Considerations:

• Must have control structure to lower water level
• Federal, state, local permits may be required
• May require temporary variance from established federal and state water level regulations
• Need reasonable assurance that water body can be refilled after drawdown is complete
• Oxidizes and compacts sediments, creating more suitable native plant habitat
• Best to draw down in winter
  • Summer rains and humidity reduce ability to draw down water body and desiccate plants
  • Kills target plants through desiccation, freezing and perhaps prescribed fire
  • Summer drawdowns may expand nuisance native plants into deeper areas of the water body
• Winter drawdowns may interfere with fish and wildlife habitat utilization and reproduction
  • Waterfowl utilization most abundant during winter
  • Fish spawn late winter / spring – may impact year class
  • Endangered species may lose habitat and forage
    • Snail kites nest in winter/spring – low water exposes nests to predation
    • Apple snails need water in winter/spring for reproduction
• Most appropriate for controlling overgrowths of native plants
  • Hydrilla resists drawdowns via underground turions (tubers)
  • Drawdown may allow hydrilla to colonize deeper water – persisting after re-flood
  • Desiccation assists water hyacinth seed germination upon re-flood
  • May need successive annual drawdowns to control hydrilla and water hyacinth
  • Torpedograss is tolerant to wet and dry soils and water depths up to 5-6 feet
• Difficult to get public support
  • Loss of recreation (boating, skiing) or associated businesses (fish camp) for several months
  • May result in property damage – collapsing seawalls
  • May reduce water supply for adjacent agriculture/aquaculture, power generation, drinking water, etc.
  • May add too much water to receiving areas like estuaries, already flooded areas, etc.
• Non-selective and will control most plants and animals in the dewatered zone
• May stimulate dense stands of transitional native plants i.e. pickerelweed or smartweed upon re-flood
• May trigger floating rafts of terrestrial plants that grew on exposed bottom, or buoyant sediments such as peat upon re-flood

Examples of Feasible Control:

• Winter drawdowns conducted every three years on Rodman Reservoir control hundreds of acres of water lettuce in areas inaccessible to harvesters and herbicide application equipment
• Winter drawdowns with prescribed fire reduce torpedograss density on Lake Okeechobee
• Winter drawdowns and disking soils enhance maidencane growth on Orange Lake
• Drawdowns and associated sediment removal in the Kissimmee Chain of Lakes offset sediment accumulation accelerated by stabilized water levels, creating more suitable emergent plant habitat

Dewatering / Lower Water Level

Reduces amount of water to be treated with herbicides or applies additional stress to plants

Considerations: Similar to Drydowns

• Reduce water volume by several inches to several feet to facilitate herbicide control
  • Reduces amount of herbicide necessary to achieve appropriate concentration
  • Creates storage to buffer against rainfall events that could flush herbicides out of the system
  • Retains treated water to prevent possible downstream impacts i.e. irrigated crops, fishery, etc.
  • Reduces cost of herbicide control

Examples of Feasible Control:

• Lower water level on Lake Istokpoga prior to large-scale hydrilla herbicide control to prevent discharging treated water that may injure downstream irrigated crops
• Reducing water volume in the Kissimmee Chain of Lakes reduces herbicide use by as much as 25% and saves millions of dollars  

Flooding / Increase Water Level

Increases water level to control plants below or flushes them out of system

Considerations:

• Need flood control structure
• Ensure that increasing water levels will not flood surrounding structures or developed property
• Flush floating plants (i.e. water hyacinth, tussocks, or floating islands) out of system or into uplands
• Reduce water level quickly to strand floating plants in upland portion of water body
• Reduce light penetration to stress submersed plants after herbicide application
• Flood controlled emergent plants to slow recovery after herbicide application or prescribed fire

Examples of Feasible Control:

• Increasing water level after hydrilla is stressed from herbicide application in Lake Istokpoga reduces light penetration, further stressing treated hydrilla as well as sprouting tubers and turions
• Re-flooding exposed marsh in Lake Okeechobee after herbicide application or prescribed fire slows torpedograss recovery and promotes germination of native plant seed bank
• Pulsing water level increase prior to a winter drawdown on Rodman Reservoir strands hundreds of acres of water lettuce in the uplands exposing them to desiccation and freezing