The Borden Park Natural Swimming Pool is the first chemical-free public outdoor pool to be built in Canada. The project replaced an existing pool, and includes a seasonal pavilion and landscaped pool precinct for 400 swimmers. The challenge was to create a large-scale pool with high-quality water control while also achieving an environmentally healthy and natural filtration process. The design process began with developing a pool technology that cleanses the water through stone, gravel, sand, and botanic filtering processes. This inspired a materials-oriented concept for the change room facility to achieve a technically rigorous and aesthetically integrated design whose gabion basket stone walls visually evokes the idea of filtration. The pool involves a balanced ecosystem where plant materials, microorganisms, and nutrients come together within a gravel and sand filtering pro- cess to create “living water.” Filtration is achieved in two ways: through a biological-mechanical system or the constructed wetland and gravel filter, and in situ, with Zooplankton.