Shipping companies say the dropping water levels in some of the Great Lakes is hurting them financially.

Wayne Smith, vice-president of Seaway Maritime Transport in St. Catharines, Ont., said his 33 freighters have had to lighten their loads by 10 per cent because they are in risk of scraping the bottom of the increasingly shallow Great Lakes.

"It certainly is significant," said Smith. "As you can imagine in a competitive world today, 10 per cent on your productivity is quite a serious impact."

Lakes Huron and Michigan are at their lowest levels since the 1960s, while Lake Superior is at its lowest since 1926, according to a report from the Canadian  Hydrographic Service at Burlington, Ont.

The lower Great Lakes, Lake Ontario and Lake Erie, remain at normal levels.

The reason for the lowering levels is being studied. Some have suggested climate change or dredging may be a factor.

Jim Given, the vice-president of the Seafarers International Union, said he has seen ships struggle with the low levels in the Welland Canal, connecting Lake Ontario to Lake Erie.

"You watch them go past and you see them stir up the bottom of the canal," he said. "You see the mud trail for miles."