About 4,200 gallons of oil from a ruptured Unocal pipeline seeped into the San Gabriel River, coating at least 50 birds, officials said.
Workers from the state Department of Fish and Game and Unocal have spread more than 2,000 feet of containment booms along the river to catch the oil.
About 36,000 gallons gushed from a 10-inch pipeline after it was clipped Monday afternoon during construction of a Norwalk Metrolink station, south of Imperial Highway near Bloomfield Avenue.
By Wednesday, the oil had flowed into nearby Coyote Creek, a concrete-lined wash, and into the river. Several birds were treated Wednesday at a makeshift site near the river in Long Beach.