Boaters on TVA’s upstream reservoirs are urged to look out for exposed rocks, stumps and sandbars this holiday weekend with lake levels averaging seven feet below normal, TVA officials said Thursday.

But the 3-year-old drought that has lowered TVA’s storage reservoirs is showing some signs of easing this summer. A year ago, lake levels on TVA’s 10 storage reservoirs averaged 13 feet below normal, six feet below their current stage, TVA spokesman John Moulton said.

“Our main stream reservoirs like Chickamauga and Nickajack in Chattanooga are at normal summertime pools, but many of our storage reservoirs (in East Tennessee, Northeast Georgia and Western North Carolina) didn’t come up to normal summertime levels this year,” Mr. Moulton said. “Even though we have held back as much water as we could this year, our lake levels are still down in many areas.”

Despite predictions of rain this weekend in Chattanooga, rainfall so far this year in Chattanooga is nearly seven inches below normal.

Dave Hotz, a meteorologist for the National Weather Service in Morristown, Tenn., said there is a 40 percent chance of rain today and Sunday in Chattanooga and a 60 percent chance of some thundershowers Saturday.

“The rain might disrupt some of the July Fourth celebrations, but any thunderstorms won’t be drought breakers,” Mr. Hotz said.

So far this year, rainfall in Chattanooga has totaled 22.5 inches, or 6.9 inches below normal. But last year at this time, Chattanooga had received less than 14.3 inches of rain.

TVA meteorologists said rainfall in the Tennessee Valley during 2007 averaged the lowest amount in the 118 years that such records have been kept.

The rainfall deficit has cut power production from TVA’s 29 hydroelectric stations to only 58 percent of normal. Hydropower generated at TVA dams is the cheapest source of electricity for the federal utility, and replacing the lost hydroelectric supplies has cost TVA more than $250 million in extra power generation and purchased power costs, according to financial filings.

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