"It was built in the early 1900s by a method we call hydraulic fill, or sluicing. Essentially, they took a sand pile on top of a hill, and ran water over it and collected it between some dikes. They would wash the sand in and let it settle out, " he said, adding many early dams in Utah were built that way and are essentially sand piles.

"They have been shown to be problematic, especially in an earthquake. They have a very high potential for liquefaction," he said. Holmes also had extra seepage, which could lead to internal erosion of a dam that could cause it to break apart. In fact, Holmes had partially failed in 1924 and 1983, with some repairs made thereafter.

"What we did is went in and completely removed the dam and completely rebuilt it. Interestingly enough, the bulk of the construction was out of the same material," but modern construction techniques provide better compacting and density of material, plus drains to control interior seepage, Marble said.

Other repairs

Another dam of highest concern was Logan First Dam. Its old spillway could pass only 1 percent of the potential maximum flood expected there. The U.S. Army Corps of Engineers say dams should pass 100 percent of such flooding and consider it an emergency situation when a dam can pass only 50 percent. (If flood waters go over the top of a dam, they often cause quick, extreme erosion and failure.)

While state inspections had for years urged upgrades, the dam's owner, Utah State University, repeatedly said it could not afford them unless the Legislature came up with money. It finally did through the upgrade program it passed for all dams in 1990. Logan First Dam now meets all minimum standards, Marble said.

Also, Mountain Dell Dam in Parley's Canyon was long one of the biggest concerns in the state and had an initial risk rating score of 250. Salt Lake City Public Utilities Director LeRoy W. Hooton Jr. remembers how that dam scared officials during heavy snowmelt runoff flooding in 1983.

Officials figured they came within an hour of having water go over the top of the dam that year (which could easily lead to quick erosion and failure). It was saved only as they released as much water as they could through outlets and spillways, even though that flooded downstream areas and turned 1300 South Street into a long "sandbagged river."

"We couldn't take a chance of overtopping the dam, but we still got awfully close," he said about that year when the mountains went from "freezer to furnace" almost overnight to produce record snowmelt runoff flooding. He said vibration at the concrete dam was so intense when all outlets were open that officials feared it would cause cracking.