The company decided to restart the line after examining its safety record and deciding it was safe, Pruessing said.
The cause of the rupture has not yet been determined, but company and government officials have speculated that high waters in recent weeks may have scoured the river bottom and exposed the pipeline to damaging debris.
The Yellowstone River at Billings had dropped nearly 2 feet by Monday from its peak Saturday morning, according to the National Weather Service. But temperatures reached the mid-90s Sunday, causing the melt of mountain snow to accelerate.
It is possible cleaned areas would become fouled again as waters rise.
Gov. Brian Schweitzer, who earlier criticized the company's inspection of the spill, planned to tour the damaged areas Tuesday.
Underscoring rising anger over the spill among some riverfront property owners, Pruessing was confronted after his news conference Monday by a goat farmer and environmental activist who said his partner was sickened by oil fumes and had to be taken to the emergency room.
"I need to know what we've been exposed to. People are sick now," Mike Scott said. Scott's partner, Alexis Bonogofsky, was diagnosed Monday with acute hydrocarbon exposure after she experienced dizziness, nausea and trouble breathing, he said.
Pruessing said air and water monitoring had not revealed any health risks. But he told Scott the company would provide the public with more information.
The Environmental Protection Agency said in a statement Monday afternoon that officials were still taking air and water samples to determine the impacts.
EPA officials said they and U.S. Fish and Wildlife Service personnel conducted an aerial assessment of the Yellowstone from Laurel to 30 miles downstream of Billings, finding oil deposits along the river banks, in slow water and in small pools in backwaters at intermittent points.
The U.S. Department of Transportation, which oversees pipelines, notified Exxon Mobil in July 2010 of seven potential safety violations and other problems along the pipeline. Two of the warnings faulted the company for its emergency response and pipeline corrosion training.
Transportation Department spokeswoman Patricia Klinger said the company has since responded to the warnings and the case was closed.
The company also was cited for "probable violations" in a February letter. Those included inadequate pipeline markers in a housing development, a section of pipeline over a ditch covered with potentially damaging material and debris, vegetation in a housing area that covered a portion of line and prevented aerial inspections, and a line over a canal not properly protected against corrosion.
The company responded in a March letter that it had corrected all of the problems, most of them within a few weeks of being notified.
Company spokesman Alan Jeffers said there was no direct connection between those problems and the pipeline failure.
"These are important things we needed to take care of, and we took care of them by the time we got the notice," Jeffers said. No fines were issued, he said.
The Yellowstone spill has amplified calls from some safety advocates and environmentalists who want the government to impose more stringent regulations on the industry.
Anthony Swift, a policy analyst with the Natural Resources Defense Council, said the fact that Exxon Mobil's Silvertip line was apparently in compliance with federal rules underscores that those rules need to be strengthened.
"These are the sort of spills that we shouldn't be tolerating," Swift said. "We need to incorporate tougher safety standards."
The company said only one case of wildlife damage — a dead duck — had been reported, but Pruessing said that could not be confirmed. A local newspaper, the Billings Gazette, has run pictures of a turtle and a group of pelicans apparently with oil on them.
If another surge of water pushes oil further into back channels as expected, it could be a potential threat to fisheries, said Bruce Farling, executive director of Trout Unlimited's Montana chapter.
Farling said there are many fish eggs and recently hatched fish in those channels.
The stretch of the Yellowstone where the spill occurred contains sauger, bass catfish, goldeye, trout and, farther downstream, below Miles City, native pallid sturgeon.
"If we get a bunch of oil in some of these backwater areas, these are precisely where these small fish rear," Farling said.