Aquatic Weed Growth Prompts Quarantine in NW Montana Lake

A Fish, Wildlife and Parks sign at Beaver Lake warns of the danger of Eurasian watermilfoil, as buoys mark out where mats have been laid to choke out the aquatic invasive weed.

A stubborn patch of Eurasian watermilfoil in a northwest Montana lake prompts the state Department of Agriculture to quarantine the area to boats. The quarantine is temporary. It will last until cleanup operations wrap up, and monitors stop spotting Eurasian watermilfoil in Beaver Lake.

The lake sits north of Whitefish with a view of the ski runs at Big Mountain, and a small outlet that drains to Whitefish Lake. The patch of weeds sits right next to the boat launch at Beaver Lake. Consulting Biologist Erik Hanson says cleanup is going quickly,  “we didn’t find very much, we haven’t done the full perimeter of the lakeshore, but we’ve done, you know, 3-to-500 yards in proximity to the patch, and only found, probably 10 little spots where there are plants growing.”  Hanson said the outlet to Whitefish Lake is on the opposite side of Beaver Lake from the weed patch, and they’ll be working their way around the lake, monitoring for Eurasian watermilfoil plants and fragments.

The weed spreads through fragmentation. A curtain is set up on the outlet, blocking fragments from floating downstream. Monitors first spotted the patch on Beaver Lake last fall. Flathead County placed barrier mats over the area to eliminate the light source and choke out the plant. This summer ongoing monitoring efforts spotted the weed floating in the lake. The existing barriers aren’t being moved until fall so as to not to stir up the weeds, and additional barriers cover an extended area.

The non-profit Flathead Lakers organization is paying for a dredging operation through a grant from the Department of Natural Resources and Conservation. Hanson was brought on board by the Flathead Lakers.

“We’re using a very small dredge. So, basically it’s a 200 pound dredge that floats on two, four-foot pontoons that can be pulled around by a diver in the water,” Hanson said, “what the diver is doing, is he’s actually using it more like a vacuum cleaner to kind of get the top of the plant, and then he moves the dredge down the plant and then helps remove the root mass from the soil, and it just kind of sucks up the plant to the surface.”

Aquatic Plant Specialist Craig McLane with the state Department of Agriculture says the weeds weren’t pulled to begin with because of timing.

“Eurasian watermilfoil goes through a period where it breaks up, and disperses, and it was found so late in the year, we were already in that period of that fragmentation,” McLane said, “they originally began to pull it, but quickly realized that it wasn’t a feasible option because they were just breaking the plants up and those little fragments can start new populations. So, it was just better to cover them up, and keep ‘em where they were, and just let them die.”

McLane said this is the only known Eurasian watermilfoil in the Flathead Basin. The Department of Agriculture lists the Toston and Fort Peck Reservoirs, Jefferson River, lower Clark Fork, and portions of the Upper and Lower Missouri River as also having the invasive weed.

“It has the ability to get really thick and dense, and if it gets- for example, in an irrigation ditch, it will slow irrigation water, cause sedimentation issues, prevents recreation of fishing, and boating, and can displace native species as well,” McLane said.

The temporary quarantine on boats on Beaver Lake went into place last week. After cleanup officials will continue to monitor, looking for any other plants or fragments “so if we can’t find any plants, and there’s no fragments floating around, and we feel comfortable that we’ve got it under control, we’ll lift the quarantine,” McLane said the use of herbicides is on the table, if necessary, but the current treatment plan does not call for the use of weed killer. He said Eurasian watermilfoil gets caught on trailers and tangled in props of boats, spreading to the next water body the boat enters.

The quarantine does not block people from fishing or swimming on Beaver Lake.

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