You may have seen the posts on social media — some concerts on the Chicago River that are uniquely Chicago.

The concerts have become a summer tradition. They draw crowds above and below the Belmont Avenue Bridge that spans the North Branch of the river between North Center and Avondale.

People gather on the bridge and below. A crowd of kayakers turned out too for the concert on Monday.

All ears are tuned to the musicians who are seemingly stranded on a concrete island. It's actually a pile, also called a dolphin — yes, a dolphin — meant to protect the bridge from being hit by ships.

"It's just some structure in the river that seems to be perfectly made," said concertgoer Joe Pelzers.

"I've been a musician myself," added concertgoer Michael Kuehler. "I think I'd just be wondering the whole time, like, 'What if I drop my guitar in the water?'"

There is really no bad seat on the Chicago River for the occasion.

"You can be here. You can be up on the bridge. You can be under the bridge," concertgoer Ari Bennardo said while onboard a boat. "You can be on the water — which we are now, which is awesome."

Boats shuttle the bands and their equipment to the stage as the crowd waits. How do they know to be there? Word about the Secret River Show spreads on social media.

"You've got to have your ear to the river," said concertgoer Connor Corr, "but I heard it straight from the River King himself."

Who is this River King of whom Corr speaks? That would be Ben Kinsinger.

Kinsinger crowned himself the River King, but he shares music with the masses as the concert organizer.

"This is the first time people are hearing these bands, and then they have this moment where 'I saw them at the River Show,'" Kinsinger said.

The Secret River Show isn't so much of a secret anymore. Looks like the word's gotten out a bit.

"Doesn't seem like it's very well kept," said concertgoer Drew Reynolds.

Indeed, people were seated several deep under the Belmont Avenue Bridge and gathered all along the sidewalk on the bridge. Some were even sitting on the bridge with their legs dangling off.

"This is the most people ever," Kinsinger said.

This is now the fourth year of music on the river. Bands play from noon until sunset.

"It's tough times, but stuff like this makes it easier to go through," Bennardo said.

With a captive audience, musicians don't seem to mind a stranded island.

"The creative spirit becoming alive," Kinsinger said.