ATLANTA — Flash flooding earlier this week overwhelmed Atlanta’s stormwater system and appears to have triggered a major fish kill along the Chattahoochee River.

Chattahoochee Riverkeeper Executive Director Jason Ulseth said thousands of fish have died along a 20-mile stretch of the river beginning south of Peachtree Creek.

“From small little bait fish, up to 30 pound striped bass. Everything in the river looks like it’s dead,” Ulseth said.

Ulseth said early analysis suggests sewage spills caused by Wednesday’s storms are the likely cause of the fish kill.

“The sewage gets broken down in the river by bacteria and it sucks all of the oxygen out,” Ulseth said.

He said the loss of oxygen in the water caused fish to suffocate.

“It sucks all of the oxygen out so we’ve had essentially thousands of fish in over 20 mile stretch of river that suffocated and are now dead,” Ulseth said.

Ulseth said he has never seen a fish kill of this scale before.

“I’ve never seen any type of fish kill of this magnitude before,” Ulseth said.

Officials with the City of Atlanta and the Georgia Department of Natural Resources are also investigating.

WSB Radio’s Jonathan O’Brien contributed to this story.