Firefighters in Calgary struggled to haul four women on board their boat Saturday night as they clung to orange buoys in the Bow River that mark what officials call the "drowning machine."

Every year, firefighters have to rescue boaters who drift too far down the river and get too close to the river's weir, or small dam.

City officials say the buoys, with chain strung in between, are where boaters or swimmers can make what they call a "last attempt" to save themselves before being sucked into the weir and drowned.

All four women were wearing life-jackets when they went into the water and grabbed onto the chain linking the buoys together.

They told firefighters they had tried to get to shore in their boat before hitting the danger zone, but had failed.

All four women, said to be in their 20s, declined to be taken to hospital to be examined.