(CNN) — A massive fish kill on the Chattahoochee River west of Atlanta was reported Friday by environmental protection non-profit, Chattahoochee Riverkeeper. Chattahoochee Riverkeeper executive director Jason Ulseth told CNN he discovered the dead fish when he embarked on a river patrol Friday morning. Ulseth estimates thousands of fish, some weighing 20 to 30 pounds, are dead along a roughly 20-mile stretch of river on the western border of Fulton County.