Get on a boat on the upper Mississippi River, and you’ll eventually come upon a looming concrete structure stretching across the river’s main channel.

It’s called a lock and dam, and it’s a distinct feature of the upper river. The construction of locks and dams, a major feat of engineering that occurred largely during the Great Depression, has transformed how the Mississippi River runs.

What are these locks and dams for? How do they work, who controls them, and what changes have they made to the river ecosystem?

Here’s what you should know.

Why are there locks and dams on the Mississippi River? How do they work?

Prior to the installment of the locks and dams, the upper Mississippi River was free-flowing, regularly cutting new paths — and sometimes it was so shallow that people could wade across it. The locks and dams were put in place so that boats hauling freight up and down the upper river could have easier passage.

In 1930, Congress approved a project that would ultimately create the current system: The upper river is divided into sections called pools, where a fixed amount of river is held back by a dam. The U.S. Army Corps of Engineers controls how much water is in a pool at a given time. Each pool must be at least nine feet deep to allow towboats hauling barges to move through.

The construction of the locks and dams provided work for thousands of people along the upper river during the Great Depression. River towns grew in population as temporary workers moved in. An oral history project from the 1980s notes that in Genoa, Wis., near Lock and Dam 8, “Business boomed, particularly taverns,” and “Anyone having a room in a home had no trouble renting it.”

Between the headwaters in northern Minnesota and St. Louis, the river falls about 420 feet in elevation. When a boat enters a lock, the lock acts like an elevator, bringing the boat up or down to the water level of the next pool. You can also picture it like a staircase of water that boats and barges climb and descend.

Do the locks and dams control flooding on the river?

The locks and dams don’t provide flood control. Downstream from Wisconsin, there are levees meant to constrain the river away from communities and high-production farmland.

How many locks and dams are there?

There are 29 locks and dams on the upper river.

The first lock and dam structure, at Upper St. Anthony Falls in Minneapolis, has been closed to barge traffic since 2015. The Army Corps is currently considering the removal of the next two structures, Lower St. Anthony Falls Lock and Dam and Lock and Dam 1, between Minneapolis and St. Paul, to return the river to its free-flowing past in the Twin Cities.

The southernmost lock and dam is near Granite City, Illinois, north of St. Louis.

Who operates them?

The Army Corps operates the locks and dams.

Why doesn’t the lower Mississippi have locks and dams?

The lower river, which stretches south from Cairo, Illinois to the Gulf of Mexico, does not have locks and dams. As major rivers like the Missouri and the Ohio join up with the Mississippi, the channel becomes deep and wide enough to naturally accommodate shipping.

Are the locks and dams in good condition?

When the locks and dams were constructed, mostly between 1930 and 1940, engineers estimated their life span to be about 50 years.

The Army Corps makes routine repairs to the structures, many of which still have original parts that are now around 90 years old. This winter, for example, they’ll drain the water out of Lock and Dam 2 near Hastings, Minnesota, to make repairs to its concrete. But the Corps reports that there’s an estimated billion-dollar maintenance backlog — and officials acknowledge that making fixes here and there may not be enough.

“At some point, we’re going to need some major rehabilitation of these structures,” Kristin Moe, navigation business line manager for the Army Corps’ St. Paul District, told the Journal Sentinel earlier this year.

Groups that represent the shipping industry contend that instead of making repairs to existing locks, the Army Corps should be constructing new ones that have a 1,200-foot chamber to more efficiently accommodate larger groups of barges than the current chambers, which are 600 feet long.

How have the locks and dams affected the river ecosystem?

Converting the free-flowing river into a series of pools has changed its natural habitats and processes. Because the dams caused more water to fill into the flood plain permanently, forest cover decreased and became fragmented, according to a 2022 study on the ecological status of the upper Mississippi.

Islands have shrunk or disappeared, attributed to wind and wave erosion across the pools, and backwater areas off the main channel have filled in with sediment, making them less hospitable to fish. Consistent high water levels have made it more difficult for some types of aquatic vegetation to survive.

In the decades since the locks and dams were constructed, the Army Corps has completed projects to try to address some of these consequences and revitalize habitat for fish and wildlife, including building new islands and dredging backwaters to restore their depth.

This story is a product of the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri in partnership with Report for America, with major funding from the Walton Family Foundation.