An unusually warm winter and a mild spring have resulted in a drop in water levels along the Saint Lawrence River, raising concern among those who use it for business and pleasure.

Near Montreal's Old Port, the river was 6.19 metres above sea level during April, a metre lower than it was at the same time last year, and that means trouble for cargo ships.

Jean-Luc Bédard, vice-president of operations for the Montreal Port Authority, says the lower water levels get, the less merchandise the ships can carry. Even a centimetre less water means reduced tonnage for ships.

"Like, we have the 4,200-tonne capacity," he said. "For each centimetre, that means about 72 tonnes."

Pleasure boaters are also keeping a close eye on the situation.

"Where it's affecting us at the moment is just in our preparations to set up launching boats," said David Speak, general manager of the Beaconsfield Yacht Club.

"Pulling boats to their docks, some of them are touching the bottom or will have to plough through the mud a little bit."

Environment Canada's senior climatologist, David Phillips, warns the dry spell might last awhile.

"We're going to see temperatures of 30 [C], 32," he said, "the kind of temperatures you see in the dog days of summer, which will cause more drying out."

The good news, Phillips said, is the forecast also calls for some rain on Thursday.