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Diver dredging is a mechanical control technology for milfoil removal that was pioneered by the British Columbia Ministry of Environment. During diver dredging operations, divers use venturi pump systems (small gold mining dredges) to suction plants and roots from the sediment. The pumps are mounted on barges or pontoon boats and the diver uses a long hose with a cutter head to remove the plants. The plants are vacuumed through the hose to the support vessel where the plants are retained in a basket and sediment and water are discharged to the waterbody. Often a silt curtain is deployed around the treatment site to control turbidity. Click here to learn more about diver dredging.
Sites suitable for diver dredging include lakes or ponds lightly to moderately infested with milfoil. Because diver dredging can be very expensive, this method is most suitable for moderate to early infestations of milfoil and for follow-up milfoil removal after an herbicide treatment. Diver hand pulling is more effective in lightly scattered patches of milfoil, whereas diver dredging may be more appropriate in denser milfoil beds. Diver dredging may also be applicable in water bodies where no herbicide use can be tolerated. Theoretically diver dredging could be used in any waterbody to eradicate milfoil; however the costs for large scale projects would become astronomical.
Development of an integrated vegetation management plan is advised prior to beginning a diver dredging project. Diver dredging projects may require a federal permit from the US Army Corps of Engineers. The necessity for the Corps of Engineers permit is site dependent. State permits for diver dredging for noxious weed removal is covered by the Hydraulic Approval pamphlet Aquatic Plants and Fish.
The littoral zone of the lake is surveyed immediately prior to starting control work and milfoil locations are mapped and Global Positioning System (GPS) points established.
Diver dredging can begin as soon as milfoil can be easily seen and identified - generally in the spring. If diver dredging is being used as a milfoil eradication method also see the milfoil eradication strategy using hand pulling and bottom barrier installation. Diver dredging can be used in conjunction with these other methods to achieve eradication; with dredging used to reduce the density of plants, followed up by hand pulling. Generally diver dredging projects continue for several years and are very expensive.
During diver dredging, the divers may use a tool to loosen milfoil root crowns before using a suction head to remove the plant. In hard-packed or rocky sediments, the plants often break off at the root crown, leaving the root behind to re-grow. In these areas, alternative control methods, such as bottom barrier installation, should be used. In locations with denser milfoil colonies, divers should make several passes through the area to ensure that all plants have been located and removed. Removed plants can be used for compost rather than having to be discarded as solid waste.
Factors that affect the success of diver dredging include: sediment type, visibility, amount of fragments created, density of native aquatic plants, and effort expended. The amount of acres covered per day is dependent on plant density, ease of removal, and number of divers. Once milfoil plants have become sparse, diver hand pulling is just as fast as dredging and has less impacts.
Sometimes diver dredging equipment is used just to transport plants to the surface. The diver pulls the plant and uses the dredge hose to suction the plant to the support boat rather than placing the plants in a bag and carrying them to the surface. Using a dredge for plant disposal is not considered dredging and does not trigger the need for Corps of Engineers approval.
In Washington, diver dredging was used in Silver Lake in Everett to contain a relatively early infestation of milfoil. Although milfoil was not eradicated in Silver Lake, dredging, in combination with hand pulling and bottom barrier installation, did remove most of the milfoil from the lake. Diver dredging is also being used in Idaho lakes and rivers to contain recently discovered milfoil populations.
No research has been conducted in Washington to quantify the impacts of diver dredging. Although the object of diver dredging is to remove milfoil, sediment is unavoidably stirred into the water. The obvious impact of diver dredging is increased turbidity in the area of plant removal with the degree of turbidity dependent on the sediment type. Fine, silty sediments produce more turbidity than sandy or rocky sediments. If turbidity interferes with the ability of the divers to see the milfoil plants, efficacy of plant removal can be affected. Diver dredging may also release buried pollutants and/or nutrients. In Silver Lake, sediment bioassays were required prior to dredging to ensure that the sediments did not contain toxic materials. Bioassays are probably more important in waterbodies with a history of mining, combined sewage outfalls, land filling, storm water outfalls, or other activities that may have contributed pollutants to the sediments.
It is very difficult to control fragment release during dredging operations. If a silt barrier is deployed around the dredging site for turbidity control, divers should make an attempt to collect milfoil fragments within the area before removing the barrier.
Diver dredging, used alone, is probably not an eradication tool, but it can be the first step to reducing the biomass of milfoil to the point where other manual methods can be used to eventually eradicate the plant.
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