Chris Berkey makes his living plying the often treacherous waters of the Great Lakes, delivering staples like cement to industries nestled in the myriad harbors that dot a coastline that's equal to nearly half of the circumference of the globe.

It's not glamorous work, but it is critical to the U.S. economy. And it's getting harder.

Water levels in Lake Michigan and Lake Huron fell to record low levels for December, and are expected to break the all-time low sometime in the next few months.

Cargo ships like Berkey's are being forced to lighten their loads, some harbors have already been forced to close and the tourist trade is bracing for an impact as well.

"In years past, there was always a buffer," he said. "That buffer's gone."

It's not a new problem. Lake levels have been below average for at least 13 years, said Keith Kompoltowicz, chief of watershed hydrology for the U.S. Army Corps of Engineers in Detroit.

But it is an increasingly serious one:

The coal trade on the Great Lakes declined 8.2% in 2012 from the previous year, and down a quarter off the 5-year-averge -- in large part due to falling water levels and a $200 million backlog in necessary dredging throughout the lakes, according to the Lake Carriers' Association. Commercial fishing boats are finding it increasingly difficult to navigate some harbors, risking a downturn in a vital part of the Great Lakes economy, said Mark Breederland, an educator with Michigan SeaGrant, which works with coastal communities on water-level issues, among other things. Charter boat operations and other businesses in coastal communities that depend on tourism fear the impact lower water levels will have from spring to fall, when tens of thousands of people flow into the state to boat, fish, eat out and shop.

Fishing lure

In Frankfort, Michigan, a popular salmon run on the Betsie River draws tourists drawn by the lure of fishing a rare naturally replenished population of the prized fish, said city manager Josh Mills.

"We see people from Texas, from Georgia, from Ohio, Illinois, other areas of Michigan," he said.

But low lake levels last year dried up the run, leaving salmon flopping in the mud, and forcing the state Department of Natural Resources to close to run to protect the population.

It appears a good number of the fish made it to their spawning grounds, but if water levels don't recover in the spring, the narrow channel through which the fish pass could dry up once again -- prompting tourists to find someplace else to go, Breederland said.

Also of concern: potential access problems at some of the private marinas dotting Betsie Bay, Mills said.

Despite efforts to diversify the city's economy in recent years, such problems would be a huge blow to the tiny community of 1,300, the city manager said.

"I'm confident the community will step up," he said. "But if there's no water, we're going to miss out on a lot of activity."

Precipitation and evaporation

The problem is a long-term cycle of too little water from melting snow and rain to counter the effects of evaporation on the lakes, Kompoltowicz.

Last winter, too little snow fell on the Great Lakes region to fully replenish the lakes. While Lake Michigan and Lake Huron typically rise a foot after the spring melt, the lakes only rose four inches last spring, Kompoltowicz said.

Add that tiny rise to a very hot, very dry summer that sucked water out of the lake like a straw, and you have a recipe for the decline in lake levels under way today, eh said.

There's too little data to say the problem is a product of global warming, he said. It's also a cycle that's been seen before.

Lake levels were nearly this low in December 1964, and it's the March 1964 record that's likely to fall in the next few months.

There is hope, he said. Records dating back to 1918 would seem to indicate a cyclical pattern that could well result in record lake levels in the next few years, he said. Such swings occurred in the 1970s and 1980s after similar low points.

Dredging backlog

But even if the water returns, what would appear to be a more intractable problem looms: Congress.