Buried in Mt. Tom: For nearly a century, reservoir has served Eau Claire | Front Page

EAU CLAIRE – Buried within Mt. Tom for nearly a century, a vast reservoir remains vital to providing water to most of Eau Claire’s population.

At the top of the mountain in the neighborhood of North Side Hill, there are benches for those tired from the climb, a few small huts, and security fences around a grassy area.

This protected turf toupee is a clue to what lies beneath—a 3.2-million-gallon reinforced cement vessel that has been pressurizing a growing city since 1925.

“It’s very important. It’s the heart of it,” said Mark Nelson, Eau Claire Waterworks Manager.

Mt. Tom’s elevation — its summit sits at an elevation of 1,025, which is more than 100 feet higher than streets and neighborhood blocks around it — is why the reservoir is as useful today as it was in the 1920s.

“Mount Tom is holding the pressure,” said Cole Cloutier, the city’s utility engineer.

A map of the city’s water pressure zones shows that Mt. Tom still generates the necessary pressure to service most of Eau Claire. On the outer edges of the city, where houses and shops were built in hilly areas during the development of Eau Claire, there are further reservoirs, water towers and pumping stations to ensure these zones have the right water pressure.

Should Mt. Tom Reservoir go offline for any reason, including the decennial inspection when the reservoir is drained, there are other water tanks in Eau Claire to shoulder the workload. The city’s largest water tank — a 5.6-million-gallon standpipe on Rudolph Road — can support Mt. Tom.

Mt. Tom Reservoir is the oldest water tank in the city’s system still in use. Others, including water towers and standpipes, were built between 1952 and 1998, according to an annual report from the city’s water company, Eau Claire.

But the condition of the reservoir belies its old age.

“It’s really amazing the shape it’s in,” Nelson said.

When it was drained for a full inspection in 2017, the interior proved amazingly well preserved. Photos from this inspection show a white paper label from the construction crew from the 1920s still firmly attached to the ceiling and legibly printed on it.

Although over the decades some of the pipes leading to the reservoir have been replaced and more modern gauges installed, the concrete structure itself has not required any work since its construction.

This is in contrast to metal water towers and standpipes that need to be repainted and repaired or have their internal coating replaced. Over the next five years, the city estimates that it will complete two $1.4 million maintenance projects at other reservoirs and water towers. But during this time, $0 worth of projects are planned for Mt. Tom reservoir.

“It’s an amazing water tank because it doesn’t require a lot of maintenance,” Nelson said. “He just lies there and does his job.”

The storage facility passed the last inspection with flying colours.

In the early 20th century, as Eau Claire grew, there were discussions about improving water supplies and fire safety.

The addition of the reservoir to Mt. Tom was seen as a key element in this.

The project took a long time to plan, as evidenced by the March 1918 date on the original designs for the reservoir, designed by city engineer JT Hurd.

In 1924 all planning came to a head.

An April inspection by the National Board of Fire Underwriters found that Eau Claire’s water pressure dropped significantly when fire hydrants were opened, according to an article published in the Eau Claire Leader newspaper.

“Generally speaking, the proposed new reservoir on Mt. Tom should be built as soon as possible since the city currently has no reserve storage,” wrote Harvey T. Munn, a water inspector, in his report.

In June 1924, Eau Claire City Council approved $165,000 in bonds for several projects to improve the city’s waterworks. The construction of Mt. Tom Reservoir and a long access tunnel for its plumbing was budgeted at $68,000 from the bonds.

A. Larson & Co. – a partnership between Andrew Larson and LG Arnold – was selected as the contractor for the project.

Arnold’s grandson, Dean Arnold, 74, has chronicled numerous projects involving his family’s Chippewa Valley business, including the reservoir.

Earlier this year, he created a video slideshow of Mt. Tom Reservoir, which can be seen on a TV at the Chippewa Valley Museum.

Dean Arnold, himself a retired civil engineer living in Chicago, plans to continue creating presentations on the many construction and infrastructure projects his grandfather helped design.

“These projects have an enduring legacy,” Dean Arnold said in a phone interview with Leader-Telegram.

Construction of the reservoir begins

According to an article by Eau Claire Leader, logging and clearing of the ground at the summit of Mt. Tom was underway on September 5th. Then came the excavation with a steam excavator shoveling slate off the mountain so it could be hauled away in horse-drawn carts.

An October 1924 article proclaimed that dynamite blasting, machinery and horse-drawn carriages “…made a most noticeable dent in the face of nature at the summit of Mt. Tom.”

Within two months the excavation was complete, containing 14,000 cubic yards of shale and sandstone, the Eau Claire Leader reported in November 1924.

Next, the reservoir’s concrete floor was poured—a foot-thick concrete slab reinforced with metal.

“The residents of Eau Claire have little reason to worry about any potential dangers that might pose from the water storage facility at Mt. Tom, given the imposing strength and solidity of the structure,” the newspaper proclaimed on November 12, 1924.

That same month, work was completed on the 220-foot tunnel bored into the side of Mt. Tom. Large pipes were laid here to connect the reservoir to the city’s water system.

As winter approached, the reservoir continued to take shape and provided an interesting sight for those who saw a photo of it in the 7th December 2024 issue of Eau Claire Leader. The dam had already been poured by this time, creating a deep, circular structure on Mt. Tom.

“No, this is not the amphitheater of the ancient Colosseum in Rome, nor is it a boxing arena or an indoor track under construction,” the newspaper jokingly wrote.

A cold snap around Christmas with a few sub-zero days put the project on hold briefly, but crews returned to the site on December 26, 1924.

On January 21, 1925, the final wooden forms for the pillars and roof were finished, but the weather was not yet warm enough to pour concrete. But that all changed later and before the end of March the last bucket of concrete had been poured to complete the roof.

With the project thus virtually complete, newspaper reports documented the rest of the spring on the completion details. This included removing the wooden forms, adding earth around the wall of the reservoir, and more piping work to connect them.

An August 1925 article about a fire at the Gillette Rubber Co. factory noted that Mt. Tom Reservoir was operational at the time.

Mentions of the reservoir surface in the following months as more of the city’s water supply systems connect to it.

An October 29 article titled “Large reservoir on Mount Tom shows value” explained how the water pressure used to fight fires has doubled since the reservoir began operating.

And on December 13, 1925—a Sunday—Mt. Tom reservoir became the only source of water for Eau Claire for most of the day. The city’s main supply was shut down for 19 hours in order to install new, larger pipelines, so the reservoir was taken over during that time. During that time, the reservoir’s water level dropped just 3 feet, leaving 2.75 million gallons in the tank, according to an article by Eau Claire Leader. Water company officials said they were pleased with the reservoir’s performance and that it was a good test of how useful it would be in an emergency.

In fact, construction of the Mt. Tom Reservoir was under budget by about $10,000 — money the city used for additional water projects.

Nelson and Cloutier are among the few people to have set foot on the reservoir since it was completed in 1925.

Both were in there when an independent engineer inspected it five years ago.

“It feels like you’re in a big cave,” Nelson recalled.

The circular reservoir is 160 feet in diameter and its walls are designed to be 22½ feet high. The floor slopes toward the center, where a 12-inch wide pipe connects it to the rest of the city’s water supply.

Prior to the inspection, the water was first drained through equipment at the city’s waterworks, lowering the overall level in the system. The remaining water is then channeled from the reservoir through a temporary chute on the side of Mt. Tom and ends in a nearby storm sewer.

A worker is lowered through a hatch to secure a ladder for the others to follow into the dark interior.

The walls are black from decades of manganese – a naturally occurring mineral the city filters out of its water due to discoloration. Even with the brightest flashlights they bring, Cloutier said it’s still difficult to see due to the dark environment.

“It’s really neat,” he said.

Photos from the inspection show the rows of dark pillars dimly lit by workers’ flashlights. At the foot of some of the pillars, the names of workers from the 1920s are carved into the cement as it dries.

While the modern workers were in the empty reservoir, they kept their voices low.

“You don’t talk loud during these tank inspections,” Nelson said.

Even if you’re talking to someone on the other side, he says, a normal conversational volume will do. Anything louder and the echo in the tank makes it hard to understand what you’re trying to say.

After the inspection, workers get out, seal the container tightly and fill it up again – so it can go on with its work unnoticed by passers-by.

Newspaper articles from the 1920s even noted that the reservoir’s aesthetic impact on Mt. Tom was due only to the loss of some trees and the ground being about four feet higher than before.

Though unseen, what is buried there continues to change the lives of the residents of Eau Claire, whether they realize it or not.

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