Bottom Barriers


One of the strategies we use to control the rampant spread of Eurasian Milfoil in Lake Fairlee is the deployment of bottom barriers. These are sheets of heavy black PVC plastic, which are spread on the bottom of the lake. Each sheet is 6 feet by 100 feet. We have about 200 of them. They are held down with epoxy coated “rebar” bars and hooks. The sheets keep the sunlight from reaching the plants, which kills them and prevents new growth.

Our permit from the Vermont Department of Environmental Conservation allows us to place up to three acres of bottom barrier at a time. We are required to use only qualified professionals to install it. Because the decaying plants release methane, which collects in large bubbles under the sheets and can dislodge them, the installed bottom barriers must be inspected regularly. The sheets cannot be installed before June 22nd, in deference to the spawning season of the resident fish. And no area of the lake can be covered for more than 28 months.

In fact we are only able to cover about two acres at any one time. We generally use the last five or six weeks of each season to remove all of our bottom barriers and move them to a new location. It takes our full crew, four divers and a boat person, that long to do the job. Our finances are stretched to the limit, and we want to keep the suction harvester operating as long as possible. Also, we have found that the most effective way to deploy the sheets we have is to overlap them 12 to 18 inches.

preparing to removeWe use the same pontoon boat for the bottom barrier operation as for suction harvesting. divers in the water remove the rebar from a section, and bring the end up to the waiting boat.

hooked to the boat There are hooks on the front of the pontoons to which the end of the sheet is attached. then the boat backs carefully over the sheet, peeling it gently off the bottom until it streams our behind.

hauling-out-sheets-1-2.JPGThen the crew on the boat pulls the sheet up onto the deck, folding it back and forth. You cannot tell from the picture how dirty a job this part is, nor how bad it smells.

stacking sheets Here they are stacking the folded sheets. Note (especially in the large picture at the top) how low in the water the boat is floating. In previous years we have pulled al the sheets, storing them on the shoreline, then done all the installing. This year, because many of the moves were not a long way, we filled the boat one day and put it all down the next, when possible.

laying sheetsIt is harder to show the process of laying down the sheets. Most of the activity takes place under the water. There are three divers working, one at each end and one in the middle, making sure that the sheets get situated properly. where the milfoil is thick, or where the water is deep, the job gets harder. Wind can also create problems, making it nearly impossible to get the sheets into position. Then each sheet is weighted with coated rebar, and hooked in place with J-hooks.

This year the largest area of bottom barrier is just east of Treasure Island — the island, not the beach. A new patch of milfoil erupted there this summer and we are trying to gain some control of it before it spreads. Some of this patch is rooted in water deeper than fifteen feet, which is very unusual. Because of financial constraints we had to let the divers go a week earlier this year, and were therefore unable to move all the bottom barrier we intended to.