In a letter to Interior Secretary Doug Burgum and national parks officials, several Democrats and a Senate independent said that members of the administration could face fines and even criminal prosecution.
The health secretary is trying to restart the work of a panel that advises the government on vaccines, after a judge froze its decisions and prevented it from meeting.
Health Secretary Robert F. Kennedy Jr. during a House hearing in April. Last June, he fired all 17 members of a vaccine advisory committee and named new ones, many of whom share his skepticism of vaccines.
The Upper Colorado River on the Grand Canyon last month. About 40 million people and 5.5 million acres of cropland depend on the Colorado for drinking water and irrigation.