(CNN) -- Montana's governor pushed for answers Monday as to why additional resources have not been devoted to clean up the Yellowstone River, days after hundreds of barrels of oil seeped into the rushing waters after a pipeline ruptured.
ExxonMobil reported Monday night that more than 280 people have converged on the area near Billings, including workers from the Texas-based oil company and the Clean Harbors environmental firm. Gov. Brian Schweitzer told CNN earlier in the day that the effort has been "pretty good" thus far, even as he criticized the speed and comprehensiveness of the response.
"Not all the assets that we wanted are here, and we're going to find out why," he said.
The governor noted that "one year ago, almost to the day," a host of state, county and local agencies held a "mock pipeline spill" drill on the Yellowstone River. Many of those organizations, including the Montana Fish, Wildlife and Parks department, the state Environmental Quality department and various Yellowstone County officials, are involved in the current effort in the southern part of the state.
The crisis began late Friday, when ExxonMobil reported that 750 to 1,000 barrels (32,000 to 42,000 gallons) of oil escaped through a crack in one of its pipelines in the Yellowstone River in Laurel, about 16 miles southwest of Billings. The company said it shut down the line within minutes, but not before the toxins had been dumped in the water and left a "pretty heavy" smell of oil hanging over the area, according to Laurel rancher Lloyd Webber.
Historically high water levels and rapid currents have made things difficult for those trying to clean up the spill. On Saturday, for instance, levels near Billings reached their peak for the season at 13.95 feet, nearly a foot above flood stage, according to the U.S. Geological Survey.
While the water conditions helped break up and dilute the oil spill, they also left clean-up workers with fewer options. Duane Winslow, Yellowstone County's emergency services director, said Sunday that crews have been forced to work from onshore for fear of being swept up in the water.
"It's too dangerous to do anything on the river, to put out any sort of boats or anything," Winslow said.
ExxonMobil said late Monday that many areas remained unsafe, adding that it has eight boats ready to go should conditions improve.
In terms of evacuations, flooding has -- and continues to -- affect more people in the region, thanks largely to the residual effects from the melting of massive winter snowpacks. About 200 residents were ordered out Friday night, and then allowed back in the next morning, due to the spill. That's significantly less than the hundreds who have been forced from their homes due to persistent, far-reaching flooding in the area.
"We haven't seen a lot of issues around here (due to the spill)," said Nathan Hammond, who works at the Yellowstone River Campgrounds in Billings. "But there has been a lot of flooding, that has been a big issue."
Still, ExxonMobil reported in a statement that it had received 94 calls to its community claims line as of Monday night -- 36 of those coming from landowners reporting oil had washed up on their property.
The current continued to move swiftly Monday down the Yellowstone, carrying with it all sorts of debris, like a tree 30 inches in diameter that Schweitzer spotted. While no official determination has been made on what caused the pipeline rupture, the governor said it is easy to see the dangers posed by fast-moving waters. (Any pipelines built now, he noted, must be buried under the riverbed in order to prevent such incidents.)
By Monday, water levels near Billings had dropped to 12.14 feet, putting the Yellowstone once again below flood stage. This helped, in that fewer brush and logs were being swept into the river. But receding waters also left behind pools with "heavy oil," Schweitzer said.
The governor called it premature to minimize the spill's potential impact by saying, for instance, that it has "only" affected 10 miles of the river. His fear is that the abundance of fish, birds and animals in the area, which is about 100 miles downstream from Yellowstone National Park, may be hurt in the short and long term.
"My biggest concern is those 1,000 barrels," Schweitzer said. "You cannot dump (that much oil) into a pristine trout stream without causing damage to the fisheries."
Besides the fish, the area is home to Canadian geese, ducks, ospreys, otters and bald eagles, said Charles Preston, an ecologist and conservation biologist who heads the Draper Museum of Natural History. The birds, in particular, might die directly or indirectly as they go after fish. Long term, and depending on factors like the quantity and nature of the leaked oil, toxins may kill critical insects -- which, in turn, could have a trickle-down effect on the "multimillion-dollar fishing industry," he said.
Preston added that, beyond the short-term consequences, "it could take years to really understand the impact of the spill."
The Yellowstone is the nation's longest river without a dam -- a plus for the spill clean-up efforts because the continuously running water more easily breaks up the toxins, but a negative because any ill effects could spread into the Missouri River and its other tributaries, Preston said.
More than 48,000 feet of absorbent boom and 2,300 absorbent pads had been used as of Monday night to soak up the oil, while "vacuum trucks" and tankers have been positioned nearby to transport them from the scene, according to ExxonMobil. The company said the air quality and municipal water systems are also being monitored, while planes are routinely flying over the river to help detect patches of oil.
David Eglinton, an ExxonMobil spokesman, said Monday that the company is committed to staying the course and studying the effects through the whole course of the river. That includes checking water quality as far downstream as Miles City (144 miles) and Glendive (222 miles) from the original spill site.
"We will stay there as long as it takes," he said.
Schweitzer, too, promised that the state government agencies likewise wouldn't rest until the clean-up is complete and all related issues -- including the cost of the effort, a tab he said should go to the oil companies -- are resolved.
"Everybody in Montana will work hard until this is done," the governor said. "We'll be on it like a stink on a skunk."
CNN's Matt Smith contributed to this report.