Candlewood Lake Drawdown

Every winter, the Candlewood Lake water level is lowered. This event is referred to as the “drawdown”. The Candlewood Lake Authority is often asked why the water level is lowered and why some years are deeper than others, who decides how far to lower the water, and when and how it happens. Like a lot of other things about Candlewood Lake, the drawdown is a complex process with many variables and many people and organizations having input.

 

Why is there a drawdown?

In a nutshell, the annual drawdown is done to control the invasive plants in Candlewood Lake. The main culprit is Eurasian Watermilfoil, which makes up the vast majority of the plant community in Candlewood. By lowering the level of the water every winter, the goal is that temperatures will get low enough to freeze the plants and their roots and prevent them from growing back as easily the following year. This means that the deeper the drawdown goes (more water removed from the lake and more shoreline exposed), more plants will be exposed to the air, and hopefully, more plants will be frozen and controlled for the next summer. However, when the water is lowered, it doesn’t automatically mean that every plant that is exposed to the air is going to be killed. There are a number of things that have to happen for the roots to get effectively frozen: the air temperature needs to get low enough and the soil has to dry out. A lot of snow can actually prevent an effective drawdown by insulating the plant roots from the cold air (this means the snow functions almost like a warm blanket for the plants!).

How does a drawdown happen? How do I know when it will happen?

There is a lot of water in Candlewood Lake that has to go somewhere else to lower the water level dramatically enough for the drawdown. Candlewood Lake is a “Pumped Storage Reservoir” owned and managed by the power company FirstLight. On the northeastern tip of the lake in New Milford, there is a large earthen dam and a canal leading to a very large pipe. That pipe stretches from the Housatonic River, up the hill to Candlewood Lake. Being a “pumped storage reservoir” means that Candlewood Lake is mostly filled with water that is pumped up the hill and stored in the lake using electricity when there is extra. When there is high demand for power, water is let back down through the pipe using gravity to spin a turbine and generate electricity. In the winter, when it is time for the drawdown, FirstLight starts letting water back out of the lake until they reach the depth range for that year’s drawdown. FirstLight controls when the water depth goes down, what depth it goes to, and when it comes back up. The specific timing decisions are made based on a number of variables, including, but not limited to: the weather and how much water is in the Housatonic River. By draining some of Candlewood into the river, FirstLight works hard to make sure they don’t flood downstream of New Milford – sometimes delaying when the lake level drops until the Housatonic River level drops to safe levels. Generally, the “drawdown season” stretches from Dec. 1 to the beginning of the next year’s fishing season (second Saturday in April). However, the specific dates are subject to change every year, which is why FirstLight always recommends removing docks and other items from the lake regardless of drawdown depth to prevent possible ice damage.

Why is the drawdown level different year to year?

So if that’s all true, why does the depth change every year? The drawdown is a really effective tool for managing aquatic plants because it exposes them all to the freezing air no matter what – but that means it also exposes everything else that lives in those shallow parts of the lake to freezing air too! Native plants, shellfish, and other critters that use that area for habitat and hunting all risk being killed by the drawdown the same way the drawdown kills Eurasian watermilfoil. To try to mitigate the impact on those helpful parts of the ecosystem, historically it’s been helpful to only do a deep drawdown every other year – giving those organisms a fighting chance to keep their population intact. The area of the lake that is exposed in both shallow and deep years is routinely devoid of almost all plants since the drawdown is such an impactful tool. However, if there was a deep drawdown every year, there would be substantially fewer plants in the lake – preventing fun sportfish (like our famous smallmouth bass) from having a place to reproduce and their young to survive being hunted. One other aspect to consider is that deeper drawdowns increase the rate of shoreline erosion in the lake. By exposing more lakebed to the air, you destabilize the sediment, and when the lake refills in the spring that destabilized sediment will fall off the shoreline and enter the water. This can contribute nutrients to the water that fuel algae growth, and harm the structure of the lake’s shore. By switching the drawdown depths every year, the rate of this increased erosion can be slowed substantially.

 

Why is this so complicated?

It can be easy to forget just how complicated of an ecosystem a lake is; each part interacting with each other in a complex “dance”. So as lake managers it’s hugely important that we try to maintain a healthy balance of each of those parts whenever we think about impacting that ecosystem. The Candlewood Lake Authority (CLA) does not control the drawdown – FirstLight, with some requirements set on them by FERC (the federal agency responsible for overseeing hydropower projects), controls the depth, timing, and extent of the drawdown every year. However, we sit on a committee with the State of CT DEEP, federal agencies, and the lakes Zoar and Lillinonah Authorities (remember, the water from Candlewood eventually reaches Lillinonah and Zoar) to discuss the plan for the drawdown each year and advocate for what we feel is best for our lake based on the science and monitoring we do. One side benefit of the drawdown, and a common topic of discussion this year (2021), is that the drawdown is also very effective at killing adult zebra mussels exposed to the air in addition to the plants.

What is the plan for this winter?

In response to the discovery of a number of solitary adult zebra mussels in the lake this past winter, the Candlewood Lake Authority and CT DEEP recommended an “intermediate-depth” drawdown for this year (2021/2022), rather than the originally planned shallow drawdown. It was felt that this would not only further control the plants next summer, but would have the additional benefit of killing any zebra mussels in those depths exposed to the air.