For Apple Valley there is an upside to December's streak of extreme cold: The frigid temperatures are ideal for the draining of Long Lake.

The city pulled the plug on the lake in September to expose its bottom to a deep freeze cold enough to kill the invasive curly-leaf pondweed that is fouling the lake's water quality.

Days of temperatures at zero degrees have gotten the project off to a good start, said Jeff ­Kehrer, the city's natural resource coordinator last week. "For December this is pretty darn cold, pretty fast."

Drawdowns deliver best results in a dry, cold winter in which the lake bottom is not insulated by snow and freezes solid.

Snow cover on the lake is light so far, Kehrer said. The last time the lake was drained, in the winter of 2009, it was covered with a thick layer of snow and the winter was not a cold one. "I think we should have much better results than we had the first drawdown. I have a better feel about this one."

But Kehrer said he is ­making no predictions of success until he sees the lake next spring.

The goal is to knock back the curlyeaf pond weed for a number of years, he said. "I am guessing that we will take care of the rough fish." Rough fish, including bullheads, also were targeted for removal.

The pondweed forms thick mats on the water surface through June and July, making it difficult to canoe and kayak. Because it is the first plant to come up in the spring, curly-leaf blocks light to more slowly growing native plants, and when it dies back at the end of July, it releases phosphorous, which promotes green algae growth.

Draining lakes for the winter is the most dramatic tool used in Minnesota's campaign to restore water clarity. The 37-acre Long Lake was drained under Pilot Knob Road into adjoining Farquar Lake. It is the fourth south metro lake to undergo a drawdown.

The cold is especially welcome after a challenging drawdown, Kehrer said. Rain in September and October kept refilling the lake. The lake level finally dropped low enough to allow the use of earth-moving machinery to remove built-up sediment conveyed into the lake by storm sewer pipes in two locations. There is about a foot of ice on the lake, and a trickle of water is still ­draining.

The valve will remain open until the end of winter, when it will be closed to allow the lake to capture snow melt and begin to fill up again, Kehrer said.

Long Lake Watershed Association President Paul Habegger said last fall at the start of the drawdown, "If this works, there are going to be a lot of happy people."