Description of Method

Lowering the water level of a lake or reservoir can have a dramatic impact on some aquatic weed problems. Water level drawdown can be used where there is a water control structure that allows the managers of lakes or reservoirs to drop the water level in the waterbody for extended periods of time. Water level drawdown often occurs regularly in reservoirs for power generation, flood control, or irrigation; a side benefit being the control of some aquatic plant species. However, regular drawdowns can also make it difficult to establish native aquatic plants for fish, wildlife, and waterfowl habitat in some reservoirs. 

Lowering the water level in the winter exposes the sediment to both freezing and loss of water. Freezing can have a dramatic impact on aquatic plants (such as Eurasian watermilfoil or Brazilian elodea) that have no overwintering structures such as viable seeds, turions, tubers, or winter buds. Prolonged exposure to freezing temperatures is often fatal. Freezing of the sediments can also impact species like frogs, turtles, and invertebrates that may over winter in the drawn down area. Drawdowns may impact aquatic mammals such as beavers and muskrats.  

Lowering the water levels in the summer can expose the sediments to desiccation and high temperatures (depending on the climate). These conditions can also kill some aquatic plants.

Drawdowns that expose greater areas of sediment (and plant beds) will be most effective in controlling aquatic plants. However, if the drawdown does not occur on a regular basis, the plants will recolonize and reestablish in these areas. Be aware that the growth of some aquatic plant species may be enhanced by water level drawdowns. Know what species you want to control before selecting this method of control. Results from Vermont's Lake Bomoseen drawdown indicate that single winter drawdowns on lakes with major deepwater wetlands can cause catastrophic and possible long-lasting changes in the plant communities. 

A winter 1981-82 drawdown of Blue Lake, Oregon reduced dense beds of Eurasian watermilfoil that had maintained position and density in the lake relatively unchanged since 1973. However regrowth of new stems from surviving rootcrowns was widespread (N. Stan Geiger - Winter drawdown for the control of Eurasian watermilfoil in an Oregon oxbow lake (Blue Lake, Multnomah County, published in Lake Restoration Protection and Management, EPA 440/5-83-001)

G. Dennis Cook in the 1980 article "Lake level drawdown as a macrophyte control technique" recommended lake level drawdown for macrophyte control in situations where prolonged (one month or more) dewatering of lake sediments in possible under rigorous conditions of cold or heat, and where susceptible  species are the major nuisances. (Water Resources Bulletin, Vol. 16, No. 2). The author points out that rigorous conditions suitable for macrophyte control may not occur with heavy snowfall or in milder, rainy winters.

In Washington, Lake Chelan is drawdown significantly on a regular basis for power generation. This exposes Eurasian watermilfoil plants to freezing/desiccation and as a result milfoil has not become a problem in Lake Chelan. The same situation occurs in Lake Roosevelt, the reservoir formed behind the Grand Coolie Dam. Although Lake Roosevelt is being continuously fed milfoil fragments from the Pend Oreille River which enters the Columbia River in Canada, milfoil has not established nuisance populations in Lake Roosevelt. We believe that extensive drawdowns for power generation have had the side benefit of controlling milfoil. However, native aquatic plants have not established to any degree either.  

Advantages

• If a water control structure is in place, drawdown can be a very cost effective way of controlling plants like Brazilian elodea and Eurasian watermilfoil.
• The expansion of native aquatic plants in areas formerly occupied by exotic species can be enhanced by drawdown.
• Game fish are reported to experience enhanced populations after drawdown.
• Drawdowns provide an opportunity to repair and improve docks and other structures.
• Loose, flocculent sediments can become consolidated after drawdown occurs.

Disadvantages

• To be cost effective, a water level control structure must be in place, otherwise high capacity pumps must be used.
• The growth of some aquatic plants may be enhanced by water level drawdowns - know the species that you want to control.
• Winter weather may influence the success in killing the target species. Snow before a hard freeze may insulate the sediment and prevent freezing to a depth that will kill the roots; milder climates may not experience the freezing or dewatering conditions needed to kill the exposed plant roots and rhizomes.
• Docks are left high and dry, water intakes may no longer be in the water, it may not be possible to launch boats, and some people will complain about aesthetics of the waterbody.
• There will be significant impacts to fish and aquatic wildlife by lowering the water and exposing the sediments.
• Algal blooms have been reported to occur after drawdowns have occurred.
• Water levels may be lower in wells during drawdowns.

Permits

Permits are required for many types of projects in lakes and streams. Check with your state and local jurisdictions before proceeding with a water level drawdown.

Costs

If a water level structure is in place, costs may be minimal.