ELECTRA LAKE – Aging Stagecoach Dam, a timber-crib and rock structure that served as a safety valve, has been razed in order to build a concrete dam and spillway.

“It outlived its useful life,” Alfred Hughes of Xcel Energy said last week as he watched an ASI Constructors Inc. crew prepare bedrock to receive the foundation of a new structure.

Stagecoach Dam – rebuilt in the 1980s – showed evidence of seepage. It was an unsettling sign but not the threat to human life that some of the nation’s other 84,000 regulated dams are, Hughes said.

Dams are classified as high-hazard if their failure will cause death downstream. In Southwest Colorado, 11 dams, including Stagecoach, which holds back Electra Lake just south of Durango Mountain Resort, have this classification.

A dam is regulated by a public agency if it meets one of three criteria – stores more than 100 acre-feet of water, has a surface area of at least 20 acres or is 10 feet or more in height.

The average age of dams across the country is 52 years – the oldest dating to 1931 – with many needing prompt attention, said Keith Ferguson, an engineer with Denver-based HDR One Co., and incoming president of the United States Society on Dams.

The American Association of Civil Engineers gives the nation’s dams a letter grade of D overall, Ferguson said in a phone interview. Colorado dams, however, are doing fairly well, he said.

Dams are classified as high-hazard, signficant-hazard or low-hazard, Ferguson said. Four thousand of 14,000 high-hazard dams are deficient, and the number is on the rise. High-hazard means that if a dam fails, at least one person would be killed.

It would cost $21 billion to bring the 4,000 deficient dams up to snuff, Ferguson said.

Dam failure is not unheard of in Colorado.

On Aug. 3, 1933, Castlewood Canyon Dam in Castlewood State Park failed, releasing water into Cherry Creek and flooding Denver 30 miles away. Two people were killed.

On July 15, 1982, Lawn Lake Dam in Rocky Mountain National Park failed and flooded downtown Estes Park. Three people lost their lives.

Federal dams such as those at Vallecito Reservoir and Lake Nighthorse are regulated from Washington, D.C. States regulate other dams, which in Colorado means the Division of Water Resources. The Federal Energy Regulatory Commission becomes involved in dams connected to hydroelectric generation, including the two dams that hold back Electra Lake, Stagecoach and Terminal.

FERC and the state inspect the Electra Lake dams, said Matt Gavin of the state Division of Water Resources office in Durango. FERC covers a broader range of issues, he said.

Only a couple of Southwest Colorado dams have known structural issues.

As a result of the seepage at Stagecoach Dam, the Division of Water Resources required the level of water behind the dam be maintained 4 feet below full.

In Montezuma County, the dam at Totten Reservoir, built in 1965, is kept 5 feet below full because of a crack in the upper embankment.

The term high-hazard doesn’t refer to the structural integrity of a dam, but to the danger to people if it failed, Gavin said. Stagecoach Dam never spilled seriously but is listed as high-hazard because it is part of an overall high-hazard project, he said.

Gavin typically inspects 60 to 70 dams a year in a territory that stretches from the western half of the San Luis Valley to the upper Dolores Basin.

Here is a breakdown of major dams regulated in Southwest Colorado:

Vallecito, Lemon, Ridges Basin, McPhee and Jackson Gulch, all high-hazard federal dams.

Narraguinnep, Totten, Summit, Stagecoach, Terminal (Electra Lake) and Durango Terminal, all high-hazard regulated by the state.

City Reservoir No. 1, a low-hazard dam in the Weminuche Wilderness, is regulated by the state.

Also, Navajo Dam, which is in New Mexico although much of the reservoir is in Colorado, is a high-hazard federal dam.

“We like to look at foundation work,” Gavin said as the ASI Constructors crew worked. “They shape the foundation surface with concrete and slush grout to make a uniform surface.”

When inspecting dams, Gavin said, he looks for signs of structure damage, sinkholes, cracks, obstructions in spillways and seepage. A little seepage is normal, but cloudy water indicates a potential problem because it shows that dirt is being carried away.

Gavin said with the completion of the Stagecoach Dam and spillway, Electra Lake will be in top-notch shape.

Electra Lake, which has a capacity of 23,324 acre-feet, is filled by diverting water from Little Cascade Creek, a tributary of the Animas River. At the south end of the lake, Terminal Dam releases water for Tacoma Station on the Animas River. Stagecoach is on the north end and would release water only in an emergency.

“We really weren’t worried about Stagecoach failing because we control the inflow,” Hughes said. “But the lower water level means less flow for Tacoma Station.”

Hughes is superintendent of Xcel’s hydroelectric plants at Tacoma Station just downstream from here and in Ophir and Salida.

Tacoma Station isn’t operating because the water level here was dropped 26 feet to allow work on Stagecoach. Hughes said the water loss is the equivalent of 4,800 acre-feet.

Mike Preston, director of the Dolores Water Conservancy District, which owns Totten Reservoir, said the district can live with the water-level restriction. The reservoir has a capacity of 3,000 acre-feet.

The crack in the dam abutment hasn’t been fixed for lack of funding, he said.

“It would be expensive, a $500,000 proposition, to get the reservoir ready for full capacity,” Preston said.

Bill McCormick, chief of the dam safety branch of the state Division of Water Resources, said no regulated dams in the state failed in the 2010-12 period.

But the division’s annual report said 14 dam-safety incidents were recorded involving seepage, embankment settlement and excessive upstream slope damage.

The state inspects high-hazard dams yearly, McCormick said. Dams that present significant hazard are inspected every other year and low-hazard dams every six years.

About 2,900 dams under division jurisdiction are in a database that keeps track of scheduled inspections. Inspections generally are done from May to October to avoid snow.

“The technology to create safe dams is there,” Ferguson said. “What is needed is funding and commitment.”

daler@durangoherald.com

An original version of this story had an incorrect name of the organization that Keith Ferguson is going to head.