Exxon Mobil's pipeline subsidiary completed the environmental cleanup work tied to a 2011 oil spill into the Yellowstone River, Montana's government said.

An estimated 1,500 barrels of oil spilled into the Yellowstone River when the company's Silvertip pipeline ruptured. The breach was attributed to river scour as the event occurred during a period of heavy flooding.

The Montana Department of Environmental Quality said Exxon had completed all of the supplementary environmental projects associated with the spill. That effort included more than $1.3 million spent on projects aimed at restoring the river's ecosystem.

"These projects, and the teams that worked on them, made a lasting contribution not only to the local communities, but to the Yellowstone River," DEQ enforcement specialist Larry Alheim said in a Thursday statement.

The department said primary reclamation activities were completed in August 2013.

The Silvertip update comes as responders continue monitoring of the January spill from the Poplar oil pipeline in Montana. About 1,000 barrels of oil spilled from Poplar and about half of the oil released had been recovered before cold weather forced a suspension of response operations in late February.

A late winter thaw freed some of the oil trapped in frozen waters of the Yellowstone River. Authorities lifted a water conservation measure this week. Some oil pollutants were discovered in a regional water treatment plant.

Bridger Pipeline, the Poplar operator, said it was closing its claims office as of Friday.