Is there any general rule of thumb as to how far down you draw the water? One foot? Five feet? Based on total or average depth of BOW? A percent of that depth? We are toying with this concept. We were thinking of doing it in an attempt to freeze/kill some invasive/problem weeds along the shoreline in shallow water each winter.
When you start looking for information about drawdowns, there are some basic concepts. Note some of the sources ALSO list the disadvantages of drawdowns; do your homework on those items. Below are some items that I found from several sources. Many of the sources repeat the basic concepts.
Depending on the size of the pond and the availability of deep water, the pond should be dropped to expose 30 to 40 percent of the pond bottom. If ponds lack extensive deep water, they should be dropped less to prevent catastrophic fish loss. But the drawdown should expose as much pond bottom as possible, restricting water to deeper sections only.
Drawdown encourages population control when forage fishes are overabundant. Lowering the water congregates the forage, and large predator fishes, preventing forage fishes from having protection in shallow water. The larger prey fishes, such as largemouth bass and catfish, will reduce the population of the smaller fishes over the winter, also allowing bigger fish to put on excess weight.
If ponds are experiencing noxious weed problems in the shallower sections, winter drawdown will directly affect these weed banks in a negative fashion. Often, weeds around the pond margins and in shallow areas become problems in the summer, growing in huge mats. While some weeds are beneficial, noxious aquatic weeds will get out of control and must be controlled. Ponds become sinks for nutrient runoff in agriculture settings, and the shallow water allows for full sunlight penetration, allowing unrestricted growth. Drawdown will expose these weed banks to air-drying and freezing temperatures, often killing many of these problem weeds. While drawdown will not completely-eradicate weed problems, winter drawdown will help combat noxious aquatic weeds in shallow areas.
https://efotg.sc.egov.usda.gov/references/public/MS/ms-ecs-399-02.pdfhttp://www.solitudelakemanagement.com/blog/winter-drawdown-reducing-nuisance-aquatic-weedsAn effective method, that is not new to lake management, is winter drawdown. This technique drops the water level to expose the dormant plants in the sediment to sub-freezing temperatures during winter months decreasing their survival. Additionally, “a drawdown is a prime opportunity to conduct a pre-emergent herbicide application for aquatic weed control,” says Sarah Miller, Aquatics Specialist at SePRO Corporation. “During this time, low rates can be strategically applied to the exposed littoral zone and prevent nuisance weeds from ever having a chance to establish or regrow after filling. This proactive approach provides a proven management option for sites with challenges such as high flow and irrigation use.” These products may not be an option during the summer, because of irrigation restrictions, but with no irrigation required in the winter, these products will be long gone before irrigation resumes in the spring.
https://www3.nd.edu/~aseriann/Lake_Drawdown.pdf
Fall/winter drawdowns can be beneficial in that some desiccation takes place as the waterbody is dewatering, but thorough freezing of the plants and the lake sediments is the key.Freezing of theplants damages the structure and integrity of the vegetative material. Freezing of the lake sediments will impact rooting systems and rhizomes, both by freeze damage, scouring, and
subsequent uplifting of the rooting systems.Scouring action of ice moving over the exposed lake bed will force tubers and rooting systems from the substrate. When the water level is again raised, these anchoring plant structures will often float downstream and discharge through the lake basin, or they can be hand removed as they float around.
A study oflake drawdown conducted by Dennis Cooke 1980 found that various aquatic plants responded differently to drawdown. Summary of Winter Drawdown Study Findings:
Decrease in Abundance - Watershield (Brasenia), Pondweed (Potamogeton), Yellow water lily, (Nuphar), White water lily (Nymphea), Spike rush (Eleocharis), Water milfoil (Myriophyllum spp), Pickerelweed (Pontedaria)
Increase in Abundance - Bulrush (Scirpus), Arrowhead (Sagittaria), 3way sedge (Dulichium)
No Change - Bladderwort (Utricularia), Tape grass (Vallisneria), Bur reed (Sparganium
