I don’t know about that watershed, but in the Sooke watershed water treatment was considered as the water was getting regularly contaminated due to hunters, people dumping cats, runoff from logging, forest fires etc.
They went a different direction, excluding public access effectively, eliminating industrial activity And restoring watercourses that led to the reservoir.
Keeping the natural system intact was (is? I haven’t been involved for 20 years) determined to be a more cost effective method of delivering clean water than treatment. Perhaps that’s the approach there. Treatment would be very expensive for a city the size of Vancouver so I could see eliminating access as a way of reducing the degree of treatment necessary.