Nightly News | June 18, 2013
>>> the great lakes contain one fifth of the fresh water supply in the surface of the earth . that may seem like a great deal of it, these days it's not enough for the level of commerce that those five truly great lakes have supported for so long. the problem is the water levels. the lakes are currently low and getting lower. our report on the impact tonight from our chief environmental correspondent anne thompson .
>> the cargo ship rides high as it docks on lake erie . and that's not good.
>> she left 8,000 tons of iron other in minnesota, that's basically a day's work for an iron ore mine.
>> reporter: the mesabi liner can't carry a full load, because water levels in the great lakes are too low.
>> it's reducing our revenue. we carry less tons, we get paid by the ton. we don't make as much money.
>> reporter: this is the 14th straight year of low water levels for the $34 billion shipping industry . this january, lakes michigan and huron hit a record low. now superior, the largest fresh water lake in the world, those three remain well below their historical averages.
>> this ship is the size of an aircraft carrier. these days the difference between success and failure is measured in a matter of mere inches.
>> we're light loading the ship by probably about two and a half feet.
>> reporter: the water in st. mary's river between superior and huron determines how much your ship carries.
>> how difficult is it to steer a ship this big when there are only nine inches to two feet of water underneath you?
>> it's extremely difficult.
>> reporter: the industry wants the army corps of engineers to do more dredging. but the problem is not enough rain and snow over the lakes in the winter, and too much evaporation, fuelled by warmer water temperatures .
>> evaporation is having a much more significant impact on the system than it used to.
>> basically, the precipitation carpet keep up with the rate of evaporati evaporation?
>> that's a good way to say it.
>> reporter: impacting boaters big and small. there are places this charter business can't go.
>> we're all hoping and confident that the water levels are going to come back.
>> and keep the great lakes a source of pleasure and profit for america. anne thompson , nbc news, toledo, ohio.