Rebar just below the water's surface ripped through a number of rafts on the Bow River this weekend, prompting the Calgary fire department to issue a warning after multiple rescues.

The fire department said they had to repeatedly rescue rafters who were stranded on an island just west of the Crowchild Trail bridge, where one of the worst hazards lurked.

"One particularly hazardous location is just upstream from where the river passes under the Crowchild Trail bridge," officials said.

"Large amounts of rebar have been discovered protruding approximately four metres from shore near the island, very near to a standing wave on the north side in the main channel of the river. It is not readily visible as it is just below the surface of the water."

Rebar is often a set of steel bars or wires using to reinforce concrete.

The fire department recommends using the south side of the river to avoid the hazard but warns the current is stronger on that side.

The rebar - steel bars used to reinforce concrete - is more of a hazard than usual due to low water levels. (www.engineeringtoolbox.com)

The area is marked with caution tape as a temporary measure until something more permanent can be installed.

Calgary fire has posted safety tips for river usage, which includes the use of life jackets.