This is an archived article that was published on sltrib.com in 2011, and information in the article may be outdated. It is provided only for personal research purposes and may not be reprinted.

BILLINGS, Mont. • State and federal officials plan to start pushing hundreds of bison back into Yellowstone National Park this week to make way for cattle that graze on surrounding lands in Montana.

A helicopter is scheduled to assist in the effort beginning Tuesday in the West Yellowstone area. Officials want to prevent any contact between bison and cattle to guard against the spread of an animal disease.

Yellowstone's bison periodically attempt to migrate outside the park in search of food at lower elevations during winter. But for the last two decades that migration has been blocked to prevent the transmission of brucellosis, a disease that causes pregnant animals to prematurely abort their young.

On the north side of the park, about 800 bison that were captured during that migration continue to be held in government-run holding pens near Gardiner.

Many of those animals could be released back into Yellowstone this week, said park spokesman Al Nash.

Officials said the success of their operations on both the west and north sides of the park is dependent on how quickly snow inside Yellowstone melts. Unless that snow is gone, exposing the vegetation that bison eat, animals that are driven into the park could turn around and leave again.

"There's a still a lot of snow, and once we get past a lot of snow, there's going to be a lot of water," Nash said.

State and federal agencies gave more leeway to migrating bison this year than in the past, allowing hundreds of the animals to linger in areas where they once faced capture and shipment to slaughter.

But that increased tolerance has been criticized by Park County commissioners and livestock groups who say hundreds of roaming bison caused property damage and threatened public safety.

A lawsuit seeking to restore restrictions on bison movements was filed Friday by the Park County Stockgrowers Association. A second lawsuit is expected to be filed this week by Park County officials, County Attorney Brett Linneweber said.

County Commission chairman Randy Taylor said agencies were violating a 2000 federal-state agreement by allowing bison to roam freely.

"We want the Park Service and everyone to go with the agreement that we had, not turning them all loose," Taylor said.