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Massachusetts Water Resources Authority
May 13, 2013

Leonard Cawley, Community Relations Manager
(617) 660-7972,

MWRA to Begin Large-Scale Removal of Invasive Aquatic Plants
from Wachusett Reservoir

Figure 1. Eurasian watermilfoil "topped out" at the height of the growing seasoon

Figure 1. Eurasian watermilfoil “topped out” at the height of growing season

Figure 2 - Wachusett Reservoir

Figure 2. Wachusett Reservoir


The Stillwater River supplies the Wachusett Reservoir via a series of basins (Stillwater, Oakdale and Thomas). The invasive plants Eurasian watermilfoil (Myriophyllum spicatumFigure 1) and Fanwort (Cabomba caroliniana) were discovered in 2001 in Stillwater Basin, the uppermost basin (Figure 2). Both plants are known to aggressively displace native vegetation and grow into nuisance densities. These plants propagate through seeding, roots and by fragments of stems which drift downstream, sink and form new plants. Because of the large areas of shallows in the Wachusett Reservoir, particularly in the North Basin near the Intake, there is great concern over invasive plants colonizing these areas. Invasive plants can quickly reach nuisance densities and create impairments to water quality.

Figure 2 - Wachusett Reservoir

Figure 2. Wachusett Reservoir

After the discovery of these plants, a study was performed to assess options for eradication. One probable solution was to dredge the basin. However, since this method was quite costly, environmentally disruptive, and required a lengthy permitting process, MWRA decided not to delay addressing the problem and began, in 2002, control efforts such as placing benthic barriers to smother the plants, seasonal hand-pulling by divers, and installation of floating fragment curtains. These combined efforts prevented the migration of these plants into the main basin of the reservoir. Success was measured by no pioneering invasive plants observed in the main body of the reservoir during seasonal surveys, and the program continued annually.

Recent Developments:

In the summer of 2012, numerous pioneering colonies of invasive plants were discovered (and removed) from the main body of the reservoir. Because plant fragments are clearly migrating downstream despite the control efforts in place, MWRA reconsidered more intensive options to eradicate these invasive plants at their source, Stillwater Basin. Since the initial invasives control work began in 2002, a more efficient method of invasive aquatic plant removal has come into recent widespread use – Diver Assisted Suction Harvesting (DASH).

Figure 3 - DASH Boat Entering Oakdale Station

Figure 3. DASH Boat Entering Oakdale Basin

Figure 4 - Plants filtered from suction harvesting on board DASH

Figure 4. Plants filtered from suction harvesting on board DASH

2013 Invasives Control:

DASH harvesting includes the use of specialized boats (Figure 3) with suction hoses and screening trays to filter out the plant matter from the water. Divers carefully pull the invasive plants, roots and all, and guide them to the suction hose underwater. The plants emerge on the screens where they are collected and temporarily stored in a hopper onboard (Figure 4). The filtered water is returned to the reservoir. Once the boat hoppers are full, the plant matter is transported to shore containers. Once these containers are filled, they will be taken to a DCR composting facility.


MWRA contractors will deploy three DASH boats utilizing six divers starting May 15, 2013.  They will work the Stillwater Basin following a grid pattern starting at the northern end of the basin and move south.  The project will occur in two phases from May 15th to October 15th to cull the initial growth and return later in the season to address likely regrowth.  MWRA will also deploy a Quality Assurance Diver to inspect and videotape the underwater work zones for documentation

MWRA’s goal is to remove the source invasive plants from Stillwater Basin to protect the Wachusett Reservoir’s water quality.  Detailed follow-up monitoring and selected diver hand-pulling in subsequent seasons after this project will be required to ensure the plants do not return.    MWRA is also developing long-term alternatives, such as dredging, if the DASH approach does not achieve the desired level of control.



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Posted May 13, 2013