The city is advising people stay out of Chedoke Creek and Cootes Paradise while it investigates the cause of "highly contaminated water" there.
"If you come into contact with the water, wash yourself off promptly with warm water and soap," the city said in an advisory Thursday.
The city says it's conducting visual inspections of sewers and other underground infrastructure in the area, investigations upstream of Chedoke Creek, and probes of the sanitary sewer and surface around the Ministry of Transportation project near Highway 403 and Cootes.
Staff also plan to remove surface material from the creek.
Last week, a resident told The Spectator he spotted a sheen of oil, plastic objects floating in the water and grey masses.
Ministry of the Environment spokesperson Gary Wheeler said Thursday the province, city and Hamilton Conservation Authority are trying to "pinpoint a source" for the odours and apparent pollution that spurred complaints.
Since then, the Royal Botanical Gardens has removed docks at Cootes and posted warning signs along the shoreline.
Anyone with questions about health risks can call Public Health Services at 905-546-2189 or email email@example.com.
If anyone suspects pollutants in the sewer system or bodies of water, they're encouraged to call 905-540-5188.