A dam that helps preserve Lawrence’s supply of drinking water while pumping up production for a hydroelectric power plant behind City Hall is poised to get a $1 million face-lift.

But the maintenance project may be only the beginning.

Consideration of the potential for more work — or even building a replacement dam that could cost up to $40 million — will be getting a hearing during the coming months, as city officials grapple with balancing the community’s needs with the challenges of keeping a 135-year-old dam in service.

The discussion comes as federal regulators are warning that the existing dam “is in imminent risk of failing,” and that recent efforts to boost the Kansas River level only increases the likelihood of a failure.

“It’s unlikely that that would happen, but I think we need to talk about different contingencies,” said David Corliss, Lawrence city manager.

The issues are gaining plenty of attention at Bowersock Mills & Power Co., which owns the dam and uses it to manipulate the level of the Kansas River. Higher river levels boost efficiency both for intakes that serve the city’s Kaw Water Treatment Plant and for turning Bowersock’s own hydroelectric turbines.

Just last week, the company and its employees finished replacing 16 of the 65 doors that line the top of the dam, restoring functionality to a system that had been down since April 2007. The doors, when raised, can lift the water level immediately upriver by about four feet.

But officials from the Federal Energy Regulatory Commission, in a letter written Friday, told Bowersock to bring the boards back down, at least until some concerns about the dam’s structure could be addressed through repairs.

While Corliss said that

$1 million in repairs were being scheduled for completion by the end of this winter, Bowersock leaders worry that the river might not come down to a level that would allow repairs for another two, three or five years.

“For us as a business, we have to be thinking: We have to batten down the hatches,” said Sarah Hill-Nelson, Bowersock’s owner and operator. “I don’t know exactly how we’ll be able to handle this. It’s extremely detrimental.”

Hill-Nelson wants the city to invest $100,000 or $200,000 into building a temporary dam just upriver from the dam’s northern end so that the repairs could be conducted properly and that more comprehensive inspections could be conducted.

Corliss said that anything outside of the $1 million in upcoming repairs — which includes filling known gaps in the dam with grout, while the river is up against the existing dam — would be considered as part of the city’s overall needs for water and sewer systems. The dam fits into such systems because it helps collect water from the river, then pumps it from the city’s Kaw Water Treatment Plant into the pipes that flow into homes and businesses in town.

The city already is working on plans to improve the river intakes that serve the plant, work that Corliss estimates will cost at least $5 million.