Question: Whatever happened to Lake Wilson after the salvinia weed onslaught?

Answer: Seven years ago, Wahiawa’s Lake Wilson looked more like a field than a lake.

But since workers cleared it in November 2003, there have been no reports of the invasive plant Salvinia molesta in the 325-acre lake, whose entire surface was covered by the weed in January 2002, said the Department of Land and Natural Resources’ Aquatic Resources Division.

The state invested more than $1 million in fighting the weed, summoning city, state and military crews and volunteers together to pull out the plant with machines and by hand, and controlling it with aquatic herbicides. Workers used excavators, dump trucks, boats and trailers to remove 50,000 cubic yards of the weed, which was taken to a disposal site.

The weed posed a threat to the lake and its ecosystem because it would cut off sunlight and oxygen to the water below. Dense mats of salvinia can also clog waterways and water intake structures. The size of the weed can double in a day.

Salvinia molesta is a popular aquarium plant, but its sale in Hawaii is prohibited. It was added to the state’s noxious weeds list by the Legislature in 2003, the same time it created the Hawaii Invasive Species Council.

Lake Wilson made a perfect home for Salvinia molesta because of its mild climate, fresh water and a source of nutrients from the 1.6 million gallons of treated sewage discharged daily in the lake from the Wahiawa Wastewater Treatment Plant.

"We do have staff up there regularly for other activities (managing the public fishing area and monitoring the lake), so if they were to spot any plants, we could take action quickly," DLNR spokeswoman Deborah Ward said.

Lake Wilson, also known as Wahiawa Reservoir, is the largest freshwater sport fishery in the state, holding more than 3 billion gallons of water with roughly 20 miles of shoreline. According to the DLNR, the lake was constructed in 1905-06 by damming the convergence of the north and south forks of the Kaukonahua Stream for sugar irrigation purposes. Swimming or water skiing in the lake is not allowed.

Ward said there is only one other location on Oahu that the DLNR knows of where Salvinia molesta is growing: a drainage ditch along Kapaa Quarry Road near the model airplane field in Windward Oahu. She said the area is managed by the city Department of Parks and Recreation, which sprays the weed with chemicals from time to time.

This update was written by Kimberly Yuen. Write to us at What Ever Happened to …, Honolulu Star-Advertiser, 500 Ala Moana Blvd., Suite 7-210, Honolulu 96813; call 529-4747; or e-mail cityeditors@staradvertiser.com.