Dead fishes were first found floating by local anglers on November 16 morning in the Lukha river, which flows into Bangladesh, about 130 km east of here.
Villagers reported discolouration of the waters from the confluence of Lunar and Lukha rivers, which was believed to be caused by acidic content and was the reason behind the death of fishes, S Lamare, Sunapyrdi headman, said.
A team of scientists from the State Pollution Control Board collected water samples to determine its quality and the reason behind the discolouration.
"We have deputed a team of experts to collect samples to ascertain the reasons behind the unnatural discolouration and death of fishes," MSPCB chairman, M Allya said.
Delhi-based Central Laboratory of Central Pollution Control Board had said the water was highly 'acidic' and unsuitable to life forms and unfit for human consumption.
Rivers downstream of coal mining areas and cement plants have remained blue for the past five years between November and February.
The MSPCB in its report last year had blamed the mine run-off and acid effluents from coal mines as the major probable cause of water pollution in the area.