A new report to Congress says pipeline spills caused by flooding dumped 2.4 million gallons of crude oil and other hazardous liquids into U.S. waterways over the past two decades.

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

The report says a review of accidents found flood-related pipeline spills since 1993 in California, Texas, Iowa, Oklahoma, Louisiana, Montana, Nebraska, South Dakota and Kentucky.

Agency officials will report back to Congress within the next year on what else should be done to prevent such accidents.