Water Level Manipulation

Dewatering / Drawdown

Reduces water level to control exposed plants and improve substrate to support plant diversity


  • 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


  • 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