JUDY WOODRUFF:Now: why some of the Great Lakes are dropping to record low levels, and the economic bite that's accompanying this environmental change.

Elizabeth Brackett of WTTW Chicago has the story.

ELIZABETH BRACKETT, WTTW: Leland Harbor is the heart of this northern Michigan town. The small town is quiet in the winter. But the population jumps 10-fold in the summer, when tourists flock to Leland Harbor, beaches and quaint shops.

That tourist economy is now in jeopardy because of the dramatic drop in Lake Michigan's water level. Harbor master Russell Dzuba says the lake is down more than two feet from its average. And that drop is threatening to close the harbor.

RUSSELL DZUBA, Harbor Master: The economic impact this harbor has on the community is strong. And when things are slow, the guy at the grocery store, the guy at the restaurant comes down and asks me what's going on.

ELIZABETH BRACKETT:The guy asking the questions from the grocery store is likely to be Joe Burda. His family runs the only grocery store in town.

JOE BURDA, Leland Mercantile: The summer business in general keeps us open for the rest of the year. The boats and the traffic that the harbor brings in is a pretty big percentage of what we do in the summer.

ELIZABETH BRACKETT:The low water also endangers Leland's historic fishing industry, a huge tourist draw.

Now these fishing boats, which were bought and are now run by the local preservation society, sit perilously close to the bottom of the lake. Across the peninsula from Leland on Grand Traverse Bay, rocks and boulders, long underwater, dot the now dry lakebed. Docks sit far from the water.

And in nearby Suttons Bay, sand flats appear. Concrete blocks that once anchored boats now sit in just inches of water.

If I had been walking along this beach in Suttons Bay in 1984, the water would have been almost a foot over my head. The Army Corps of Engineers confirms that Lake Michigan and Lake Huron water levels have hit an all-time low. On this chart, the blue line shows the long-term average for the two lakes, the red line the actual monthly water levels.

Scientists at the Great Lakes Environmental Research Laboratory in Ann Arbor, Mich., blame evaporation and less precipitation for the dropping lake levels.

ANDREW GRONEWOLD, Great Lakes Environmental Research Laboratory: When the water temperatures increase, which they are right now, especially through the summertime; then, in the fall, when we have the cool air masses coming over the lakes, we have increased evaporation, and that evaporation rate has been exaggerated, particularly this year.

We're also in a year where there's been extremely low precipitation, so over the last year very little rain was coming in to the system, both in the form of snow melting in the springtime and then also direct rainfall onto the lakes themselves.

ELIZABETH BRACKETT:Last winter was the fourth warmest winter on record. And those warmer temperatures lead to less ice formation and still more evaporation.

ANDREW GRONEWOLD:And when the lakes are changing that dramatically, that is a change in the climate. Now, what is causing the lakes to warm so much, that is something that is going to require some additional research.

ELIZABETH BRACKETT:With Great Lakes water levels at historic lows, the only way to keep harbors open is by excavating or dredging the lake bottom. But Chuck May, who chairs the Great Lakes Small Harbors Coalition, said the money for dredging has dried up.

CHUCK MAY, Great Lakes Small Harbors Coalition: You got almost a perfect storm hitting the Great Lakes harbors. You've got low water and you've got lack of maintenance, lack of dredging, lack of infrastructure.

And you combine those two, and you've got situations where we truly face a crisis throughout the Great Lakes, harbor after harbor, and it's just growing.

ELIZABETH BRACKETT:It's not just tourist towns that are suffering. So are commercial harbors, like one in Ludington, Mich., 100 miles south of Leland. Ludington is one of the 139 commercial and recreational harbors around the Great Lakes.

Under new federal regulations issued by the Obama administration, only commercial harbors that handle one million tons are eligible for dredging. Today, only 15 of the Great Lakes harbors meet that criteria.

That worries Chuck Leonard, the chief operating officer for Pere Marquette Shipping, which operates car ferries and barges out of Ludington. He is concerned about what low water levels will mean to the amount of tonnage he can ship.

CHUCK LEONARD, Pere Marquette Shipping: We're starting to see with our vessel where we're having to light-load her, and I'm afraid we could see that increase moving forward.

ELIZABETH BRACKETT:For every inch the water level drops, carriers for fit 8,000 tons of cargo. As loads shrink, the one million ton rule becomes harder to meet.

CHUCK LEONARD:We can't get the vessels loaded to where we'd like to get them in the harbors. The tonnage in the harbors diminishes. And then they become unfundable because the tonnage isn't adequate for the funding. It's -- being in the shipping industry right now is a very frustrating experience.

ELIZABETH BRACKETT:Leonard's company pays into a harbor maintenance tax. He and Chuck May think that money should be used for dredging.

CHUCK MAY:The federal government actually owns these harbors, these channels. And they actually have a tax called a harbor maintenance tax that they put in place the beginning of 1985 to take care of these harbors. So far, in the past 15 years, they have collected $8 billion dollars that they have not spent on harbors.

ELIZABETH BRACKETT:Legislation introduced to force all the tax money to be spent on maintaining the harbors has not gained traction in Congress.

So, Leland Harbor master Dzuba, frustrated at seeing otters play on a beach that shouldn't exist, has looked elsewhere for money.

RUSSELL DZUBA:In '07 the appropriations stopped, so we started fund-raising at a local level. And that's where we are today.

ELIZABETH BRACKETT:He raised $120,000 dollars last year to pay for dredging to keep its harbor open, but says he doesn't know how long the community can support those costs.

And scientists predict that lake levels will drop further this winter, with ever greater consequences for the 30 million people who live in the Great Lakes Basin.