WHEELING, W.Va. (AP) _ The city began depending on the Ohio River for all its water again today after levels of diesel fuel from a million-gallon spill fell to treatable levels, but authorities warned that business as usual could still drain reserves.

''We're just going to stay on top of it and keep working on it,'' said Lloyd Adams, the city engineer and public works director.

City Manager Mike Nau said reservoir levels were rising today as the river water began flowing.

Nau said the the city's main reservoir, with a 3.5 million-gallon capacity, contained slightly more than 2 million gallons this morning. The tank was down to 500,000 gallons Sunday morning.

Despite the improvement, Nau said businesses have been asked to remain closed and residents have been asked to avoid baths and showers to only use water stockpiled before the slick reached Wheeling. Schools were open.

Adams said Wheeling's Ohio River water intakes were turned on fully overnight, but barges that had been pumping clean water will be kept on standby and will resume work if pollution levels in the river rise. Adams said the plant is running at 30 percent to 40 percent capacity.

''It will be a while until we get out of the woods,'' Adams said.

A makeshift system had kept drinking water flowing since the fuel reached Wheeling's water intakes Friday. Officials in the city of 43,000 resumed pumping water Sunday from the Ohio through sponge filters, then mixed it in a reservoir with water pumped from the barges.

Assistant City Manager Nancy Vapner and other officials used the word ''critical'' to describe the situation.

Wheeling was the focus of water concerns Sunday as the last Pennsylvania residents affected by the Jan. 2 spill had their tap water restored. A civil emergency in Steubenville, Ohio, was lifted Saturday, and river water was being treated and pumped to customers there.

The next town expected to be affected is Sistersville, about 50 miles downstream from Wheeling, but authorities don't know when. Cold weather has caused ice in the Ohio River, keeping the fuel from dispersing and slowing its movement.

Sistersville Mayor Lester Leach said Ashland Oil Co. has dispatched a barge carrying more than 1 million gallons of water to the town of 2,200 people.

The fuel spilled into the Monongahela River south of Pittsburgh after a 40- year-old Ashland Oil tank crumpled as it was being filled. The slick has traveled more than 100 miles on the Monongahela and the Ohio through Pennsylvania, Ohio and West Virginia.

Ashland Oil has offered to pay all cleanup costs.

''We're asking everyone to keep an accounting,'' West Virginia Gov. Arch Moore said after a helicopter tour Sunday morning. ''We expect to make demands and we have to have an accurate accounting of those demands.''

In Steubenville, the civil emergency that had ordered all non-essential businesses closed Friday was lifted ''after we took another look at things and figured that the worst of it was over,'' Mayor David Hindman said.

''I am not satisfied that the situation is back to normal. For the following weeks, we're going to have to watch everything, monitor everything,'' Hindman said. ''But I am sure, absolutely sure, that the water is safe. I went home last night and drank a whole quart of it and I didn't taste a thing.''

A suburban Pittsburgh water company opened its pumps Sunday for the first time in a week, bringing relief to the last of the Pittsburgh-area residents who had endured dry spigots.

''This is the best news I've ever had in my life,'' said Joe Wiesner, general manager of the Robinson Township Water Authority. ''There's been tremendous improvement, but we're asking people to conserve water for another day until we fill our storage tanks.''

Classes resumed today for about 5,000 pupils in the West Allegheny and Montour school districts near Pittsburgh. Students had been sent home last week in an effort to conserve water.

Wheeling reserves swelled by Sunday evening as Ohio River water was allowed to flow into the water system for the first time since Friday. But conservation measures were in effect. No fires or water shortages were reported in the city Sunday, officials said. Few residents took advantage of 14 National Guard water tanks scattered throughout the area, said Gene Barker, an assistant fire chief.

''No one took a shower this morning and we skipped the coffee at breakfast,'' Phyllis Stewart, 45, said Sunday as she left a church service.