The fluorescent green colouring appeared to start about 500 metres on the Victoria side of the entrance to the park and, over the course of an hour, the substance flowed down into the environmentally sensitive estuary.
By 5:30 p.m. the river, known for its dramatic salmon runs, eagles and other wildlife, was back to its normal colour.
Ministry of Environment teams were immediately sent to the area to investigate and members of Langford Fire Department collected samples for analysis.
No dead fish or animals had been found by early evening.
Earlier in the day a fountain beside Veterans Memorial Parkway in Langford also turned bright green, said Langford Fire Chief Bob Beckett.