(Reuters) - The Department of Transportation has fined Exxon Mobil Corp
Exxon's Silvertip pipeline, which carries 40,000 barrels per day of crude in Montana, leaked about 1,500 barrels of oil into the river in July 2011 after heavy flooding in the area.
Exxon has previously said the accident cost the company about $135 million.
"The risk of flooding on Yellowstone River was a known threat that could cause the pipe in the river to lose physical support and potentially rupture," the U.S. Pipeline and Hazardous Material Safety Administration said in a report on Monday.
In January, it said the spill would have been about two-thirds smaller had company workers responded quickly.
Exxon has 30 days to contest the violations in the notice.
Exxon could not be reached immediately for comment by Reuters outside of business hours.
(Reporting by Mridhula Raghavan in Bangalore; Editing to Paul Tait)