When a levee works properly, it creates a barrier between water and towns or farmland. A river wants to seep into its flood plain, but the levee prevents that. It narrows and straightens the river, forcing water to flow higher and faster.
In some ways, it helps create a water highway. But a river isn't a highway and it's always looking for an exit -- a break in a levee. And when a levee breaks, disaster happens.
Dana Werner is the man at the U.S.Army Corps of Engineers who oversees the St. Paul District levees. The district spans from Minot, N.D. to Guttenberg, Iowa, and includes the Mississippi and the Minnesota rivers. The Corps manages the 68 levees in this region that it knows about, says Werner.
"But then you also have levees which are out there that a private party may have constructed, an ag levee or a ring levee around their homestead to protect it. And those types of levees the Corps really doesn't regulate," Werner said.
Levees allow communities to develop as close to the river as possible, turning flood plains and wetlands into farm fields, housing developments and shopping malls. That could be dangerous, but a levee is designed with flooding in mind, Werner says.
"Once [the river] passes the community, if the levees themselves are non-continuous the water again has the opportunity to spread out over the land before it gets to that community, and then passes that levee system as well," said Werner.
However, continuous levees and nearly continuous levees are common today as more communities build along riverbanks, meaning the river has no place to spread. There's another reason for levees, too. A controlled river is easier to navigate.
When you combine frequent heavy rainfalls with a walled-off river, communities are borrowing trouble, according to Washington University hydrologist Bob Criss. Levees designed to protect against 100-year or 500-year floods create a sense of security that borders on fraudulent, Criss says.
"And I say fraudulent because it's this language that allows insurance to be purchased at public expense, and allows development to proceed in a flood plain. And all this activity continues to make the problem worse," said Criss.
Rivers want to flood, but communities want to stay dry. University of California Berkeley civil engineering professor Bob Bea says in this battle of man versus nature, "our piles of dirt (are) even higher. And of course, rivers are a lot of natural things."
"So they can always outwit you," Bea continued. "As flood stages continue to increase we finally ran out of energy, sometimes money, so now we're faced with over-topping."
Staff from the regional Army Corps wasn't able to comment on the Corps' levee policy, and headquarters staff in Washington D.C. was not available for comment.
But the Corps' Dana Werner did explain that when the Corps raises one levee it does consider how that will effect communities up and downstream.
"For the river or for the communities along the Mississippi, where you've got those levees on both sides, it may be that you have to raise miles of levee rather than a little short levee around a community or adjacent to a community," Werner said.
However, when one of those levees is undermined it acts like a drain, and all the pressure built up on that swollen river pours into town and surrounding land, Werner says.
One solution to that problem is to create designated drainage locations, so the river has a place to spread out.
Another is to determine how many levees the nation actually has, and do away with some of them. That would mean some communities would not only be in a flood plain, they would know they might get flooded when storms roll in and the rivers rise.