ALONG THE RIVER — The Mississippi runs the spine of America, touching 10 states and draining waters from 21 more, a vast waterway with a rich mythology, a sometimes powerful beauty and an always alarming propensity to flood.

Nearly 30 locks and dams hold back water in the river’s upper reaches. Every river bend to the south is lined by concrete to slow the water’s corrosive force. Levees corset thousands of miles of riverbanks and 170 bridges run above. All of this infrastructure is aimed at permitting barge traffic and protecting farms and cities. Most of it is decrepit.

Now, with President Trump’s push for a $1.5 trillion infrastructure plan, there are hopes of billions to fix up the Mississippi. But there are clashes over which projects to pursue, and no agreement on how to pay for any of it.

A move to tame one portion of the river can create chaos for people somewhere else along its 2,350-mile path, and in that precarious balance is the key to understanding the competing interests and enduring problems that vex the entire country.

“To understand America at this time,” says R.D. James, a Missouri farmer and new Army assistant secretary overseeing its Corps of Engineers, “you have to understand the river.”

At the same time, it’s clear that the river itself has changed.

“It doesn’t behave like it used to,” said John Carlin, a towboat pilot who has worked the Hannibal, Mo., riverfront for more than 40 years. “Seems like it doesn’t take much to get out of control.”

Now, the Mississippi is flooding again. Last week, after a deluge of late-winter rain, the Corps opened a massive floodway just above New Orleans, an emergency relief valve that it has been forced to use with increasingly regularity — three times in just the last seven years.