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 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, in which a severed Exxon Mobil Corp. pipeline released 63,000 gallons, a 4,200 gallon anhydrous ammonia spill into the Missouri River in Nebraska, and a 28,350 gallon gasoline spill 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 report back to Congress within the next year on plans to ensure federal rules for pipeline crossings are sufficient.
There are at least 2,841 locations across the U.S. where hazardous liquid pipeline cross rivers and other bodies of water, according to the report.
In recent years, many 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.