"It's just kind of like . . . " began Emily Byllott, 22, as she moved her hoop in circles.
" . . . fun," concluded her friend, Shaye Smith, 24.
Byllott, of Long Island, teaches HoopYogini, a fitness program that combines modern hoop dance with yoga.
Both she and Smith, of Brooklyn, said they were determined to be at Penn's Landing on Sunday, the lure of neither the beach nor the city enough to keep them from the daylong show.
"It's my three favorite artists" - Lotus, Papadosio, and GRiZ - "all in one place on the same day," Byllott said.
Added Smith, "I just really like the whole scene."
The scene on Sunday was friendly, and the weather gorgeous, with clear skies and temperatures in the 80s. Speedboats and duck boats plowed the Delaware River, and the skyline of Camden seemed close enough to touch.
Valentina Marie, 19, of Voorhees, said she came to Penn's Landing for one reason: "The music."
As the tones blared from the stage, she created a separate, free-flowing kind of art, moving long burgundy scarves in time to the music.
The Jam returned after a six-year hiatus, reclaiming its role as the unofficial kickoff of the summer concert season. Over the years, Jam on the River has hosted top entertainers such as the Flaming Lips, Bob Weir, Cypress Hill, and Al Green.
The first Jam took place nearly 30 years ago, in 1986, a Louisiana-centric festival called the Jambalaya Jam. A decade later, that event merged with another called RiverBlues, giving birth to the Jam on the River. Over time, it moved away from roots music and into a jam-band lineup. Then it disappeared from the concert calendar.
On Sunday, people spread blankets to soften their seats on the concrete steps in front of the River Stage at the Great Plaza.
"Where's the shade, man?" one friend called to another.
There was little to be found. Between acts, people scurried to what shade could be located among trees and embankments.
It was a laid-back afternoon when friends caught up to share memories of concerts past, and some families made new ones.
Justin Carey, 35, of Philadelphia, was there with his 5-year-old son, Dylan, who has been to at least five concerts. They have the same favorite band, Phish.
Father and son came to the Jam to hear the bands and because family friends were among the musicians.
"We wanted to avoid the beach," said Carey, who lives nearby. "We didn't want to be anywhere else - hanging out with our friends on the river."