The federal Bureau of Reclamation, which manages hundreds of dams in 17 Western states, has closed the dams to public tours.
Among the dams affected are Flaming Gorge on the Green River in Utah, Glen Canyon on the Colorado River near the Utah-Arizona border, and Hoover Dam on the Colorado near Las Vegas.
The closures result from fears of terrorism, prompted by the "orange" alert issued last week by Homeland Security Secretary Tom Ridge. An orange alert means the terrorist threat is "high," one step from "red" alert, issued when a terrorist attack is deemed imminent.
To protect the dams, Reclamation officials keep visitors away from areas where terrorists could disrupt the dams' operations. Parking on or near the dams will be prohibited.
"We want to make sure that water deliveries take place as scheduled, that power is generated and delivered as scheduled, and that water quality is maintained," said Barry Wirth, spokesman for the bureau's Upper Colorado River office.
Wirth said the possibility of a terrorist destroying a dam to create a catastrophic flood is minimal.
Although tours of the dams will be curtailed, most visitors centers will remain open. Wirth said the centers have been modified to provide more photographs and videos showing the dams' internal operations.