Colin Meloy performs with the Decemberists at ACL Fest on Sunday, Oc. 4, 2015. JAY JANNER / AMERICAN-STATESMAN

That the Decemberists became one of the most popular bands in America remains mystifying, in a supremely satisfying way. They’re nerdy, as they fully acknowledged with their 10-minute ACL Fest finale about a mariner’s revenge, complete with giant cardboard-cutout whale. They don’t feel the need to floor it all the time: Midset, leader Colin Meloy announced he was “going to try to play a quiet song … you know, the old festival killer.” And fifteen years in, they’re hardly the newest and shiniest indie it-thing anymore.

But nobody on the current indie landscape writes better songs than Meloy, and on a big stage with an expanded lineup at a major festival, they stood out precisely for that strength. This year’s “What a Terrible World, What a Beautiful World” affirmed the Portland band still has a lot left to give, nearly a decade after their major-label breakthrough “The Crane Wife” and 2011’s chart-topping album “The King is Dead.”

Standouts from the new material included “Cavalry Captain,” with multi-instrumentalist Chris Funk supplying buoyant blasts of trumpet, and “Why Would I Now,” a folk-pop gem from the band’s new “Florasongs” EP due out next week that was brightened by the harmony vocals of longtime Neko Case cohort Kelly Hogan. Crowd favorites included “Down By the Water,” a 2011 Grammy nominee for Best Rock Song, and the aforementioned “Mariner’s Revenge Song” from 2005’s “Picaresque,” with the band’s many longtime devoted fans following detailed instructions about when to “scream like you’re being swallowed by a whale,” Meloy requested. “You’re not going to get asked that a lot this weekend.”

Meloy’s distinctive vocal helps set the Decemberists apart, and they wisely don’t wash it out with overdriven instrumentation. It helps that his lyrics are dependably emotional and evocative, as evidenced on “A Beginning Song” and “Make You Better” from the new record. Core members Funk, keyboardist Jenny Conlee, bassist Nate Query and drummer John Moen play so well as a unit behind Meloy that he’s free to lose himself in the magic of the melody. Out in the audience, it’s effortlessly easy to do the same.