More Studies Needed on Impact of Drawdowns

The following article by the late Ned Hatfield was published by Ossipee Lake Alliance in 2005. It is one of a series of articles we’re reposting to provide context for the August 25th public meeting at which the state will present its plan to replace the Ossipee River Dam. 

In a previous article in the “Ossipee Lake Report,” Bob Smart explained how the water level of Ossipee Lake is lowered each fall to reduce overflow at winter’s end due to ice melt and spring rains. Otherwise greater amounts of damage due to extreme high water would take place to shorelines and property. These are practical reasons for managing the water level, but governing lake levels through dam control is unnatural and begs the question of what effects there might be on the lake’s plants and animals? The purpose of this article is to consider the potential ecological effects and review available information.

Impact of Drawdowns
Limnological studies have shown that predictable associations of interacting plants and animals (ecosystems) develop in lakes under different physical and chemical conditions. With natural water levels, sediment types, temperatures and other properties, species adapted to these conditions survive. When conditions are changed, such as by manual control of water levels, species more tolerant of this stress are favored with detrimental ecological effects to the natural communities.

Species intolerant of desiccation, for example, are reduced or eliminated in those areas which are uncovered. Aquatic ecosystems are disrupted and many of the lost natural species produce more food than the species which take over. Their absence thereby eliminates or reduces the number of dependent microorganisms, invertebrates, fishes and wildlife. Reduced interconnecting food webs can affect fisheries in open and deep water as well.

Drawdowns directly affect the littoral, or Ossipee Lake Report shallow water, plant communities in a lake. Freezing or drying out of roots and the resulting compaction of the sediment kill most aquatic plants. The reduced plant productivity and number of niches (fewer plant species means decreased cover and food supplies) results in a decrease in the number of microorganisms and invertebrates. Release of nutrients from sediments can reduce future shallow water plant (macrophyte) growth.

As phosphorus is a nutrient in limited supply in most freshwater lakes, when it is released into the water it could cause phytoplankton overgrowth (blooms). This would reduce water clarity and, although temporarily increase oxygen supply through photosynthesis, the death and accumulation of these organisms could reduce oxygen levels through their decay.

Although Ossipee Lake has a “healthy” flushing rate, this could result in low oxygen levels at depth, and affect cold water fish populations (trout) and invertebrates. An exception to most macrophytes dying as the result of lowering the water level below the littoral zone, is the situation where the exposed sediments remain wet due to groundwater or other seepage, or excessive precipitation. In such a case, more plants might survive than expected and there would be less change to the littoral communities.

It would seem that this would be an exception in a northern latitude like New Hampshire, where normal precipitation would not keep shallow soils wet and freezing temperatures last long enough to solidify sediments to a depth where roots would be damaged. Mussels and insect larvae living in the littoral zone would be exposed to drying or freezing and to excessive wildlife predation. Decreased invertebrates means less fish food (small fish have been shown to feed on these and invertebrates on the microorganisms). Larger fish feed on smaller fish and therefore the biological productivity of the lake as a whole is decreased.

This in turn affects wildlife. Whether these effects would be long term or if productivity would return to levels prior to drawdown is not known. Other possible effects of winter drawdowns include reduction of brood-rearing cover for ducks or fishes, exposure of muskrat houses, or destruction of their food supply. Amphibians and reptiles from these shallow areas could be preyed upon, freeze, or be killed on roads while leaving their habitat. Natural aquatic food supplies for birds and wildlife might also be reduced. On a positive note, these could also help reduce cattail communities which could increase the eutrophication of the lake.

Vermont Study
Over the winter of 1988-89, a drawdown was conducted in Lake Bomoseen, Vermont. The legislature required a study of effects on fish, wildlife and aquatic plants. The number of exposed watermilfoil plants was decreased by 88% but there was no effect in deeper waters where most watermilfoil plants grew. (It seems this would also be the case in Ossipee Lake.) Five out of ten abundant native plant species decreased, but others remained stable or even increased. These resulting plant communities tend to become less productive monocultures and decrease cover for invertebrates and fish. Plant diversity decreased in the exposed areas by 44%, but lake-wide plant diversity did not change.

Frequent drawdowns result in a decrease in plant diversity, and species which tolerate the repeated drying out tend to become dominant. Therefore, it is possible that ecosystems could change to less productive and structurally simple habitats for extended periods of time and cause a decrease in species diversity of ecological or recreational (economic) concern.[1]

Timing of Drawdowns
One consideration is the early versus late fall timing of the drawdown. Although higher water for the summer and early fall means more boating, is this the best thing for the lake? Persons around Ossipee Lake in the 1950s remember low lake levels, even during some summer periods. The channel near Spindle Point has been known to be wadable at times. These naturally lower levels resulted in less shoreline damage from wave action due to boats or wind and, in addition, protected shoreline plant communities along the big lake’s southeastern shore peat layer, known to include unique plant species.[2] Unnaturally high water results in shoreline erosion above the normal water line.[3] Keeping the level too high during the fall could increase this process, especially in the parts of the lake most exposed to strong autumn winds.

Another reason to initiate drawdown earlier is to prevent lake trout from spawning in shallow water which otherwise later would be exposed. These fish need stable water levels from the time of spawning (possibly late October in Ossipee Lake) through the winter so their eggs will remain covered.[4] An even later drawdown, such as in late November, would expose already buried invertebrates and amphibians.[5]

A contrasting view suggests that if a drawdown “is conducted late in the season, especially after initial ice and snow cover, that usually provides insulating qualities to protect the near-shore habitat.”[6] It might be predicted, however, that dropping the water level after ice formation would cause shoreline damage with sediments and vegetation sticking to the ice as it tore away.

Slow drawdown seems favored by all the persons I spoke with in preparing this article. It provides plants with water for a longer period of time and a less sudden change with time to adjust physiologically to a state similar to dormancy. Mobile invertebrates would at least have the chance to “follow the water down,” but in at least one study mussels proved ineffective at doing this. “Most mussels moved in circles, with no specific orientation. Consequently, they did not move with the receding water.”[7]

What’s Needed
With all these considerations in mind, how should the annual drawdowns of Ossipee Lake be managed? This article is a preliminary collection of ideas and information. It would be additionally worthwhile to review the status of Ossipee Lake from recreational, economic and ecological points of view. Studies could be carried out to document the make-up of the lake’s littoral zone and the condition of recreational fish populations.

Among the questions that could be addressed are: How do the ecological aspects in the literature pertain to Ossipee Lake? How do the preferences of land or boat owners interact with the lake’s ecology? What is the best blend of management decisions to protect the majority of interests of all the stakeholders as well as the sustained health of the lake?

Ned Hatfield was a retired teacher and long-time resident of Freedom, also serving on the town’s zoning board. He died in July 2017. 

(1) The Lake Bomoseen Drawdown, Vt. Agency of Nat. Res., Waterbury, Vt.
(2) Hellquist, C.B. 1972. Vascular flora of Ossipee Lake, NH and its shoreline. Rhodora 74: 445-452.
(3) DES Env. Fact Sheet DB-16.
(4) Pers. com. Don Miller, NH Fish & Game.
(5) Pers. com. Jody Connor, NH DES
(6) Pers. com. Dave Courtemanch, MEDEP
(7) Samad F. & Stanley J.. 1986. Loss of Freshwater Shellfish After Water Drawdown in Lake Sebasticook, Maine. J. Freshwater Ecology, v. 3, No. 4, p 519 – 523.



  1. Bob Smart 6 years ago August 13, 2018

    Candlewood Lake in Danbury, CT is a “Pump Storage” facility built in 1930 by the electric company. The lake became infected with milfoil. T40 years ago the power company decided to try a deep drawdown of 10′ every other year and a 5′ during alternate winters in an attempt to freeze out the weeds. Only the 10′ winter with no snow before freeze up killed some of the weeds. The fish population continues to grow and Candlewood is one of the favorite locations for fishing derbies.

    Do not design for more than a drawdown to 404′ to avoid having my driven well go dry.

    As for Ossipee, both summer and winter mother nature plays games with the water heights. At least once during the winter temperature and rain will take the level from 404-5′ to over summer level of 407.25′. Spring is usually the most dangerous time with high spring melt and wind driven ice chunks. I believe the main reason for draw downs is to protect the dam structures and machinery so that it is ready for summer fun.

    The one fear I have of this project is that the new dam will allow a drawdown below 404′. Many of us rely on point driven wells. So far my well has been OK but I do not know how low the level has gone because the gauge bottoms out at 404′. There have been a few reports of dry wells from some residents.

  2. Steve Foley 6 years ago August 14, 2018

    Interesting points I’ve never considered.


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