Consultants hired by the Athabasca Chipewyan First Nation say the algae bloom is probably the result of high water from record-breaking rainfall combined with unusually warm temperatures.
The scientists say dead fish floating on the river probably died as a result of oxygen starvation from the out-of-control algae growth.
The sheen was first spotted Friday night and covered more than 100 kilometres of the lower Athabasca.
Fort Chipewyan's water intake was closed, but is expected to reopen later Thursday.
Area residents say the sheen is still visible in some stretches of the river.