Ontario’s Greater Golden Horseshoe is experiencing rapid growth that if unchecked could perpetuate "sprawl" and threaten the Region’s sustainability. To manage this growth, the Province adopted a program of "Smart Growth" and prepared a regional Growth Plan amidst a suite of complementary legislation. Municipalities are now expected to accommodate high levels of growth with an adequate supply of water and the necessary infrastructure to support increased demand. This invites the question of whether growth can be sustained through infrastructure upgrades, or whether absolute hydrologic limits will reshape regional growth. To investigate this, two strands of research are merged, which have traditionally been carried out individually - Smart Growth and "Planning by the Pipe". This paper argues that Ontario should align its growth management strategy with the servicing capacity and lifespan of water and wastewater infrastructure as well as the finances required for their maintenance and expansion. This consideration must not only reflect preferred areas for growth but the region’s hydrological capacity to support the increased demand in these areas.