Milfoil can sometimes effectively be controlled when waterbodies are dewatered by releasing water via a water level control structure (dam or weir) or by pumping. The effectiveness of milfoil control is determined by several factors including the amount of the waterbody bottom exposed, duration of exposure, presence of springs, and the weather at the time of drawdown. The success or failure of drawdowns in controlling milfoil can be highly variable from lake to lake and from year to year within the same waterbody (Vermont Agency of Natural Resources, 1989). G. Dennis Cook (1980) recommended lake level drawdown for macrophyte control in situations where prolonged (one month or more) dewatering of lake sediments is possible under rigorous conditions of cold or heat; a key factor being desiccation. The author pointed out that those conditions suitable for macrophyte control may not occur with heavy snowfall or during milder, rainy winters. Click here for more information about water level drawdown.
In Washington, milfoil control has usually been a side benefit of drawdown regimes occurring in waterbodies and reservoirs for other purposes such as for power generation, irrigation, or flood control. The impacts of fluctuating water levels are severe on a natural waterbody so this activity rarely occurs solely for milfoil control in Washington. Waterbodies suitable for water level drawdown are those with infestations of milfoil where drawdown occurs on a prolonged and regular basis. Because western Washington is so much wetter and milder than eastern Washington, drawdown is generally more successful in controlling eastern Washington milfoil populations. However, in some western Washington reservoirs, such as Tapps Lake and Riffe Lake, prolonged annual drawdowns have helped control milfoil infestations. Since milfoil survives in deeper water, drawdowns will not eradicate milfoil from the waterbody. Generally waterbodies with fluctuating water levels such as reservoirs are highly perturbed systems.
Because water level drawdown impacts the entire waterbody, it should be conducted only under the direction of an integrated aquatic vegetation management plan. Few waterbodies in Washington, except for reservoirs, have water control structures and the means to lower the water level to the extent necessary to achieve significant milfoil control. Some lakes with water level controls also have court adjudicated water levels. Because impacts to habitat are severe, drawdown should only be considered as a milfoil control in waterbodies where the habitat value is not considered important by resource agencies.
Factors to consider when evaluating water level drawdown as a possible control for milfoil include:
As the water recedes, docks and other shoreline structures, such as retaining walls and irrigation or potable water intakes, are exposed and shallow wells may run dry. It may become impossible to launch boats, and boating and other recreational activities may be curtailed or restricted during drawn down periods. On the plus side, lowered water levels may allow repairs to be more easily made to near-shore structures. Sometime drawdown can consolidate flocculent sediments and results in firmed sediments when the water returns.
Water level drawdown exposes the sediment and affects the habitat for emergent and submersed plants, fish, benthic invertebrates, waterfowl, and aquatic mammals. Vermont concluded that drawdown did major damage to deepwater wetland communities at Lake Bomoseen. It caused decreases to two rare plant species and provided only short-term control of milfoil. Greening and Gerritsen (1987) noted that frequent drawdowns result in a reduction in species diversity and favor tolerant plants which eventually come to dominate the lake.
The impacts to animals by the Lake Bomoseen winter drawdown (September 1988 to March, 1989) were also significant. The drawdown “decreased habitat suitability for species that require stable water levels such as beaver and muskrat by preventing them from using their winter food supplies and exposing them to adverse weather and predation. Habitat suitability was decreased for species that overwinter in the bottom sediments such as frogs, turtles, and macroinvertebrates because freezing the sediment kills these animals.” The Vermont report also concluded that the drawdown of Lake Bomoseen had an adverse impact on all the littoral zone macroinvertebrate communities (snails, mussels, aquatic insects). The impacts to fish by the Lake Bomoseen drawdown were difficult to measure because only one year of data was collected.
Other impacts that may occur after drawdown include:
There is some anecdotal evidence in Washington to suggest that milfoil seeds may germinate after summer lake bottom desiccation. In two small natural lakes in Thurston County where milfoil had been eradicated, milfoil appeared in abundance after drought conditions contributed to partial or whole lake drawdown. The fall/winter following the drought, the lakes, refilled and an abundant population of milfoil was observed in the spring/summer, particularly in the areas where the lakes had been dewatered.
Cook, G. D. 1984.Lake level drawdown as a macrophyte control technique. Water Resources Bulletin, Vol. 16, No. 2.
Greening, H.S. and Gerritsen, J. 1987. Changes in macrophyte community structure following drought in the Okefenokee Swamp, Georgia. USA. Aquatic Botany, 28:113-128.
A report prepared for the Vermont Legislature by the Vermont Agency of Natural Resources, Waterbury, Vermont. 1989. The Lake Bomoseen drawdown: An Evaluation of its Effects on Aquatic plants, wildlife, fish, invertebrates, and recreational uses.
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