A weir, also known as a low head dam, is a structure across a river that causes river water to back up and pool behind the structure. This build-up of water may be used for power generation or recreation, but in Calgary, the purpose is to divert water from the Bow River into a canal system that provides irrigation water for agriculture. The weir performs an important task, but as a side effect it creates a powerful recirculating hydraulic wave, known to rescue professionals as a “drowning machine.”