On the surface of Lake Byllesby, anglers cast lines into calm, walleye-stocked water. But vibrating 57 feet below, there’s trouble.

In the basement of the 103-year-old dam that formed this lake near Cannon Falls, Minn., hulking blue turbines and generators — lined with rust — whir and churn, and they could fail at any moment.

Byllesby is one of hundreds of dams in Minnesota that are decades past their 50-year life span and that need emergency — or just constant — repair. And as costs mount, the question is rising: are these dams worth it?

Nationally, dam removals have multiplied sixfold since the 1980s, to an average of once a week. Ecologists champion this solution, which can help restore native species.

But for some local governments, demolition isn’t an option. Dams supply communities with drinking water or, like Byllesby Dam on the Cannon River, recreation and hydropower — a renewable energy source.

Either way, it’s getting more costly to deal with the aging infrastructure. The average U.S. dam is 52 years old, according to the American Society of Civil Engineers, which gave them a “D” grade in last year’s national report card.

“There’s an increasing risk of them failing and that becomes expensive in a lot of ways. You have downstream loss of life and infrastructure that’s destroyed,” said Luther Aadland, a river ecologist with the Minnesota Department of Natural Resources (DNR) who wrote a book on removing dams. “That’s something that, nationwide, is going to have to be evaluated more and more because we have more of these dams that are falling apart.”

In Dakota County, which has one of the older and larger dams in the state, officials plan to trade deterioration for major savings.

County leaders hope to save millions by updating the creaky Byllesby turbines and potentially making them the power source for every single one of their parks and buildings, from Hastings to Apple Valley.

A teardown?

The future of the dam — and of the lake — was in jeopardy in the 1960s. The dam’s owner, then Northern States Power, considered tearing it down.

Neighbors, especially those with a slice of lakeshore property, were outraged. They pushed the surrounding counties — Dakota and Goodhue — to buy the dam.

After decades of joint ownership, Dakota County took over the dam in 2010. Around that time, dam owners were again confronted with whether to keep it after federal regulators said that it would not withstand a massive flood and that it needed major upgrades.

By then, county parks existed on both sides of the lake. Removal was no longer an option.

“The costs of doing that would be so detrimental to property values and to investments both sides have put into it,” Dakota County Commissioner Mike Slavik said.

In other communities, dams no longer serve a purpose and have become liabilities.

In Montevideo, a dam that blocked the Chippewa River was costing more to maintain than it was worth, City Manager Steve Jones said. Floods kept forcing repairs, and the city was worried after hearing about deaths at other dams across the state.

“We got to thinking, ‘Why are we keeping rebuilding this dam, and do we really need this dam?’ ” Jones said.

In 2012, the 64-year-old structure was torn down. That cost $350,000 and was covered by state and federal grants.

It removed what was long a choke point — for fish and people.

“The day it was open, there were kayakers there,” Jones said.

At Byllesby Dam, officials opted for a $5.8 million overhaul, including adding a spillway with concrete gates that can release large amounts of water when necessary.

But the dam’s hydroelectric machinery did not change.

“We have this 100-year-old turbine that at any moment could fail. Then we’re out the power portion of it,” said Josh Petersen, a county water resources engineer.

Water is pushed through huge blue tubes that could double as water slides. The flow creates enough energy to power roughly 1,000 homes, Petersen said. The county sells that to Xcel Energy. With more efficient turbines, the dam could generate enough electricity to power twice as many homes, he said.

The county recently hired consultants to evaluate how to maximize the sale of hydroelectric power. Last month, they suggested upgrading the equipment to produce more electricity and using that to power the county’s various offices and parks.

It would be an uncommon program and a big cost saver for Dakota County, which spends $1.58 million on electricity each year, according to county documents.

Officials are just starting to discuss the idea with their current energy providers, Xcel Energy and Dakota Electric. To save money, the consultants suggested that the county use the utility companies’ power lines and substations to distribute the electricity.

Building boom

Byllesby Dam is far older than the majority of the state’s dams, the bulk of which were built between 1960 and 1979, according to the U.S. Army Corps of Engineers.

In the next 15 years, Minnesota will face an unprecedented number of dams reaching the end of their 50-year life spans. While the structures can safely operate long after they reach 50, dam owners must follow the motto of the National Park Service Dam Safety Program: “Maintain ’em or drain ’em.”

And maintaining ’em is an expensive task.

This year the Minnesota Legislature channeled $6.5 million to dam removal, repair and reconstruction.

The DNR keeps getting more requests for help from local governments that manage dams, said Jason Boyle, state dam safety engineer. Every two years, the DNR sends the Legislature a list of needs. That list grew from 73 to 108 requests over the past eight years.

Some projects linger on the list for years.

“Obviously there’s not enough money to replace everything at once,” Boyle said.

When possible, the DNR champions removing dams, even those that generate electricity.

“While hydropower is often viewed as ‘green’ or ‘clean’ energy, hydropower has unseen detrimental impacts associated with the turbines in addition to general dam-related effects,” Aadland says in his book, “Reconnecting Rivers: Natural Channel Design in Dam Removals and Fish Passage.”

Turbines can kill migrating fish. Decaying organic matter in dam reservoirs releases methane that may exceed fossil-fuel greenhouse-gas emission limits, Aadland said.

But the publicly owned dams that generate funding, through hydroelectricity or by providing a water supply, are not often removed, said Mark Ogden, project manager for the Association of State Dam Safety Officials.

“Those owners are definitely, for the most part, planning and budgeting for whatever needs there are,” Ogden said

At Byllesby Dam, Slavik said, the county hopes the increased cash flow from hydroelectricity will allow for future repairs. “The county is committed to the dam.”