Lake Drawdown Defined

Many questions were raised recently about lowering the Lake in the winter. The following information was taken from an Environmental Fact sheet published by the NH Department of Environmental Services. Remember that “lake management is lake specific” and what is right for one lake could be harmful to another. The key is to understand the biological identity of your lake or pond before developing a management program.

What is drawdown?

Lake level drawdown and the subsequent exposure of sediments to prolonged freezing and drying is an inexpensive means of aquatic weed control. By exposing the sediments to prolonged freezing and drying (35 to 50 % of the bottom area should be exposed), some rooted plant species are permanently damaged and the entire plant, including roots and perhaps seeds, are killed if exposed to freezing for several weeks.

Effectiveness

The effectiveness of a winter drawdown is dependent upon a deep frost and complete dewatering of the sediments. These conditions may not occur with heavy snow cover or milder, rainy winters. The technique is species-specific. Some species of aquatic weeds will not be affected and some species will increase in abundance. Freezing and desiccation are required; wet, cold lake sediments, or wet sediments covered with snow may have little negative effect on plants. Drawdown should be alternated every two years with no drawdown, so that resistant species do not become firmly established.

Advantages of Drawdown

-Inexpensive means of aquatic weed control. -Absence of machinery or toxic chemicals. -Improvements to docks, dams and swimming areas can be made.

Disadvantages of Drawdown

-Possible algae blooms after reflooding, due to nutrient release from sediments. -Reduction in diversity and abundance of benthic invertebrates that are essential to fish and waterfowl diets. -Consolidation of fish population into a more centralized area of deep water, making them more susceptible to overharvesting. -Oxygen in remaining pool can be depleted leading to a fish kill. -Amphibians and other fauna of the littoral zone may exhibit great changes in species composition and density. -Early Spring herbicides may need to be applied if weeds persist and begin to sprout.

Summary

Water level drawdown is an effective technique for at least the short-term control (1-2 years) of susceptible aquatic weeds, and can be accomplished at low cost without the introduction of chemicals or machinery. However, this technique is species specific and requires careful identification of the target plants before drawdown to avoid rapid establishment of resistant species.

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