The effects of Virginia's fall fire season could range from a greater threat of flooding in southwest streams and rivers to smaller harvests of shellfish in the Chesapeake Bay, a state hydrologist said this week.

"Our greatest concern is where the 'duff,' or forest litter, burns down to the soil," Garnet Wood said from his office at the Department of Forestry's Charlottesville office. "It takes about two to three years for that surface to begin accumulating again."

The forest litter consists of everything from dead ground vegetation to twigs and fallen leaves. When the ground is covered by it, rains soak in without creating much runoff.

But in some spots where the 156 Virginia forest fires burned 13,021 acres under state jurisdiction this fall, the forest litter burned right through to the soil.

Wood has surveyed the damage in southwest Virginia. He said the danger is hard to estimate.

"It's variable. I can't say it's critical or that there's no concern," he said.

But forestry officials said the fires, most of them set by arsonists in Virginia's coal country, came at the worst time of the year.

"You've got the whole year-end and the snow season ahead of you before any vegetation grows back," forestry spokesman Lou Southard said. "You do have several months when rains could fall before there's new vegetation."

That could lead to problems from the southwest all the way to the bay, Wood said. The trouble could start when rains hit the soil directly, forcing rapid erosion and filling mountain streambeds with too much dirt.

"It reduces the carrying capacity of our state's streams and rivers, and that way, flooding can occur more often with less rain," Wood said. "Over time, it ends up in the Chesapeake Bay. It increases the nutrient load in nitrogen and phosphates."

Both hurt fish and shellfish, he said. The phosphates are toxic to marine life. The nitrates cause an increase in algae, which can crowd the bay's waters, using much of the oxygen supply and cutting off sunlight filtering through the water.

"It reduces the environment for shellfish because there's less light getting to the bay floor," Wood said.

Wood said further damage could come in forests where trees were weakened but not killed by the fires.

"These less-healthy trees are more susceptible to diseases and insects," he said. That could create breeding grounds for diseases and insects that might move on to harm other areas of Virginia's forests, he said.

But while the ultimate environmental impact of Virginia's worst fall fire season since the 1950s remains to be seen, so do the costs of fighting the blazes.

"It will be months before we can come up with that," Southard said. "I don't have any ballpark."

He said three arson arrests had been made in connection with the late October fires that were not brought under control until rain began falling on Nov. 10.