Grouley, the acting facilities manager for the Glen Canyon Dam on the Utah-Arizona state line, said the plumb line is measured periodically from fixed points to determine if the concrete structure is shifting.

At large federal dams, so-called "tenders" - as the name implies - watch over the structures.

They rely on plumb lines and a sophisticated array of monitoring wells and strain and compression gauges. They also conduct routine soil, hydrological and concrete testing.

At Glen Canyon, just across the Utah line in northern Arizona, technicians can plug in instruments into the workings of the dam to determine internal stresses on the structure. Many of the monitoring systems were built into the dam during construction in the early 1960s.

The deadly Teton Dam collapse in southern Idaho in 1976 led to the creation of a central Dam Safety Office for the U.S. Bureau of Reclamation, the chief overseer of federal dams.

Reclamation now monitors 350 high-hazard dams across the nation. Fifty of those are in Utah.

High-hazard dams are inspected yearly and in more depth every three and six years. All information collected goes to the Dam Safety Office in Denver.

"We¹re the technical backbone of Reclamation," said Brian Becker, acting chief of the Denver office.

mhavnes@sltrib.com