The reason behind the Leichhardt River going up in flames on Saturday evening remains unknown, as local authorities continue to investigate.
Queensland Fire and Emergency Services were called to two vegetation fires in the Leichhardt River at 5.40pm on Saturday December 8.
QFES media spokesperson said six fire crews attended the two blazes that came close to threatening residential properties near Grace Street bridge and Alma Street bridge.
"Crews conducted structure protection as a precaution and as a result the grassfires were contained within riverbed area," the spokeswoman said.
"The blazes were classed as under control one hour later at 6.40pm and crews were released at 7.30pm.
"The area burnt exceeds the size of a football field."
Mount Isa Police Duty Officer said a report was made to local police around the same time the blaze started.
"We were notified of a group of juveniles leaving the riverbed the same time the fire started," the duty officer said.
"We are continuing investigations."
While you are here, subscribe to our weekly email newsletter delivered to your inbox at 6am every Friday.