calls for urgent action by the government in order to know exactly what caused the pollution of the water," he said when he inspected the river.
Redempta Samuel, a senior official with Tanzania’s National Environment Management Council (NEMC), said experts had collected water samples from the river and Lake Victoria, where the river drains its water.
The Mara River basin covers 13,504 square kilometres, about 65 percent of which is located in Kenya and 35 percent in Tanzania.
Narok National Environment Management Authority (Nema) Director Arasa Mosago told the Nation that he was not 'aware' of the river's pollution but promised to investigate the matter.
"We have not received any information on the Mara River's pollution but once we receive (it we will get to) the bottom of the matter," he said.