Despite concerns about runoff rates in the Yellowstone River, authorities believed Exxon Mobil\'s Silvertip pipeline was buried deep enough, an official said. U.S. lawmakers heard testimony from energy and federal officials on the July 1 oil spill near Billings, Mont. Exxon Mobil said initially that around 1,000 barrels of oil leaked from the 12-inch pipeline into the Yellowstone River. Heavy flooding along the river hampered recovery efforts. Cynthia Quarterman, administrator for the Pipeline and Hazardous Material Safety Administration, said officials in December confirmed Silvertip was buried 5 feet below the riverbed, meeting the 4-feet-depth-of-cover requirements of pipeline safety regulators. A review of the pipeline was conducted in October because U.S. and city officials \"were concerned with the risks to the Silvertip pipeline due to high runoff and possible river bottom scour and erosion of the river bank,\" she testified. Officials in Montana have called on Exxon Mobile to release data about the contents of the crude oil pipeline. Exxon officials told U.S. lawmakers they were still investigating the cause of the July 1 leak. Douglas Inkley, a senior scientist at the National Wildlife Federation, expressed concern over the long-term effects of oil spills like the one in Yellowstone River. \"Because the impacts of toxic oil can take months or even years to reveal themselves, it is clear that we won\'t know the full impacts of this oil spill for years to come,\" he testified.