Montana's governor called on the Obama administration Friday to strengthen rules that require oil pipelines to be buried just 4 feet beneath major waterways, after two breaches that spilled a combined 93,000 gallons of crude into the Yellowstone River. River scouring due to flooding or an ice jam is being investigated as a possible cause of a spill last month that dumped about 30,000 gallons into the Yellowstone upstream of Glendive, temporarily contaminating the city's water supply.