BILLINGS, Mont. — Pipeline spills caused by flooding and riverbed erosion dumped 2.4 million gallons of crude oil and other hazardous liquids into U.S. waterways over the past two decades, according to a new report from federal regulators.

The Department of Transportation report to Congress was crafted in response to a 2011 spill into Montana's Yellowstone River. The spill highlighted gaps in federal pipeline rules that require lines to be buried just 4 feet below riverbeds — scant cover that can quickly be scoured away by floodwaters.

The Associated Press obtained the report this week before its public release.

Regulators found flood-related pipeline spills since 1993 in California, Texas, Iowa, Louisiana, Montana, Nebraska, South Dakota and Kentucky. Of the 2.4 million gallons of oil, gasoline, propane and other hazardous liquids released, less than 300,000 gallons were recovered.

Although those accidents account for fewer than 1 percent of the total number of pipeline accidents, the consequences of a release in water can be much more severe because of the threats to drinking water supplies and the heightened potential for environmental damage.

The most recent accidents came during flooding in 2011 throughout the Missouri River Basin.

Those include the Yellowstone River spill that saw a severed Exxon Mobil Corp. pipeline release 63,000 gallons. A NuStar Energy LP pipeline also spilled 4,200 gallons of anhydrous ammonia into the Missouri River in Nebraska, and an Enterprise Products Partners LP pipeline released 28,350 gallons of gasoline into the Missouri River in Iowa.

U.S. Sen. Max Baucus, who requested the report with fellow Montana Democratic Sen. Jon Tester, said the results reveal "some pretty clear holes in pipeline oversight when it comes to flooding."

But Baucus said the report leaves unanswered basic questions about what steps can be taken to prevent future accidents.

Transportation Department officials said in the report that they will return to Congress within the next year on plans to ensure federal rules for pipeline crossings are sufficient.

An industry representative cautioned against imposing stringent new regulations. Such rules could force companies to divert money from other safety initiatives, such as reducing accidents caused by corrosion or excavation damage, said John Stoody, of the Association of Oil Pipe Lines.

The report showed riverbed scouring around pipelines caused by flooding "is a real but rare occurrence," Stoody said.

"It's our hope the focus remains on efforts that will provide the most public safety and protection for the environment," he said, adding that companies spent at least $1.1 billion on evaluations, inspections and maintenance geared toward safety in 2011.

In recent years, some pipeline companies have voluntarily buried their lines deeper than federal rules require. Using a technique called horizontal directional drilling, pipelines can be installed dozens of feet beneath riverbeds, minimizing the chances they could be exposed to damaging floodwaters and debris.

The technique can cost millions of dollars for a single water crossing.