NASHUA – You’re not imagining it: Despite all the recent rain, the Nashua River is disappearing downtown.

Not disappearing entirely, and only temporarily. After weeks of delay caused by the wet weather, preparations for work on the Jackson Dam began Thursday, with the gradual lowering of the water level between Jackson and Mine Falls dams.

“I noticed it right away,” said Nick Bower, who lives on Nevada Street in Nashua. Bower overlooked the river Thursday afternoon while smoking a cigarette on a break from his job at the Peddler’s Daughter. “You can see a lot more of that bank.”

Over the next week or so, the level in the downtown river will drop 8-10 feet as water flows through the outflow pipe underneath Margaritas Mexican Restaurant, without being replenished from further upstream.

The water is being removed to expose much of the dam so it can be lowered and altered as part of the Cotton Mill Square project, upstream on the opposite side of the river from Clocktower Place.

That project will turn a former mill building into more than 100 units of
market-rate and subsidized low-income housing. Lowering and altering the dam will make the development eligible for federal tax credits by removing it from the 100-year floodplain.

Under current plans, the dam work will be finished by Oct. 1.

The river was supposed to be lowered in late June, but continued heavy rain has kept water levels so high that the work was repeatedly put off.

After the level of the Nashua River downtown is reduced, in a week or so, work will begin on the dam.

Over the next couple of months, the top 7 feet of the concrete dam will be removed, 2 feet will be replaced in better condition, and adjustable “crest gates” that are controlled by inflatable bladders will be placed on top to reduce the chance of upstream flooding.

The Jackson Dam was built in the 1830s and upgraded many times to power Jackson Mills, where Margaritas is now housed.

Nashua bought the dam in the 1980s, and it has generated hydropower since then, with part of the river flowing through a long tube that has a turbine at one end.

That tube is usually mostly underwater but will become visible as the water is lowered. A smaller bypass tube, designed to provide turbulence to help guide salmon up a fish ladder, can be seen above the usual river level.

The crest gates, controlled by inflatable tubes, allow much more control to deal with flash floods and should reduce the chance of water levels rising upstream, including near the Cotton Mill development.

Similar crest gates exist on the large Merrimack River dam in Lawrence, Mass., and have produced debate on a large dam in Lowell, Mass., where opponents decry their appearance in the Lowell National Historical Park.

David Brooks can be reached at 594-6531 or dbrooks@nashua
telegraph.com. Also, follow Brooks on Twitter (@GraniteGeek).